Class 7 Science Motion and Time Exam Notes

Class 7 Science Motion and Time Exam Notes. Please refer to the examination notes which you can use for preparing and revising for exams. These notes will help you to revise the concepts quickly and get good marks.

Covers the following topics:

Motion and time are basic concepts of Class 7 Science. This exam notes page covers the CBSE Class 7 Science chapter on Motion and Time, including units and physical quantities, rest and motion, types of motion, uniform and non-uniform motion, distance and displacement, speed, velocity, acceleration, acceleration due to gravity, measurement of time and speed, and the distance-time graph, followed by the chapter exercise with answers.

Topics Covered

1. Unit
2. Physical quantities
3. Rest and motion
4. Types of motion
5. Uniform and non-uniform motion
6. Terms used to define motion
7. Measurements
8. Distance-time graph

UNIT

Measurement of any physical quantity is expressed in terms of an internationally accepted basic standard called unit.

Length, time and mass are measured in metre, second and kilogram.

PHYSICAL QUANTITIES

The quantities which can be measured by an instrument, and by means of which we can describe the laws of physics, are called physical quantities.

Examples: length, velocity, acceleration, force, time, pressure, mass, density, etc.

REST AND MOTION

An object is said to be at rest if it does not change its position with respect to its surroundings with the passage of time.

A body is said to be in motion if its position changes continuously with respect to its surroundings (or with respect to an observer) with the passage of time.

TYPES OF MOTION

(a) Linear motion: The motion of a moving car, a person running, or a stone being dropped.
(b) Rotational motion: The motion of an electric fan, or the motion of the Earth about its own axis.
(c) Oscillatory motion (to and fro motion): The motion of a simple pendulum, or of a body suspended from a spring, are examples of oscillatory motion.

UNIFORM AND NON-UNIFORM MOTION

(a) Uniform motion:
A body has uniform motion if it travels equal distances in equal intervals of time, no matter how small these time intervals may be.
For example, a car running at a constant speed of 10 m/s covers equal distances of 10 metres in every second, so its motion is uniform.

(b) Non-uniform motion:
A body has non-uniform motion if it travels unequal distances in equal intervals of time.
For example, if a ball is dropped from the roof of a building, it covers unequal distances in equal intervals of time. It covers:
4.9 metres in the 1st second
14.7 metres in the 2nd second
24.5 metres in the 3rd second, and so on.

TERMS USED TO DEFINE MOTION

(a) Distance and displacement

(i) Distance: Distance is the actual path travelled by a body in a given time. If a body travels from A to B along any path, the actual length of that path is the distance. If the body goes from A to B via C, the distance travelled is ACB. Distance is a scalar quantity, and it is different for different paths between A and B.

[Figure: Points A and B on a horizontal line, with a point C above the line between them, showing a curved or bent path from A through C to B]

(ii) Displacement: The distance travelled in a given direction is the displacement. Thus, displacement is the shortest distance between the given points. The S.I. unit of distance and displacement is metre.

If an object moves from O to B and then from B to A, the displacement is x2 - x1.

[Figure: A number line with points O, A, B and X on the X-axis; x1 marks the position of point A and x2 marks the position of point B, with the label x2 - x1 = positive]

If Suresh walks from point A to B and then again from B to C, the distance he has travelled is AB + BC, while his displacement is the straight line from A to C.

[Figure: Three points A, B and C forming a bent path A to B to C, with a straight line joining A and C]

Note: If a body comes back to its starting position, its displacement is zero. However, the distance travelled is never zero.

(b) Speed

(i) Speed: Speed of a body is the distance travelled by the body in one second.

Speed = \( \frac{Distance travelled}{Time taken} \)

Distance is measured in metre and time in second, so the unit of speed is metre per second (m/s). Speed can also be expressed in kilometre per hour (km/h).

If we know the speed of an object, we can find the distance covered by it in a given time. Distance covered = speed × time.

(ii) Average speed: The speed of a bus during a journey may vary. Its speed decreases near a bus stop, it is greater on highways, and it is less in a city because of heavy traffic. Such a body has variable speed, and for such a body we calculate the average speed.

