Selina Concise Solutions for ICSE Class 7 Physics Chapter 2 Motion

Points to Remember

Motion - Motion is the change in position with respect to a set of stationary landmarks.
Types of motion are translatory, rotatory and oscillatory.

• A body is said to move in a translatory motion if the whole body moves through the same distance in the same interval of time.
• A body is said to be in rotatory motion if it moves about a fixed point without changing the radius of its motion.
• A body is said to be in oscillatory motion if it moves to and fro about its mean position.
• A body is said to be in vibratory motion if the body moves to and fro in a particular style.
• Oscillation of pendulum is one complete to and fro motion.
• The maximum displacement on either side of a mean position is called amplitude.
• A motion which repeats itself at regular intervals of time is called periodic motion.
• A physical quantity which has only magnitude and no direction is called a scalar quantity.
• A physical quantity which has magnitude as well as direction is called a vector quantity.
• Speed is the distance travelled per unit time. It is a scalar quantity.
• The displacement per unit time is known as the velocity of the body. It is a vector quantity.
• The rate of change of velocity is called acceleration. It is a vector quantity.
• Frequency is the number of complete oscillations in one second.
• Unit of frequency is Hertz.
• Motion is of two types (a) uniform (b) non-uniform motion.

Test yourself

A. Objective Questions

1. Write true or false for each statement

(a) Two trains going in opposite directions with the same speed are at rest relative to each other.
Answer: False.
Relative motion depends on both speed and direction. When objects move in opposite directions, their relative speed is the sum of their individual speeds, so they appear to move very fast past each other.
Teacher's Tip: Think of relative rest as "matching." To be at rest relative to each other, both speed and direction must be exactly the same.
Exam Tip: In "True or False" questions about relative motion, always look for the word "direction."

(b) A ball is thrown vertically upwards. Its motion is uniform throughout.
Answer: False.
Gravity constantly pulls the ball downward, causing its speed to decrease as it rises and increase as it falls. Because the speed changes at every point, the motion is non-uniform.
Teacher's Tip: Uniform motion means "constant speed in a straight line." If speed changes, it's non-uniform.
Exam Tip: Mention that gravity causes acceleration or deceleration whenever an object is thrown up or down.

(c) The motion of a train starting from one station and reaching at another station is non-uniform.
Answer: True.
A train must speed up from rest when leaving a station and slow down to stop at the next one. During the journey, it also changes speed for turns or signals, making the motion non-uniform.
Teacher's Tip: Real-world travel is almost always non-uniform because we cannot maintain the exact same speed for the whole trip.
Exam Tip: If a body starts from rest or comes to a stop, it is always an example of non-uniform motion.

(d) A motion which repeats itself after a fixed interval of time is called periodic motion.
Answer: True.
Periodic motion follows a strict schedule where every cycle takes the exact same amount of time. Examples include the ticking of a clock or the orbit of the Earth around the Sun.
Teacher's Tip: Use a "calendar" analogy—anything that happens on a regular, predictable repeat is periodic.
Exam Tip: Do not confuse "periodic" with "oscillatory"; while many oscillatory motions are periodic, not all periodic motions go to-and-fro.

(e) A ball thrown by a boy from a roof-top has oscillatory motion.
Answer: False.
A ball thrown from a roof follows a curved or straight path toward the ground without swinging back and forth. This is actually a type of translatory or curvilinear motion.
Teacher's Tip: For motion to be oscillatory, it must return to its starting point over and over, like a swing.
Exam Tip: Always identify the path of the object before labeling the type of motion.

(f) Mass has both magnitude and direction.
Answer: False.
Mass is a scalar quantity that only tells us the amount of matter in an object. It stays the same no matter which way the object is facing or moving.
Teacher's Tip: Think of mass as a "count" of atoms; you can't have "5 kg to the North."
Exam Tip: Remember that Weight is a vector, but Mass is always a scalar.

(g) Weight always acts vertically downwards.
Answer: True.
Weight is the force of gravity pulling an object toward the center of the Earth. Since the center is directly below us, the force always points straight down.
Teacher's Tip: Imagine a plumb line; weight always follows that perfectly vertical path toward the ground.
Exam Tip: Use the phrase "towards the center of the Earth" to get extra marks when defining weight direction.

(h) Mass varies from place to place but weight does not.
Answer: False.
Mass is constant because the amount of matter doesn't change, but weight depends on gravity. Weight changes if you go to the moon or a different planet, while mass stays identical.
Teacher's Tip: Remember: Mass is "Me" (what I am made of), Weight is "Wait" (the pull on me).
Exam Tip: This is a common trick question; always remember that mass is constant and weight is variable.

