ICSE Class 7 Physics Chapter 2 Motion

Motion

Syllabus

1. Motion - change in position with respect to a set of stationary landmarks.

Types of motion and examples of the same - translatory (rectilinear and curvilinear), rotatory, oscillatory, vibratory, periodic, non-periodic motion and random motion.

2. The motion of a simple pendulum - definitions of one oscillation, amplitude, time period, rest or mean position, frequency, factors affecting time period of simple pendulum.

Finding the time period of simple pendulum, seconds pendulum.

- Finding the time period of a simple pendulum (E).

- Acceleration due to gravity, uniform acceleration

- Making a seconds pendulum (E).

- Exploring the relationship between time period and the length of the pendulum/the mass of the bob (E). (Optional - may be taken as additional project work).

3. Uniform and non-uniform motion.

4. Speed, velocity - the difference between them. Calculating speed from the formula:

S = D - T and using the correct units.

Acceleration.

v = u + at and simple problems based on this equation. Students need to identify clearly initial and final velocity, and acceleration.

- Observing examples of different kinds of motion and classifying them (E).

Teacher's Note

Motion is fundamental to everyday life - from the cars we ride to the planets moving in space, understanding rest and motion helps us predict and control movement.

Rest And Motion

If we look around, we observe that many objects do not appear to move, rather they are considered to be at rest. A body is said to be at rest if it does not change its position with respect to a fixed point in its surroundings.

Examples: A book lying on the table will not change its position if it is not disturbed and will be considered to be in a state of rest.

A bench fixed under a tree is at rest as there is no change in the position of the bench with respect to the tree and other stationary objects. Therefore, when the position of a body with respect to its surroundings does not change with time, the body is said to be at rest.

Many things around us move from one place to another. A flying bird, a moving bus, a boy playing football, motion of the pendulum of a wall clock, a moving train, a sailing ship, a walking man, etc. are some of the examples of motion. A moving object keeps on changing its position continuously with time.

A moving car changes its position with respect to a tree or a lamp post by the side of the road. So, the car is said to be in motion.

Similarly, a flying bird is also said to be in motion as it changes its position with respect to the stationary objects. Hence, when the position of a body with respect to its surroundings changes with time, the body is said to be in motion.

Teacher's Note

Whether an object is at rest or in motion depends on what we compare it to - a passenger on a bus is at rest relative to the bus but in motion relative to a tree outside.

Rest And Motion Are Relative Terms

Imagine yourself sitting inside a moving bus. When you look outside, you will observe that you are moving. Now look up at the roof of the bus. With respect to the roof, you will find yourself in a position of rest. Hence we conclude that rest and motion are relative terms.

You will further observe that a boy sitting on a bench near the road is at rest while you are moving with the bus.

To conclude, an object is in motion with respect to a set of objects but at the same time, it may be in a state of rest with respect to the other objects (moving with the same speed and in the same direction). It is the observer and the surroundings that decide whether a given object is at rest or in motion.

Teacher's Note

A person standing still on a moving train is moving relative to someone on the platform, but at rest relative to others on the train.

Types Of Motion

Various objects have different types of movement. To name a few, a train moves straight along its track, a fan rotates, the earth revolves around the sun. There can be many more such examples. These different kinds of motions can be classified into the following types.

1. Translatory Motion

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 an object is called translatory motion. There are two types of translatory motions:

a. Rectilinear Motion

If an object moves along a straight line, its motion is called rectilinear motion. For example, a stone falling straight towards the surface of the earth, a car moving on a straight road and a coin moving over a carom board.

Rectilinear motion is also called linear motion.

b. Curvilinear Motion

If an object moves along a curved or circular path, its motion is called curvilinear motion. For example, a car moving along a curved path or a javelin thrown by an athlete are in curvilinear motion. A ball thrown at an angle also has curvilinear motion.

Circular motion: The movement of a body along a circular path is called circular motion. Circular motion is a special type of curvilinear motion.

Whirling of a stone tied to a string is an example of circular motion.

2. 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. For example, the blades of a fan, a spinning top, a spinning wheel or potter's wheel, a merry-go-round, rotation of the earth about its axis, etc.

A rotatory motion is different from a translatory motion, because in a rotatory motion, different parts of the object move through different distances during the same time.

3. Oscillatory Motion

Observe the motion of the pendulum of a wall clock. The to and fro motion of the bob is called oscillatory motion. The motion of a swing and the piston of an engine perform oscillatory motion.

The to and fro motion of a body along the same path about its mean position is called oscillatory motion. The oscillatory motion is a periodic motion.