Average speed = \( \frac{Total distance travelled}{Total time taken} \)

[Figure: Bus journey showing 20 km travelled in the first hour, 35 km in the second hour, 15 km in the third hour, 60 km in the fourth hour and 40 km in the fifth hour, marked on a time line from 0 to 5 hours, labelled Variable speed]

[Figure: Bus journey showing 40 km travelled in every hour from 0 to 5 hours, labelled Uniform speed]

If a body moves with the same speed at all times, we say that it has uniform speed.

(c) Velocity

Paths from a house to a school can have different lengths, so the distance is not the same for every path. If you travel from your house to school in a straight line, that is the shortest distance between them, called displacement.

Velocity is the displacement of a body in one second. Velocity is nothing but speed in a definite direction.

Velocity = \( \frac{Displacement}{Time taken} \)

Its unit is m/s.

(d) Acceleration

If you ride a bicycle to school and you are late, you would increase your velocity, or accelerate.

Acceleration is the change of velocity in one second.

Acceleration = \( \frac{Change in velocity}{Time taken} \)

Its unit is m/s². If a car has an acceleration of 5 m/s², its velocity increases by 5 m/s every second. If the velocity of a moving body decreases, it has negative acceleration, which is also called retardation or deceleration.

Acceleration due to gravity

When a ball is thrown up vertically, its velocity gradually decreases until it becomes zero, so the ball is retarded. As the ball falls down, its velocity gradually increases, so it is accelerated. This retardation or acceleration is due to the Earth's gravitational force, and it is known as acceleration due to gravity.

The velocity of a body decreases by 9.8 m/s every second when it is thrown up, and increases by 9.8 m/s every second when it falls down.

MEASUREMENTS

(a) Measurement of time

Many events in nature repeat themselves after definite intervals of time. The sun rises every morning, and the time between one sunrise and the next is called a day. A month was measured from one new moon to the next, and a year is the time taken by the Earth to complete one revolution around the Sun. Clocks and watches, the most common time measuring devices, make use of some periodic motion.

One of the most well-known periodic motions is that of a simple pendulum. A simple pendulum consists of a small metallic ball, or a piece of stone, suspended from a rigid stand by a thread. The metallic ball is called the bob of the pendulum.

When the pendulum is at rest in its mean position, and the bob is released after taking it slightly to one side, it begins to move to and fro. This to and fro motion is an example of periodic or oscillatory motion.

[Figure: Simple pendulum with its bob swinging between the extreme positions A and B, with the mean position O in between; the bob is fastest at O and at rest at A and B]

The pendulum completes one oscillation when its bob, starting from its mean position O, moves to A, then to B, and back to O. The time taken by the pendulum to complete one oscillation is called its time period.

Activity: Gently hold the bob and move it slightly to one side. Release the bob from its displaced position, and measure the time taken to complete 20 oscillations. Repeat the activity a few times and record your observations. A slight change in the initial displacement does not affect the time period of the pendulum.

Unit of time: The basic unit of time is the second, whose symbol is s. Larger units of time are the minute (min) and the hour (h). Before pendulum clocks became popular, many time measuring devices were used in different parts of the world, such as sundials, water clocks and sand clocks.

(b) Measurement of speed: By calculating the distance travelled in a given time, we can calculate speed. Speed is generally measured in m/s. In vehicles, speed is measured in km/h by an instrument called the speedometer.

(c) Odometer: An odometer measures the distance moved by a vehicle directly.

[Figure: A speedometer dial and an odometer display, showing the instruments fitted in a vehicle]

DISTANCE-TIME GRAPH

Rajesh was travelling with his father in their car from Kota to Bundi. He noted the distance travelled by the car every 5 minutes. This is what he noted in the first 30 minutes:

Steps to draw the graph:
1. Take a graph sheet and draw two lines perpendicular to each other.
2. Mark the horizontal line as OX (x-axis) and the vertical line as OY (y-axis). Time is taken on the X-axis and distance on the Y-axis.
3. Choose scales for distance and time. For example, X-axis: 1 cm = 5 minutes, and Y-axis: 1 cm = 5 km.
4. Mark the values on the axes according to the chosen scales.
5. Mark the points for the observed values on the graph sheet, and join the points.