2. Fill in the blanks

(a) Two boys cycling on the road with the same speed are at rest relative to each other.
Answer: at rest.
When two objects move in the same direction at the same speed, the distance between them never changes. Because they don't move further apart or closer together, they appear still to one another.
Teacher's Tip: This is like sitting next to a friend in a moving car; you both move, but you stay in the same spot relative to each other.
Exam Tip: Relative rest requires identical velocity (same speed AND same direction).

(b) The motion in a straight line is rectilinear motion.
Answer: rectilinear.
Rectilinear motion describes an object moving along a single, straight path without turning. It is the simplest form of translatory motion.
Teacher's Tip: Look at the word: "recti" means straight (like a rectangle's sides) and "linear" means line.
Exam Tip: Use the term "linear" or "rectilinear" interchangeably unless your teacher specifies otherwise.

(c) One to and fro motion of a clock pendulum takes time = 2 s
Answer: 2 s.
Standard tall "grandfather" clocks are designed so that each swing takes exactly one second in each direction. A full "to and fro" cycle covers two seconds total.
Teacher's Tip: Think of a clock's tick-tock; "tick" is one second, "tock" is the second second.
Exam Tip: Remember that one oscillation is the full trip from start, to the other side, and back again.

(d) 36 km h^-1 = 10 m s^-1
Answer: 10 m s^-1
To convert km/h to m/s, you multiply by 5/18. Here, 36 × 5/18 = 2 × 5 = 10.
Teacher's Tip: Use the "5/18 trick" for quick conversions in your head.
Exam Tip: Always show the conversion steps (36 ×1000/3600 ) to ensure full marks for calculation.

(e) Total distance travelled = average speed × total time taken.
Answer: average speed.
Total distance is found by looking at the entire journey's duration and the overall rate of travel. Even if the speed varied, the average speed represents the steady rate needed to cover that distance in that time.
Teacher's Tip: Rearrange the triangle: D = S × T.
Exam Tip: If the motion is non-uniform, you must use "average speed," not just "speed."

(f) The weight of a girl is 36 kgf. Her mass will be 36 kg.
Answer: 36 kg.
The unit "kgf" (kilogram-force) is defined as the weight of a 1 kg mass on Earth. Therefore, the numerical value of mass in kg is equal to the numerical value of weight in kgf.
Teacher's Tip: Kgf is a shortcut unit designed to make the math between mass and weight easy on Earth.
Exam Tip: Be careful with units; "kg" is for mass and "kgf" or "N" is for weight.

(g) The weight of a body is measured using a spring balance.
Answer: spring balance.
A spring balance works by measuring how much a spring stretches under the pull of gravity. Since weight is a force, the more it pulls, the more the spring extends.
Teacher's Tip: Think of a "S" for Spring and "S" for Stretching force (Weight).
Exam Tip: Do not confuse this with a beam balance, which compares masses.

Activity

Examples of motion - Type/types of motion

Soldiers in a march past on a straight road:
Answer: Rectilinear.
Soldiers marching on a straight path move in a fixed direction without turning. This is a classic example of straight-line translatory motion.
Teacher's Tip: "Marching" is repetitive, but the path they take defines the motion type.
Exam Tip: Always check if the road is described as "straight" or "curved" before answering.

The movement of our chest while breathing:
Answer: Vibratory.
Breathing involves the ribcage expanding and contracting in a repeated, style-specific rhythmic motion. Because the parts are fixed at one end but move back and forth, it is vibratory.
Teacher's Tip: Vibratory motion is a subset of oscillatory motion where the object might change shape slightly.
Exam Tip: Internal organ movements are often categorized as vibratory or non-periodic depending on the context.

Hands of an athlete in a race:
Answer: Oscillatory.
When running, an athlete's arms swing back and forth relative to the shoulder joint. This repeated movement about a central point is oscillatory.
Teacher's Tip: Anything that "swings" is usually oscillatory.
Exam Tip: Identify the "mean position" (the resting position of the arm) to confirm it is oscillatory.

Pedal of a bicycle in motion:
Answer: Circular.
The pedal moves in a perfect circle around the center of the crankset. Every point on the pedal maintains a constant distance from the axle.
Teacher's Tip: Don't confuse the pedal's motion with the bicycle's motion; focus only on the pedal's path.
Exam Tip: If it moves in a loop around an external center, it is circular.

Motion of earth around the sun:
Answer: Circular.
The Earth travels in a nearly circular (elliptical) orbit around the Sun. Because it follows a path around an external body, it is categorized as circular or revolutionary motion.
Teacher's Tip: "Around" usually implies circular/revolution, while "about its axis" implies rotatory.
Exam Tip: You can also call this "Periodic" motion because it happens every 365 days.

