Force and Pressure important questions with answers

NCERT Solutions for Class 8th Science Chapter 9 Force and Pressure Textual questions and answers based on the latest syllabus of CBSE and JKBOSE

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?

• Push:

1. Pushing a stalled car to move it.
2. Pushing a door to open it.

• Pull:

1. Pulling a rope to draw water from a well.
2. Pulling a chair to sit on it.

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

• Squeezing a rubber ball.
• Stretching a rubber band.

3. Fill in the blanks in the following statements:

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

• Pull

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

• Attracts

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

• Push or pull

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

• Repels

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 _ force.

• Muscular

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

• Contact

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

• Gravity, friction

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 exerting the force: Fingers
• Object on which it acts: Lemon
• Effect of the force: The lemon is compressed, and juice is extracted.

(b) Taking out paste from a toothpaste tube.

• Agent exerting the force: Fingers
• Object on which it acts: Toothpaste tube
• Effect of the force: The tube is squeezed, and paste comes out.

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

• Agent exerting the force: Load
• Object on which it acts: Spring
• Effect of the force: The spring stretches due to the load.

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

• Agent exerting the force: Athlete’s legs
• Object on which it acts: Ground
• Effect of the force: The athlete pushes against the ground to lift off and clear the bar.

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 iron piece, molding it into the desired tool.

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?

• Electrostatic force due to the transfer of charges.

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 forces acting are gravitational force pulling the bucket downwards and the muscular force of the hand holding it up. These forces are balanced, so there is no change in the 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.

• Gravitational force pulling it down and the force of thrust pushing it upwards.

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

• (d) Atmospheric pressure

Extended Learning — Activities and Projects

1. Make a 50 cm x 50 cm bed of dry sand about 10 cm in thickness. Make sure that its top surface is leveled. 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 the 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.

• Observe and record the depth of the stool legs in the sand in both situations.
• Compare the pressure exerted by the stool in both cases.

1. 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 it firmly. Make sure there are no gaps between the tumbler and the card. Gently remove the hand pressing the card. Note and record your observations.

• Perform the experiment.
• Record and discuss your observations regarding the pressure exerted by water and atmospheric pressure.