ICSE Class 7 Physics Chapter 02 Force and Motion

Force and Motion

In your previous class you have learnt that a force is needed to set a body in motion or to bring a moving body to rest. A force can also change the speed of a moving body or change its direction of motion. The force acting on a body has a definite relationship with its motion, about which you will learn later. In this chapter, we will only discuss motion.

Motion

Everything that we see around us is either in motion or at rest. Thus, while roads, houses, trees, etc., are stationary, vehicles and people on the road are usually moving. All this is so common that we never stop to think about the exact nature of what we call motion.

Motion is Relative

Usually, we figure out whether an object is in a state of rest or of motion by comparing its position with stationary landmarks in its surroundings. When the position of an object with respect to the other objects around it does not change, we say that it is at rest. For example, the distance of your study table from the walls of the room, or from the roof or floor does not keep changing. Therefore, you conclude that it is at rest. On the other hand, when the position of an object relative to its surroundings changes, we say that it is in motion. For example, when you walk about in your room, your position relative to the things in the room keeps changing. You are then in motion.

To take this idea a little further, think of a boy sitting in a moving bus. Is he in motion or at rest? His position relative to the bus does not change. He is, therefore, stationary relative to the bus. On the other hand, his position relative to the road, and to the houses along the road, changes continuously. He is in motion relative to these landmarks. In general, we can say that all motion is relative.

We may look upon this idea in a different manner. While looking out of a moving train, you must have noticed that the trees outside seem to be moving away rapidly in the opposite direction. The trees appear to be in motion because an observer judges his surroundings assuming that he himself is at rest. Thus, when he moves in a certain direction, he feels that everything around him is moving in the opposite direction. This is precisely why the sun appears to move from the east to the west when, in reality, it is the earth that rotates from the west to the east.

Types of Motion

Things can move in many different ways, or motion can be of many different types. When the motion of a body does not have any pattern, e.g., the motion of a kite or of a mosquito, we call it random motion. On the other hand, any motion that follows some sort of pattern is called regular motion. Regular motion can be of three types, viz., (1) translatory motion, (2) rotatory motion, and (3) oscillatory motion.

Translatory motion

When an object moves from one point to another along a straight line or a smooth curve, its motion is called translatory. Usually, this term is used when the object moves in only one direction along the path. Translatory motion can be of two types - rectilinear and curvilinear, depending on the path followed.

The motion of a body along a straight line is called rectilinear motion. Examples of this are vehicles moving on a straight road, athletes running on a straight track, and any object thrown straight up or falling vertically down under gravity.

The motion of a body along a curved path is called curvilinear motion. Whenever a vehicle turns from one road into another, it follows a short curved path. Any object thrown in any direction other than the vertical also follows a curvilinear path.

Rotatory motion

When an object turns or rotates or spins about an axis, its motion is called rotatory. The rotation of the earth about its own axis is of this nature. Electrical fans, food processors, wheels, pulleys and ferris wheels are some things that show rotatory motion.

Many things around us have two different types of motion at the same time. For example, the wheels of a moving car have the same translatory motion as the car. In addition, each wheel has a rotatory motion about its own axis. Other common examples of a body showing more than one type of motion at the same time are a spinning top and a drill.

Oscillatory motion

When a body moves to and fro between two points, repeatedly, its motion is called oscillatory. A child on a swing and the pendulum of a clock are common examples of oscillatory motion.

Simple Pendulum

A simple pendulum consists of a small, heavy sphere, called the bob, suspended by a light string from a fixed support in such a way that it can oscillate freely. The sphere is about 1 cm to 2 cm in diameter, and has a hook to which the string can be tied. The length of the string can be altered, and is usually kept between 30 cm and 120 cm.

When the pendulum is at rest, with the string vertical, it is said to be at its mean position. The distance from the point of support to the centre of the bob is called the effective length, or simply the length (l) of the pendulum. To set the pendulum oscillating, the bob is pulled to one side gently, such that the string makes a small angle (less than 4 degrees) with the vertical. This angle is called the amplitude of the oscillations.

When the bob is released, the pendulum oscillates between two extreme positions on either side of the mean position. In Figure 2.5, the two extreme positions are marked A and B and the mean position is marked C.

One oscillation of the pendulum can be described as its motion

1. from one extreme position to the other and then back to its initial position, i.e., from A to B and back to A, or

2. between two consecutive crossings of the mean position while moving in the same direction, i.e., from C to A to B to C.

Time Period

The total time taken by a pendulum to complete one oscillation is called its time period (T). It can be shown, both through experiments and by calculations, that the time period of a pendulum is related to its characteristics as follows.

1. T depends on the length of the pendulum. It increases when l is increased. It is found to be about 1 s for l = 30 cm, which increases to about 2.2 s for l = 120 cm.

2. T does not depend on the amplitude, as long as the amplitude is small. When a pendulum is allowed to oscillate for some time, the amplitude of its oscillations decreases gradually due to air resistance. However, its time period does not change due to this decrease in amplitude.

3. T does not depend on the mass of the bob. This means that so long as the length of a pendulum remains constant, its time period will be the same even if bobs of different masses are used.

To find the time period of a simple pendulum, set it up as shown in Figure 2.4. In addition, you will require a stop watch to record time.

Choose any length for the pendulum. Make a chalk mark on the side of the table to mark the mean position of the pendulum. Push the bob to one side through a few centimetres and release it. When it crosses the chalk mark, moving in the same direction, count "one". Continue this for 20 oscillations and stop the watch. The reading of the watch divided by 20 will give you the time period.

If you want to do this activity at home, use a lock tied to a string as a pendulum and a watch that has a seconds hand instead of a stop watch. Hang the string off the edge of a table by placing its free end under a couple of heavy books. Alternatively, hang it off a door knob or handle.

Teacher's Note

Pendulums are used in grandfather clocks and metronomes, making them essential timekeeping devices that demonstrate how regular motion helps us measure time accurately.

Seconds Pendulum

A pendulum with a time period of exactly 2 s is called a seconds pendulum. Its length is very close to 1 m. Such pendulums take exactly 1 s to swing from one side to the other, and were used in clocks at one time.

You can make a seconds pendulum by keeping the length of the string close to 99 cm. Find the time period of the pendulum in the same way as in the preceding activity. If it is slightly different from 2 s, alter the length of the string by a few millimeters at a time until you get the exact value.

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