Motion of a swing:
Answer: Oscillatory.
A swing moves to and fro about its resting position when pushed. It repeats this path until friction brings it to a stop.
Teacher's Tip: A swing is just a large human-carrying pendulum.
Exam Tip: Oscillatory motion is the most common answer for "back and forth" movements.

Motion of a pendulum:
Answer: Oscillatory.
The bob of a pendulum moves from one extreme to the other, passing through the center each time. This is the scientific standard for oscillatory motion.
Teacher's Tip: If you see "pendulum," your first thought should always be "oscillatory."
Exam Tip: Pendulums are also "periodic" because each swing takes a set amount of time.

A stone falling from a certain height:
Answer: Rectilinear.
Gravity pulls the stone straight down toward the ground. Since it travels in a straight vertical line, its motion is rectilinear.
Teacher's Tip: Falling objects follow the shortest path to the ground: a straight line.
Exam Tip: Note that while the path is rectilinear, the speed is non-uniform (it accelerates).

A plucked string of a sitar:
Answer: Vibratory.
Plucking a string makes it move back and forth very rapidly while its ends remain fixed. This high-speed to-and-fro motion creates sound.
Teacher's Tip: Vibratory motion is just "fast" oscillatory motion in objects that are fixed at ends.
Exam Tip: Strings of instruments always exhibit vibratory motion.

A car moving on a straight road:
Answer: Rectilinear.
If the road has no turns, the car travels in a single direction along a straight path. This is a type of translatory motion.
Teacher's Tip: Most textbook "car" examples assume a straight road unless mentioned otherwise.
Exam Tip: Distinguish between the car's body (rectilinear) and its wheels (rotatory).

Motion of a train on a straight bridge:
Answer: Curvilinear.
(Note: While the OCR says Curvilinear, a straight bridge usually implies Rectilinear. If the bridge has a slight curve, it is Curvilinear.)
Teacher's Tip: If the path is a curve, use Curvilinear; if it is a line, use Rectilinear.
Exam Tip: Read the description carefully; "straight" means rectilinear.

Motion of a spinning wheel:
Answer: Rotatory.
A spinning wheel turns around a fixed axle that passes through its center. The wheel stays in one place while its parts revolve around the axis.
Teacher's Tip: Think of a "Spindle" or an "Axle" for rotatory motion.
Exam Tip: Rotatory motion occurs "about an axis" that is usually inside the body.

The movement of the wheel of a bicycle:
Answer: Translatory.
(Note: This refers to the movement of the bicycle wheel as it travels from point A to point B. It actually has Mixed motion: rotatory + translatory).
Teacher's Tip: A bicycle wheel rolls, meaning it turns (rotates) and moves forward (translates) at the same time.
Exam Tip: If asked for the "whole" movement of the bicycle, "translatory" is often the focus, but "mixed" is more accurate.

3. Match the following

Column A
(a) Circular motion
(b) Periodic motion
(c) Vibratory motion
(d) Rotatory motion
(e) Non uniform motion
Column B
(i) a running fan
(ii) a car moving in a market
(iii) revolution of earth around the sun
(iv) motion of wire of a guitar
(v) motion of pendulum of a clock
Answer:
(a) Circular motion (iii) revolution of earth around the sun
(b) Periodic motion (v) motion of pendulum of a clock
(c) Vibratory motion (iv) motion of wire of a guitar
(d) Rotatory motion (i) a running fan
(e) Non uniform motion (ii) a car moving in a market

4. Select the correct alternative

(a) A book lying on a table is an example of
1. a body at rest
2. a body in motion
3. a body neither at rest nor in motion
4. none of these
Answer: 1. a body at rest.
The book does not change its position relative to the table or the room. Therefore, it meets the scientific definition of being at rest.
Teacher's Tip: Rest is the absence of motion relative to a specific surroundings.
Exam Tip: Always identify the "stationary landmark" (the table) to prove an object is at rest.

(b) The motion of a pendulum is
1. rotatory
2. oscillatory
3. curvilinear
4. rectilinear
Answer: 2. oscillatory.
A pendulum moves to and fro about its central equilibrium position. This repeated swinging motion is the definition of oscillatory motion.
Teacher's Tip: Pendulum and Oscillatory are "best friends" in physics questions.
Exam Tip: Curvilinear is also technically true for the bob's path, but "oscillatory" is the more specific and correct physics term here.