[Figure: Distance-time graph with time in minutes on the X-axis from 0 to 30 and distance in km on the Y-axis from 0 to 35, with points (0, 0), (5, 5), (10, 10), (15, 15), (20, 20), (25, 25) and (30, 30) joined in a straight line]

For uniform speed, the distance-time graph is always a straight line. For variable speed, it can be of any shape.

Example: Three cars A, B and C travel from Delhi to Agra. The time taken and the distance covered are given in the table below.

Plot the distance-time graph of the three cars on the same graph sheet.

[Figure: Distance-time graph of the three cars drawn from the origin 0, with Time (hour) on the X-axis from 0 to 4 and Distance (km) on the Y-axis from 0 to 60; three straight lines start from 0, the steepest one marked with a car, the middle one with a motorcycle and the lowest one with a person]

Greater the speed, steeper will be the graph.

EXERCISE

Single Correct Answer type Questions

Q.1 A boy whose position with respect to surroundings does not change, is said to be in a state of -
(A) Rest
(B) Motion
(C) Vibration
(D) Oscillation

Answer: (A) Rest

1. A body whose position does not change with respect to its surroundings is at rest.
2. Hence, the correct option is (A).

Teacher's Note:
a) Rest and motion are always judged with respect to the surroundings.
b) A passenger sitting still in a moving bus is at rest with respect to the bus, but in motion with respect to the road.

Q.2 In case of a moving body-
(A) Displacement > Distance
(B) Displacement < Distance
(C) Displacement ≥ Distance
(D) Displacement ≤ Distance

Answer: (D) Displacement ≤ Distance

1. Distance is the actual length of the path travelled.
2. Displacement is the shortest straight-line distance between the start and the end point.
3. The shortest line can never be longer than the actual path, so displacement is less than or equal to distance.
4. Hence, the correct option is (D).

Teacher's Note:
a) Displacement equals distance only when the body moves in a straight line.
b) Displacement can be zero, but distance cannot be zero for a moving body.
c) Students often write displacement greater than distance, which is impossible.

Q.3 A distance is always -
(A) Shortest length between two points
(B) path covered by and object between two points
(C) product of length and time
(D) none of these

Answer: (B) path covered by and object between two points

1. Distance is the actual path covered by a body between two points.
2. The shortest length between two points is displacement, so option (A) is wrong.
3. Hence, the correct option is (B).

Teacher's Note:
a) Distance is a scalar quantity, so it has magnitude only.
b) The shortest length between two points is the displacement, not the distance.

Q.4 Which of the following is not characteristic of displacement ?
(A) It is always positive
(B) It has both magnitude and direction
(C) It can be zero
(D) Its magnitude is less than or equal to the actual path length of the object

Answer: (A) It is always positive

1. Displacement is a vector quantity with magnitude and direction, so option (B) is a characteristic.
2. Displacement is zero when the body returns to its starting point, so option (C) is a characteristic.
3. Displacement is never more than the path length, so option (D) is a characteristic.
4. Displacement can be negative when the direction is opposite, so option (A) is not a characteristic.
5. Hence, the correct option is (A).

Teacher's Note:
a) Displacement carries a direction, so its sign depends on the direction of motion.
b) Zero displacement is common in round trips, where the body ends at its starting point.
c) Distance, not displacement, is always positive.

Q.5 In five minutes distance between a pole and a car changes progressively. What is true about the car ?
(A) Car is at rest
(B) Car is in motion
(C) Nothing can be said with this information
(D) None of the above

Answer: (B) Car is in motion

1. The distance between the pole and the car changes with time.
2. A change in position with respect to a fixed object (the pole) shows that the car is moving.
3. Hence, the correct option is (B).

Teacher's Note:
a) A body is in motion when its position changes with respect to a reference object.
b) The pole is treated as the fixed reference point here.

Q.6 A distance-
(A) Is always positive
(B) Is always negative
(C) May be positive as well as negative
(D) Is neither positive nor negative

Answer: (A) Is always positive

1. Distance is the length of the path covered by a body.
2. A length can never be negative, so distance is always positive or zero.
3. Hence, the correct option is (A).

Teacher's Note:
a) Distance is a scalar, so only its size matters, not its direction.
b) Displacement, unlike distance, can be negative.