4. Vibratory Motion

There is another type of oscillatory motion in which a certain part of the body always remains fixed while the rest of the body moves to and fro in a particular style about its mean position. This is called vibratory motion.

A metallic wire tied between two fixed rods on a table is stretched (Fig. 2.7). When the wire is plucked, it starts moving to and fro. It must be noted that during this motion, there is a change in the shape and size of the body. All musical instruments like guitar, violin, sitar, etc., have strings attached to them which produce vibratory motion.

When we breathe, our chest expands and contracts. This motion is also called vibratory motion.

5. Periodic Motion

A motion which gets repeated after regular intervals of time is called a periodic motion. The earth moving around the sun takes 365 and 1/4 days to complete one revolution and this motion gets repeated after every 365 1/4 days. Similarly, the swinging pendulum of a wall clock, the needle of a sewing machine and the heart beat of a normal person are examples of periodic motion.

Movement of the moon around the earth, a branch of a tree moving to and fro, the surface of the drums (tabla) being played are some more examples of periodic motion.

6. Non-Periodic Motion

The motion which does not repeat itself after regular interval of time is called non-periodic motion.

7. Multiple Motion

Sometimes, a moving object performs two or more types of motions simultaneously. Such a motion is called multiple motion. Some of the examples of multiple motion are:

A person drawing water from a well is an example of multiple motion. Here, the pulley on which the rope runs has rotatory motion while the bucket and the rope have translatory motion.

A rider on a bicycle is another example of multiple motion. The wheels and the pedals perform rotatory motion and the bicycle as a whole moves in a curved or straight path (translatory motion).

A drill used by a carpenter for drilling a hole in wood has both translatory and rotatory motions. When the drill is being rotated (rotatory motion), it also gets pierced into the wood (translatory motion).

Observe the motion of a ball on the ground. Here the ball is rolling on the ground - rotating as well as moving forward on the ground. Thus, the ball undergoes a rectilinear motion as well as a rotational motion.

The earth rotates about its axis (rotatory motion) and at the same time it revolves around the sun in a curved path (curvilinear motion) in a fixed time interval (periodic motion).

8. Rolling Motion

The motion in which a body undergoes both translatory as well as rotatory motion is called rolling motion.

When a spin bowler (in the game of cricket) delivers a ball, the ball has a motion of spin (rotatory motion) while it moves towards the batsman (translatory or curved motion).

Example: (1) The motion of a cylinder on an inclined plane.

(2) The movement of a bicycle wheel.

(3) Movement of a drill.

9. Random Motion

When an object in motion has no specific path and which suddenly changes its motion is said to have a random motion. Some examples are given below.

(i) A mosquito while flying has translatory motion at one moment and rotatory motion at another moment.

(ii) A flying kite may have translatory motion at one instant and may have rotatory motion at the very next moment.

(iii) Motion of a football or hockey player on the ground.

Some examples of different types of motion are given below. Name the type of motion in each case.

Teacher's Note

Different objects move differently - a car moves in a straight line, a fan blade rotates, and a pendulum swings back and forth, showing us that motion takes many forms.

Uniform And Non-Uniform Motion

Uniform Motion

If an object covers equal distances in equal intervals of time, we say that the object is in uniform motion.

When the motion is uniform, the body remains at a constant speed throughout the period of its journey.

Example: Figure 2.14 shows the positions of a cyclist who is covering a distance of 2 metres in every 1 second.

Example: A flying aeroplane in a particular direction at a constant speed, a train moving straight in a particular direction at constant speed.

Look at the following table which displays the distance covered by a car in given time.

The table shows that the car is moving with a constant speed. It is covering 5 km in every 5 minutes. We can also say that it is covering 1 km in every 1 minute.

The graph shows that when we plot a uniform motion on a graph it is always a straight line.

Non-Uniform Motion

If an object does not cover equal distances in equal intervals of time, the object is said to move in non-uniform motion.

If the cyclist is covering 2 m in the first second, 3 m in the next second and 4 m in the next second, then the motion of the cyclist is non-uniform.

Example: Figure 2.15 shows the positions of a cyclist who is covering a distance of 2 m in the first second, 3 m in the next second

Look at the following table which displays the distance covered by a car in given intervals of time.

The above mentioned table clearly shows that the car is covering unequal distances in equal intervals of time i.e. 5 minutes. It means the car is in non-uniform motion.

Teacher's Note

A car moving at a steady speed on a highway travels uniformly, but a car in city traffic that speeds up and slows down repeatedly moves non-uniformly.

Simple Pendulum

An ideal simple pendulum consists of a small metal ball called the bob suspended from a rigid support by a weightless and inextensible string. But practically, we cannot construct a simple pendulum with weightless

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