(c) A car moving on a straight road is an example of
1. rotatory motion
2. rectilinear motion
3. oscillatory motion
4. periodic motion
Answer: 2. rectilinear motion.
Since the road is straight, the car follows a single linear path. This type of translatory motion is called rectilinear.
Teacher's Tip: Straight path = Rectilinear; Curved path = Curvilinear.
Exam Tip: Look for keywords like "straight" or "curved" to choose between linear types.

(d) A ball falls down vertically. Its motion is
1. periodic
2. linear
3. circular
4. vibratory
Answer: 2. linear.
A falling ball travels in a straight line toward the ground. "Linear" is another term for rectilinear motion.
Teacher's Tip: Gravity pulls in a straight line, so free-fall is always linear.
Exam Tip: Even if the ball speeds up, its *path* remains a straight line.

(e) If a body covers equal distance in equal interval of time, the motion is said to be
1. uniform
2. non-uniform
3. oscillatory
4. rotatory
Answer: 1. uniform.
Uniform motion means the speed never changes. No matter how small the time interval you check, the distance covered is always the same.
Teacher's Tip: Uniform = Boring and Predictable. Nothing changes.
Exam Tip: The definition must include "equal distance" AND "equal interval of time."

(f) A boy goes from his house to school by bus at a speed of 20 km h¹ and returns back through the same route at a speed of 30 km h¹. The average speed of his journey is
1. 24kmh¹
2. 25 km h¹
3. 30 km h¹
4. 20 km h¹
Answer: 1. 24kmh¹
Teacher's Tip: Don't just add and divide by 2! Use the proper harmonic mean formula for round trips.
Exam Tip: Always use the total distance divided by total time if you forget the shortcut formula.

(g) The earth attracts a body of mass 1 kg with a force of 10 N. The mass of a boy is 50 kg. His weight will be
1. 50 kg
2. 500 N
3. 50 N
4. 5 N
Answer: 2. 500 N.
Weight is calculated as Mass × Gravity. Since 1 kg is pulled with 10 N, a 50 kg mass is pulled with 50 × 10 = 500N.
Teacher's Tip: On Earth, just add a zero to the mass in kg to get the weight in Newtons (assuming g=10).
Exam Tip: Check your units; Weight must be in Newtons (N) or kgf, never just kg.

B. Short/Long Answer Questions

Question 1: Explain the meaning of the terms rest and motion.
Answer:
Rest - A body is said to be at rest if it does not change its position with respect to its immediate surroundings.
Motion - A body is said to be in motion if it changes its position with respect to its immediate surroundings.
Rest and motion are relative states that depend entirely on the observer's frame of reference. For example, a person sitting in a chair is at rest relative to the floor but in motion relative to the Sun.
Teacher's Tip: Use a "Reference Point" to decide if something is moving; if the distance to that point changes, there is motion.
Exam Tip: Always include the phrase "with respect to its surroundings" for a complete definition.

Question 2: Comment on the statement ‘rest and motion are relative terms’. Give an example.
Answer:
Imagine you are sitting inside a moving bus. When you look outside you will observe that you are moving. Now look to the roof of the bus. With respect to the roof of bus, you are at rest. Hence it is concluded that rest and motion are relative terms.
This means that an object can be at rest and in motion at the same time, depending on what you compare it to. There is no such thing as "absolute rest" in the universe.
Teacher's Tip: Think of a passenger on a plane; they are at rest to their seat but moving at 500 mph to the ground.
Exam Tip: Use the "moving bus" example in exams as it is the most standard and accepted illustration.

Question 3: Fill in the blanks using one of the words : at rest, in motion.
(a) A person walking in a compartment of a stationary train is ________ relative to the compartment and is ________ relative to the platform.
(b) A person sitting in a compartment of a moving train is ________ relative to the other person sitting by his side and is ________ relative to the platform.
Answer:
(a) A person walking in a compartment of a stationary train is in motion relative to the compartment and is in motion relative to the platform.
(b) A person sitting in a compartment of a moving train is at rest relative to the other person sitting by his side and is in motion relative to the platform.
These examples show how movement is judged by the change in distance between two objects. In the first case, the person moves away from the train walls; in the second, the passenger stays the same distance from their neighbor.
Teacher's Tip: If you can reach out and touch the other person without reaching further over time, you are "at rest" relative to them.
Exam Tip: Read carefully whether the train is "stationary" or "moving" before filling the blanks.

Question 4: Name five different types of motion you know.
Answer:
The different types of motion are :
1. Translatory motion
2. Rotatory motion
3. Oscillatory motion
4. Vibratory motion
5. Periodic motion
6. Multiple motion
7. Random motion.
Objects in nature often exhibit more than one of these at once. For example, a rolling ball shows both translatory and rotatory motion.
Teacher's Tip: Memorize the first four as your "primary" types.
Exam Tip: If the question asks for five, providing six or seven ensures you get full marks even if one is slightly off.