Q.7 A particle is travelling with a constant speed. This means-
(A) Its position remains constant as time passes
(B) It covers equal distance in equal interval of time
(C) Its acceleration is zero
(D) It does not change its direction of motion

Answer: (B) It covers equal distance in equal interval of time

1. Constant speed means the distance covered in every second is the same.
2. Covering equal distances in equal intervals of time is the definition of uniform motion.
3. Hence, the correct option is (B).

Teacher's Note:
a) A particle with constant speed still changes its position.
b) Constant speed does not guarantee a constant direction; a body moving in a circle has zero change in speed but non-zero acceleration.
c) Option (C) is therefore not always true.

Q.8 In 10 minutes, a car with speed of 60 kmh^-1 travels a distance of
(A) 6 km
(B) 600 km
(C) 10 km
(D) 7km

Answer: (C) 10 km

1. Time = 10 minutes = \( \frac{10}{60} \) hour = \( \frac{1}{6} \) hour.
2. Distance = speed × time = 60 × \( \frac{1}{6} \) = 10 km.
3. Hence, the correct option is (C).

Teacher's Note:
a) Convert minutes into hours before multiplying with a speed in km/h.
b) Option (B) comes from using 10 minutes as 10 hours, which is a common unit error.

Q.9 A car acceleration uniformaly from 18 km/h to 36 km/h in 5s. The acceleration in ms^-2 is-
(A) 1
(B) 2
(C) 3
(D) 4

Answer: (A) 1

1. Initial velocity u = 18 km/h = 18 × \( \frac{5}{18} \) = 5 m/s.
2. Final velocity v = 36 km/h = 36 × \( \frac{5}{18} \) = 10 m/s.
3. Acceleration = \( \frac{v - u}{t} \) = \( \frac{10 - 5}{5} \) = 1 m/s².
4. Hence, the correct option is (A).

Teacher's Note:
a) Convert km/h to m/s by multiplying with 5/18 before using the formula.
b) Using 18 and 36 directly, without conversion, gives a wrong answer.

Q.10 The brakes applied to a car produce a negative acceleration of 6ms^-2. If the car stops after 2 seconds, the initial velocity of the car is-
(A) 6ms^-1
(B) 12 ms^-1
(C) 24 ms^-1
(D) Zero

Answer: (B) 12 ms^-1

1. Final velocity v = 0, acceleration a = -6 m/s², time t = 2 s.
2. Using v = u + at, 0 = u + (-6)(2).
3. So u = 12 m/s.
4. Hence, the correct option is (B).

Teacher's Note:
a) A negative acceleration means the velocity is decreasing, which is retardation.
b) Substitute the negative value of acceleration carefully to get a positive initial velocity.

Q.11 A body is moving with uniform velocity of 10 ms^-1. The velocity of the body after 10 s is-
(A) 100 ms^-1
(B) 50 ms^-1
(C) 10 ms^-1
(D) 5 ms^-1

Answer: (C) 10 ms^-1

1. Uniform velocity means the velocity does not change with time.
2. Since the acceleration is zero, the velocity after 10 s is still 10 m/s.
3. Hence, the correct option is (C).

Teacher's Note:
a) A body moving with uniform velocity has zero acceleration.
b) Time does not multiply the velocity; it only matters when the velocity changes.

Q.12 In 12 minutes a car whose speed is 35 kmh^-1 travels a distance of-
(A) 7 km
(B) 3.5 km
(C) 14 km
(D) 28 km

Answer: (A) 7 km

1. Time = 12 minutes = \( \frac{12}{60} \) hour = 0.2 hour.
2. Distance = speed × time = 35 × 0.2 = 7 km.
3. Hence, the correct option is (A).

Teacher's Note:
a) Always convert the time into hours when the speed is given in km/h.
b) Option (B) corresponds to a time of 6 minutes, so it does not match the question.
c) A quick check: 35 km in one hour, so 12 minutes must give a little more than one-fifth of that, which is 7 km.

Q.13 A body is moving along a straight line at 20 ms^-1 undergoes an acceleration of 4ms^-2. After 2 s. its speed will be-
(A) 8 ms^-1
(B) 12 ms^-1
(C) 16 ms^-1
(D) 28 ms^-1

Answer: (D) 28 ms^-1

1. Initial velocity u = 20 m/s and acceleration a = +4 m/s².
2. Velocity after t = 2 s is v = u + at = 20 + 4 × 2.
3. So v = 20 + 8 = 28 m/s.
4. Hence, the correct option is (D).

