Science STD 8 Chapter 8: Force and Pressure

Science STD 8 Chapter 8: Force and Pressure - Exercises

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

Pushing:

Pushing a stationary football to make it move.
Pushing a swinging door to close it.

Pulling:

Pulling a rope to draw water from a well.
Pulling a drawer to open it.

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

Two examples of situations where an applied force changes the shape of an object are:

Squeezing a plastic bottle changes its shape.
Pressing a ball of dough to make a chapati changes its shape.

3. Fill in the blanks in the following statements.

(a) To draw water from a well we have to pull at the rope.
(b) A charged body attracts an uncharged body towards it.
(c) To move a loaded trolley we have to push it.
(d) The north pole of a magnet repels 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 shape.
(b) The force applied by the archer to stretch the bow is an example of muscular force.
(c) The type of force responsible for a change in the state of motion of the arrow is an example of a non-contact force.
(d) While the arrow moves towards its target, the forces acting on it are due to gravity and that due to friction 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.
Agent: Fingers. Object: Lemon. Effect: Change in shape.
(b) Taking out paste from a toothpaste tube.
Agent: Fingers. Object: Toothpaste tube. Effect: Change in shape.
(c) A load suspended from a spring while its other end is on a hook fixed to a wall.
Agent: Gravity. Object: Spring. Effect: Change in shape (stretching).
(d) An athlete making a high jump to clear the bar at a certain height.
Agent: Athlete's leg muscles. Object: Athlete's body. Effect: Change in state of motion.

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?

The force due to hammering changes the shape of the hot piece of iron, turning it into a desired tool. This is a plastic deformation that occurs because the iron is hot and pliable. The hammering force can be controlled to flatten, bend, or stretch the metal into a new shape.

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?

The force responsible for the attraction is the **electrostatic force**. When the balloon is rubbed with a synthetic cloth, it acquires a static electric charge. When this charged balloon is brought near the wall, it induces an opposite charge on the wall's surface, causing the balloon to be attracted to the wall and stick to it.

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.

The two main forces acting on the bucket are the upward **muscular force** from your hand and the downward **gravitational force** exerted by the Earth. The bucket's state of motion does not change because these two forces are equal in magnitude and opposite in direction, balancing each other out. This results in a net force of zero, so the bucket remains stationary.

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.

The two forces acting on the rocket immediately after leaving the launching pad are:

The upward **thrust force** from the rocket's engine.
The downward **gravitational force** of the Earth.

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.

Suggested Activities and Projects

1. Make a 50 cm × 50 cm bed of dry sand about 10 cm in thickness. Make sure that its top surface is levelled. Take a wooden or a plastic stool. Cut two strips of graph paper each with a width of 1 cm. Paste them vertically on any leg of the stool - one at the bottom and the other from the top. Now gently put the stool on the sand bed with its legs resting on the sand. Increase the size of sand bed if required. Now put a load, say a school bag full of books, on the seat of the stool. Mark the level of sand on the graph strip. This would give you the depth, if any, to which the legs of stool sink in sand. Next, turn the stool upside down so that now it rests on its seat on the sand bed. Note the depth to which the stool sinks now. Next, put the same load on the stool and note the depth to which it sinks in the sand. Compare the pressure exerted by the stool in the two situations.

Outline: This activity demonstrates the relationship between pressure, force, and area. When the stool rests on its four legs, the weight (force) is distributed over a small area, so the pressure is high, and it sinks deep into the sand. When the stool is inverted, the same weight is spread over a much larger area, resulting in lower pressure, and it sinks less. This proves that pressure is inversely proportional to the area over which a force is applied.

2. Take a tumbler and fill it with water. Cover the mouth of the tumbler with a thick card similar to that of a postcard. Hold the tumbler with one hand while keeping the card pressed to its mouth with your other hand. Turn the tumbler upside down while keeping the card pressed to its mouth. Make sure that the tumbler is held vertical. Gently remove the hand pressing the card. What do you observe? Does the card get detached allowing the water to spill? With a little practice you will find that the card continues to hold water in the tumbler even after it is not supported by your hand. Also try this activity by using a piece of cloth to hold the tumbler in an upside down position (Fig. 8.21).

Outline: This experiment demonstrates the existence of atmospheric pressure. The card stays in place and holds the water because the atmospheric pressure acting on the card's surface from below is greater than the pressure exerted by the water column from above. The force of the atmospheric pressure is enough to counteract the force of gravity on the water, keeping the water from spilling out.

3. Take 4-5 plastic bottles of different shapes and sizes. Join them together with small pieces of glass or rubber tube as shown in Fig. 8.22. Keep this arrangement on a level surface. Now pour water in any one of the bottles. Note whether the bottle in which water is poured gets filled first or all the bottles get filled up simultaneously. Note the level of water in all the bottles from time to time. Try to explain your observations.

Outline: This activity demonstrates that a liquid exerts pressure equally at the same depth, and its level remains the same in interconnected containers regardless of their shape or size. When water is poured into one bottle, it will flow through the connecting tubes and fill all the bottles simultaneously until the water level is equal in all of them. This shows that the liquid pressure depends only on the height of the liquid column, not on the shape of the container.