Question 5: What do you mean by translatory motion ? Give one example.
Answer:
If an object like a vehicle, moves in a line in such a way that every point of the object moves through the same distance in the same time, then the motion of the object is called translatory motion.
Example : The motion of an apple falling from a tree, the motion of a man walking on a road, the motion of a box when pushed from one corner of a room to the other, are all the translatory motion.
In this motion, the object as a whole moves from one position to another. It can be in a straight line (rectilinear) or a curved line (curvilinear).
Teacher's Tip: "Translatory" means "shifting" the whole object from point A to point B.
Exam Tip: Emphasize that "every point" of the object moves the same distance to distinguish it from rotatory motion.

Question 6: Explain the meanings of (i) rectilinear motion, and (ii) curvilinear motion. Give one example of each.
Answer:
(i) Rectilinear motion - If the motion of a body is along a straight line, it is said to be the rectilinear or linear motion. The motion of bullet fired from a gun.
(ii) Curvilinear motion - If the motion of a body is along a curved path, it is said to be the curvilinear motion. For example, the motion of a cycle while taking a turn on the road, a car moving along a curved path, a ball thrown by an athlete are in curvilinear motion.
Both are types of translatory motion defined by the geometry of the path. Rectilinear is a 1D path, while curvilinear is a 2D or 3D path.
Teacher's Tip: Think "R" for Rectilinear (Rigid straight line) and "C" for Curvilinear (Curves).
Exam Tip: Use simple diagrams of a straight arrow and a curved arrow to help illustrate your answer.

Question 7: What is rotatory motion ? Give two examples.
Answer:
Rotatory motion - A body is said to be in a rotatory motion or a circular motion if it moves about a fixed axis without changing the radius of its motion.
Examples : The blades of a fan, a spinning wheel.
The axis of rotation is the imaginary line around which the object spins. In rotatory motion, the body stays in its general location while spinning.
Teacher's Tip: Rotatory motion is like a "spinning top" — it stays on its point but turns around.
Exam Tip: Mention the "fixed axis" as it is the most important part of the definition.

Question 8: What is meant by circular motion ? Give one example.
Answer:
The motion of a body along a circular path is called circular motion.
Example : A girl is whirling a stone tied at the end of a string in a circular path.
The distance of the body from the center point remains constant throughout the motion. Unlike rotatory motion, the body itself travels around an external point.
Teacher's Tip: Think of a race track; the car is doing circular motion, but its wheels are doing rotatory motion.
Exam Tip: Note that in circular motion, the axis of revolution is outside the body.

Question 9: How does a rotatory motion differ from the circular motion?
Answer:
(i) In rotatory motion, the axis of rotation passes from a point in the body itself whereas in circular motion, the axis of revolution passes through a point outside the body. Thus the motion of earth around the sun is the circular motion whereas the motion of earth about its own axis is the rotational motion.
(ii) In the circular and rotatory motions, the distance of a point of the body from a fixed point always remains same, whereas it is not same in curvilinear motion.
The key difference is the location of the pivot point. Rotation is an "internal" spin, while circular motion is an "external" loop.
Teacher's Tip: Remember: Rotation = Spin; Circular = Orbit.
Exam Tip: Use the Earth as your example for both to show you understand the difference clearly.

Question 10: Explain oscillatory motion by giving one example.
Answer:
Oscillatory motion - The to and fro motion of a simple pendulum is an oscillatory motion.
Example : 1. The motion of a swing, 2. Piston of an engine.
This motion occurs when a body moves back and forth repeatedly over the same path. It passes through a central "mean" position in every cycle.
Teacher's Tip: Oscillatory motion is like a "seesaw" or a "swing" — it keeps coming back to where it started.
Exam Tip: Always mention that the motion is "to and fro" about a "mean position."

Question 11: What is vibratory motion ? Give one example.
Answer:
In vibratory motion, a part of the body always remains fixed and the rest part moves to and fro about its mean position. During the vibratory motion, the shape and size of the body changes.
Example : When we breath, our chest expands and contracts. This motion is vibratory motion.
Vibratory motion is often very fast and involves elastic objects. It is the basis for all musical instruments like drums and violins.
Teacher's Tip: Vibration is just "fast, small oscillations."
Exam Tip: Highlight that in vibratory motion, the object's shape often changes during the movement.

