NCERT Book Class 8 Science Force And Pressure

FORCE AND PRESSURE

In Class VII, you have learnt how objects move. Do you recall how we can decide whether an object is moving faster than the other? What does the distance moved by an object in unit time indicate? You also know that a moving object like a ball rolling on the ground slows down. Sometimes it may change its direction of motion. It is also possible that the ball may slow down and also change its direction. Did you ever wonder what makes an object to slow down or go faster, or change its direction of motion?

Let us recall some of our everyday experiences. What do you do to make a football move? What do you do to make a moving ball move faster? How does a goalkeeper stop a ball? How do fielders stop a ball hit by a batsman? A hockey player changes the direction of the moving ball with a flick of the stick (Fig. 11.1). In all these situations the ball is either made to move faster or slower or its direction of motion is changed. We often say that a force has been applied on a ball when it is kicked, pushed, thrown or flicked. What is a force? What can it do to bodies on which it is applied? We shall seek answers to such questions in this chapter.

11.1 Force – A Push or a Pull

Actions like picking, opening, shutting, kicking, hitting, lifting, flicking, pushing, pulling are often used to describe certain tasks. Each of these actions usually results in some kind of change in the motion of an object. Can these terms be replaced with one or more terms? Let us find out.

11.2 Forces are due to an Interaction

Suppose a man is standing behind a stationary car (Fig.11.2). Will the car move due to his presence? Suppose the man now begins to push the car, that is, he applies a force on it. The car may begin to move in the direction of the applied force. Note that the man has to push the car to make it move. Fig. 11.3 shows three situations that may be familiar to you. Can you decide who is pulling and who is pushing in these cases? In Fig. 11.3 (a), both the girls appear to push each other while the pair of girls in Fig. 11.3 (b) are trying to pull each other. Similarly, the cow and the man in Fig. 11. 3(c) appear to pull each other. The girls in the two situations shown here are applying force

11.3 Exploring Forces

Let us try to learn more about forces. Have you ever seen a game of tug-of war? In this game two teams pull at a rope in opposite directions (Fig. 11.5).Members of both the teams try to pull the rope in their direction. Sometimes the rope simply does not move. Is it not similar to the situation shown in Fig. 11.3 (b)? The team that pulls harder, that is, applies a larger force, finally wins the game.What do these examples suggest about the nature of force? Forces applied on an object in the same direction add to one another. Now recall what happened when you and your friend pushed the heavy box in the same direction in Activity 11.2. If the two forces act in the opposite directions on an object, the net force acting on it is the difference between the two forces. What did you observe in Activity 11.2 when both of you were pushing the heavy box from opposite directions?

Exercises

1. Give two examples each of situations in which you push or pull to change the state of motion of objects.

2. Give two examples of situations in which applied force causes a change in the shape of an object.

3. Fill in the blanks in the following statements:

(a) To draw water from a well we have to ——— at the rope.

(b) A charged body ———— an uncharged body towards it.

(c) To move a loaded trolley we have to ———— it.

(d) The north pole of a magnet ———— the north pole of another magnet.

4. An archer stretches her bow while taking aim at the target. She then releases the arrow, which begins to move towards the target. Based on this information fill up the gaps in the following statements using the following terms:

muscular, contact, non-contact, gravity, friction, shape, attraction

(a) To stretch the bow, the archer applies a force that causes a change in its ——.

(b) The force applied by the archer to stretch the bow is an example of — —— force.

(c) The type of force responsible for a change in the state of motion of the arrow is an example of a ———— force.

(d) While the arrow moves towards its target, the forces acting on it are due to ———— and that due to ———— of air.

5. In the following situations identify the agent exerting the force and the object on which it acts. State the effect of the force in each case.

(a) Squeezing a piece of lemon between the fingers to extract its juice.

(b) Taking out paste from a toothpaste tube.

(c) A load suspended from a spring while its other end is on a hook fixed to a wall.

(d) An athlete making a high jump to clear the bar at a certain height.

6. A blacksmith hammers a hot piece of iron while making a tool. How does the force due to hammering affect the piece of iron?

7. An inflated balloon was pressed against a wall after it has been rubbed with a piece of synthetic cloth. It was found that the balloon sticks to the wall. What force might be responsible for the attraction between the balloon and the wall?

8. Name the forces acting on a plastic bucket containing water held above ground level in your hand. Discuss why the forces acting on the bucket do not bring a change in its state of motion.

9. A rocket has been fired upwards to launch a satellite in its orbit. Name the two forces acting on the rocket immediately after leaving the launching pad. 

10. When we press the bulb of a dropper with its nozzle kept in water, air in the dropper is seen to escape in the form of bubbles. Once we release the pressure on the bulb, water gets filled in the dropper. The rise of water in the dropper is due to

(a) pressure of water

(b) gravity of the earth

(c) shape of rubber bulb

(d) atmospheric pressure

Please refer to attached file for NCERT Class 8 Science Force And Pressure