Teacher's Note:
a) A positive acceleration increases the speed of the body.
b) Subtracting the product of acceleration and time gives option (B), which is wrong here.
c) Check the sign of the acceleration before choosing between addition and subtraction.

Q.14 A car increases its speed from 36 km/h to 54 km/h in 10s. Its acceleration is-
(A) 30 ms^-2
(B) 3 ms^-2
(C) 18 ms^-2
(D) 0.5 ms^-2

Answer: (D) 0.5 ms^-2

1. Initial velocity u = 36 × \( \frac{5}{18} \) = 10 m/s.
2. Final velocity v = 54 × \( \frac{5}{18} \) = 15 m/s.
3. Acceleration = \( \frac{15 - 10}{10} \) = 0.5 m/s².
4. Hence, the correct option is (D).

Teacher's Note:
a) Convert both speeds to m/s before finding the acceleration.
b) The change in speed is 5 m/s, and it is spread over 10 seconds.

Q.15 The device used to measure speed of a vehicle is -
(A) Odometer
(B) Speedometer
(C) Thermometer
(D) Voltmeter

Answer: (B) Speedometer

1. A speedometer shows the speed of a vehicle, usually in km/h.
2. An odometer measures the total distance moved by the vehicle, not its speed.
3. Hence, the correct option is (B).

Teacher's Note:
a) The speedometer and the odometer are both fitted in a vehicle's dashboard.
b) Remember: speed is measured by the speedometer, distance by the odometer.

Q.16 The duration of the day from the moment the sun is over head today to the moment the Sun is overhead tomorrow is determined by-
(A) the rotation of Earth around the sun
(B) the revolution of Earth on its axis
(C) the inclination of the axis of rotation of the Earth from its plane of revolution
(D) The rotation and revolution of Earth around the sun.

Answer: (B) the revolution of Earth on its axis

1. The day is the time the Earth takes to turn once on its axis, which is the spin of the Earth.
2. The spin of the Earth on its axis is called rotation, so option (B) describes the correct cause, although its term is printed as "revolution".
3. Hence, the correct option is (B).

Teacher's Note:
a) Rotation is the spin of the Earth on its axis, and it produces day and night.
b) Revolution is the Earth's orbit around the Sun, and it takes one year.
c) The printed text of option (B) uses the word "revolution" for the spin on the axis, which is a wording error in the question.

Q.17 The increase in the speed of a car is proportional to the additional petrol put into the engine. Is it possible to accelerate a car without putting more petrol or less petrol into the engine ?
(A) Yet it is possible, if we add additional petrol
(B) Not possible, the statement is wrong
(C) May be possible if there is a leakage in the car
(D) Yes it is possible provided the car moves in circular path.

Answer: (D) Yes it is possible provided the car moves in circular path.

1. Acceleration means a change in velocity, and velocity includes direction.
2. A car moving in a circular path changes its direction continuously, so it accelerates even at constant speed.
3. This acceleration does not need any extra petrol, so option (D) is correct.
4. Hence, the correct option is (D).

Teacher's Note:
a) Velocity is speed in a definite direction, so a change of direction is also an acceleration.
b) Speed alone does not show acceleration; a steady speed can still accelerate.
c) The statement in the question links acceleration only to speed, which is incomplete.

Length, time & mass are measured in meter, sec, and kilogram

physical Quantites

Ø TERMS USED TO DEFINE MOTION

(a) DISTANCE & DISPLACEMENT :

The speed of a bus during a journey may vary. When the bus is nearing a bus stop, its speed decreases. On the highways the bus travels with greater speed but in a city or town it travels with less speed due to

The bus has different speeds at different times. So we say that it has variable speed.

for such bodies, we can calculate the average speed.

Every day when you go to school from your house, you could take path 1 or path 2 or path 3. Do these paths have the same distance? No, the distance is not the same; it varies with the path taken. Imagine that you travel from your house to school in a straight line. This will be the shortest distance between them, called displacement. In the picture, it is represented by a dotted line. Displacement is the shortest distance between two points.

Velocity = Disp laceme nt