Question 12: Differentiate between periodic and non-periodic motions by giving an example of each.
Answer:
Periodic motion : A motion which gets repeated after regular intervals of time is called a periodic motion.
Examples : The earth moving around the sun takes 365 days to complete one revolution and this motion gets repeated after every 365 days.
Non-periodic motion : The motion which does not repeat itself after regular interval of time is called non-periodic motion.
Examples : A footballer running on a field, application of brakes in a moving vehicle, a ball rolling down the ground gradually slows down and finally stops, motion of tides in the sea, etc.
Periodic motion is predictable and follows a "clockwork" rhythm. Non-periodic motion is random or changes based on external forces and human choice.
Teacher's Tip: Periodic = Scheduled; Non-periodic = Random.
Exam Tip: Use time intervals (like 365 days or 24 hours) to prove a motion is periodic.

Question 13: What is random motion. Give one example.
Answer:
Random motion - When an object in a motion has no specific path and which suddenly changes its motion is said to have a random motion.
Example : A flying kite.
In random motion, the direction and speed of the object change constantly without any pattern. It is often caused by external forces like wind or collisions.
Teacher's Tip: Think of a "Fly" or a "Kite" — you never know where they will turn next.
Exam Tip: Contrast this with "Periodic" motion to show you understand that random motion lacks a regular interval.

Question 14: Name the type/types of motion being performed by each of the following:
(a) Vehicle on a straight road
(b) Blades of an electric fan in motion
(c) Pendulum of a wall clock
(d) Smoke particles from chimney
(e) Hands of a clock
(f) Earth around the sun
(g) A spinning top.
Answer:
(a) Rectilinear motion
(b) Rotatory motion
(c) Oscillatory motion, periodic motion
(d) Non-periodic motion (or Random motion)
(e) Uniform circular and periodic motion
(f) Rotatory motion, circular motion and periodic motion
(g) Rotatory motion.
Most of these objects exhibit "multiple motion," meaning they fit into more than one category at the same time. For example, a clock hand is both circular and periodic.
Teacher's Tip: For complex objects like the Earth, always list all three (Rotatory, Circular, Periodic).
Exam Tip: If you aren't sure, identifying the most obvious one (like "Rotatory" for a fan) usually gets you marks.

Question 15: Give two examples to illustrate that a body can have two or more types of motion simultaneously.
Answer:
Sometime a body can have more than one type of motion. Such a motion is called the mixed motion.
Example :
(i) The wheels of a moving train have both the translatory as well as the rotatory motions as it moves from position A to position B while rotating.
(ii) The earth rotates about its axis (rotatory motion) and at the same time it revolves around the sun in a curved path (curvilinear or circular motion) in a fixed time interval (periodic motion).
Mixed motion is very common in machinery and celestial bodies. It occurs when an object is doing something locally (like spinning) while the whole object moves globally.
Teacher's Tip: Think of "Rolling" as the ultimate mixed motion: it's Rotation + Translation.
Exam Tip: Always define what "mixed motion" is before giving your examples.

Question 16: State the types of motion of the following :
(a) The needle of a sewing machine
(b) The wheel of a bicycle
(c) The drill machine
(d) The carpenter’s saw
Answer:
(a) Periodic motion (Oscillatory/Vibratory)
(b) Rotatory motion (and Translatory when riding)
(c) Mixed = Translatory and Rotatory motion
(d) Mixed = Translatory and Oscillatory motion.
In a sewing machine, the needle goes up and down rapidly, which is oscillatory and periodic. Tools like drills must move forward into the wood (translatory) while spinning (rotatory).
Teacher's Tip: Tools usually have "Mixed" motion because they have to "move" and "work" at the same time.
Exam Tip: For the bicycle wheel, specify "Rotatory" if it's spinning on a stand, and "Mixed" if it's on the road.

Question 17: Distinguish between uniform and non-uniform motions, giving an example of each.
Answer:
Uniform motion:
1. When a body covers equal distances in a straight line, in equal intervals of time, however small these time intervals may be.
2. In this case direction of motion remains the same.
3. Example : A body moving with a constant speed in a straight line has uniform motion.
Variable motion or Non-uniform motion:
1. When a body covers unequal distances in equal intervals of time in a straight line.
2. In this case direction of motion changes.
3. Example : Circular motion is example of non-uniform motion.
Uniform motion requires a constant velocity, meaning speed and direction never change. Non-uniform motion occurs if either speed or direction changes, such as a car slowing down for a turn.
Teacher's Tip: Uniform motion is very rare in nature; it's mostly a concept for physics problems.
Exam Tip: Draw a distance-time graph (a straight line for uniform, a curve for non-uniform) to make your answer stand out.

Question 18: How do you determine the average speed of a body in non-uniform motion ?
Answer:
In a non-uniform motion, the average speed of a body is calculated by dividing the total distance travelled by the body, with the total time of its journey. Thus,
Average speed = Total distance travelled by the body / Total time of journey
This provides a single value that summarizes the entire trip, even if the object stopped or changed speeds many times. It is the most useful way to describe travel between two points.
Teacher's Tip: Think of it as the "steady speed" that would have gotten you there in the same amount of time.

Question 19: Define the term weight and state its S.I. unit.
Answer:
The weight of a body is the force with which earth attracts the body i.e. the weight of a body is the force of gravity on it. The weight of a body is not constant, but it changes from place to place. It is represented by the symbol W. The S.I. unit of weight is newton (N).
Because weight is a force, it is measured in the same units as any other force. It is directly proportional to both the mass of the object and the strength of gravity.
Teacher's Tip: Weight is a "Pull," not an "Amount."
Exam Tip: Always specify that the S.I. unit is the Newton (N), even if you often use kgf in class.

Question 20: How are the units of weight, kgf and newton related ?
Answer:
1 kg F = 10 N
Specifically, 1 kgf is the force of gravity on a 1 kg mass. Because gravity on Earth is approximately 9.8  m/s² (rounded to 10 for simplicity), 1 kgf equals about 10 Newtons.
Teacher's Tip: "kgf" stands for "Kilogram Force."
Exam Tip: If your teacher uses 9.8 instead of 10, write 1kgf} = 9.8 N}.

Question 21: State three differences between mass and weight.
Answer:
Mass
It is the quantity of matter contained in a body.
Its S.I. unit is kilogram (kg).
It is constant for a body and it does not change by changing the place of the body.
It is measured by a beam balance.
Weight
It is the force with which the earth attracts the body.
Its S.I. unit is newton (N) and other unit is kilogram-force (kgf) where 1 kgf = 10 N (nearly).
It is not constant for a body, but it changes from place to place.
It is measured by a spring balance.
Mass is an intrinsic property that depends on the atoms inside an object. Weight is an interaction between the object and a planet's gravity.
Teacher's Tip: Use a table to present these differences for better clarity.
Exam Tip: The "constant vs. variable" difference is the most important one to mention first.

Question 22: Which quantity : mass or weight, does not change by change of place ?
Answer:
The mass of a body is constant and it does not change by changing the position of the body.
Whether you are at the bottom of the ocean, on top of Mt. Everest, or on the Moon, your mass remains exactly the same. Only your weight would change due to variations in gravity.
Teacher's Tip: Mass is "loyal" — it follows you everywhere without changing!
Exam Tip: Use the word "constant" to describe mass in your answer.

Question 23: State which of the quantities, mass or weight is always directed vertically downwards.
Answer:
Mass is the quantity of matter contained in a body. Weight is the force with which the earth attracts the body. Weight is always directed vertically downwards.
Since mass is a scalar, it has no direction at all. Weight is a vector force that pulls toward the center of gravity, which is always "down" for us on Earth.
Teacher's Tip: Only vectors have directions. Mass has no direction, so it can't be "downward."
Exam Tip: Use the definition of Weight as a "force" to justify why it has a direction.

C. Numericals

Question 1: A car covers a distance of 160 km between two cities in 4 h. What is the average speed of the car ?
Answer:
Distance = 160 km
Time taken = 4 h
Speed = ?
Speed = Distance covered / Time taken
= 160km / 4h = 40 km h^-1
This means that, on average, the car covered 40 kilometers for every hour it was driving. This is a straightforward application of the speed formula.
Teacher's Tip: Always write the formula first before putting in the numbers.
Exam Tip: Don't forget the units (km/h or kmspace h^-1}) in your final answer.

Question 2: A train travels a distance of 300 km with an average speed of 60 km h^-1. How much time does it take to cover the distance?
Answer:
Speed = 60 kmh^-1
Distance covered = 300 km
Speed = Distance covered/Time taken
therefore Time taken = Distance covere/speed
= 300 text km/60  kmh^-1 = 5 h
By dividing the total distance by the rate of speed, we find the duration of the trip. Since the speed is in km/h and distance in km, the result is naturally in hours.
Teacher's Tip: Rearrange the speed formula triangle: Time = Distanc/Speed.
Exam Tip: Check that your units match (km and km/h) before you start calculating.

Question 3: A boy travels with an average speed of 10 m ^s-1 for 20 min. How much distance does he
travel ?
Answer:
Average speed of boy = 10 m s^-1
Time taken = 20 min
Distance travelled = Speed × Time taken
Convert minutes into seconds
1 minute = 60 sec.
20 minutes = 20 × 60 = 1200 sec.
Distance travelled = 10 m ^s-1 × 1200 sec.
= 12000 m Or 12 km
Because the speed is given in meters per second, the time must be converted to seconds to find the distance. Multiplying these gives the total meters, which can then be converted to kilometers for a cleaner answer.
Teacher's Tip: "Units must match!" If speed has seconds, time must be in seconds.
Exam Tip: 12000 m is a correct answer, but converting to 12 km shows extra expertise.

Question 4: A boy walks a distance 30 m in 1 minute and another 30 m in 1.5 minute. Describe the type of motion of the boy and find his average speed in m ^s-1.
Answer:
As the speed does not remain constant throughout the journey the motion is non-uniform
Total distance travelled in going and coming back
d = 30 m + 30 m = 60 m
Total time taken in going and coming back
t = 1 min + 1.5 min = 2.5 min
= 2.5 × 60 s = 150 s
Average speed = Total distance travelled/Total time of travel
= 60m/150 s = 0.4 m s^-1
Since the boy took different amounts of time to cover the same distance, his speed changed, making it non-uniform. The average speed is the total path length divided by the total duration of the walk.
Teacher's Tip: "Average Speed" is the only fair way to talk about a journey where speed changes.
Exam Tip: Always state the "type of motion" (non-uniform) if the question asks for a description.

Question 5: A cyclist travels a distance of 1 km in the first hour, 0.5 km in the second hour and 0.3 km in the third hour. Find the average speed of the cyclist in (i) km h^-1, (ii) m s^-1.
Answer:
(a) Distance travelled in first hour = 1 km
Distance travelled in second hour = 0.5 km
Distance travelled in third hour = 0.3 km
Total time taken = 3 hr
Total distance travelled = 1 + 0.5 + 0.3 = 1.8 km
(i) Average speed in km h-1
Speed = Distance/Time taken = 1.8/3 = 0.6 km h^-1
Average speed in m s-1
1 km = 1000 m
1.8 km = 1.8 × 1000 m
= 1800 m
1 hour = 3600 seconds
3 hour = 3600 × 3 = 10800 sec.
Average speed = D/T = 1800/10800 = 0.167 m s^-1nearly
The cyclist is slowing down over time, so we calculate the overall average by summing all distances and dividing by the total three hours. For the second part, converting both units to meters and seconds is essential.
Teacher's Tip: For m/s, you can also just take your km/h answer and multiply by 5/18.
Exam Tip: Keep at least two or three decimal places for m/s results to ensure accuracy.

Question 6: A car travels with speed 30 km h^-1 for 30 minute and then with speed 40 km h^-1 for one hour. Find :
(a) the total distance travelled by the car
(b) the total time of travel, and
(c) the average speed of car
Answer:
Speed of car for first 30 minutes = 30 km h^-1
Speed of car for next 1 hour = 40 km h^-1
(a) Total distance travelled by the carIst case, Speed = Distance/Time
Distance = Speed × Time
(because 30 minutes = 0.5 hours)
Distance = 30 × 0.5 = 15 km ...(i)
IInd case Speed = Distance/Time
Distance = Speed × Time
Distance = 40 km h^-1 times 1 hr = 40 km ...(ii)
Adding (i) and (ii): Total Distance = 15 km + 40 km = 55 km
(b) Total time of travel = 0.5 hr + 1.0 hr = 1.5 hr
(c) Average speed = Total distance travelled/Total time taken
= frac55 km1.5 hr = 36.67 km h^-1
To solve multi-part trips, find the distance for each segment separately first. Then, sum the totals and divide to find the overall average speed for the whole journey.
Teacher's Tip: Always convert minutes to hours (divide by 60) if your speed is in km/h.
Exam Tip: Label each case (Case 1, Case 2) clearly to make your work easy for the examiner to follow.

Question 7: On earth the weight of a body of mass 1.0 kg is 10 N. What will be the weight of a boy of mass 37 kg in (a) kgf (b) N ?
Answer:Weight of a body of mass 1.0 kg body = 10 N
(a) Weight of a boy of mass = 37 kg
(b) Weight of a boy of 37 kg in newton will be 1 kgf = 10N
∴ 37 kgf = 37 × 10 N
= 370 N

Question 8: The weight of a body of mass 6.0 kg on moon is 10 N. If a boy of mass 30 kg goes from earth to the moon surface, what will be his (a) mass, (b) weight ?
Answer:(a) Mass remains same it does not change
So mass of boy 30 kg on earth = 30 kg on moon surface
(b) Weight of boy on moon becomes 1 / 6
∴ 30 kg boy will weight 30 x 1/6 = 5kg
1 kg = 10 N ⇒ 5 × 10 N = 50 N
∴ Weight of boy on moon surface = 50 N