NCERT Solutions for Class 8 Science Chapter 11 Force And Pressure

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

Answer:

• Push –
  • Heavy box is pushed to move from one place to another
  • Car is pushed if it fails to starts
  • To close a drawer, it is pushed.
• Pull –
  • Rope is pulled to take out water from the well
  • Rope is pulled in game of tug of war
  • To open a drawer it is pushed.

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

Answer: 1) Making chapati from dough. 2) Changing the shape of clay by pressing it in between the hands.

3. Fill in the blanks in the following statements.

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

Answer: pull

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

Answer: attracts

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

Answer: push or pull

(d) The north pole of a magnet __________the north pole of another magnet.

Answer: 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 __________.

Answer: shape

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

Answer: muscular

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

Answer: contact

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

Answer: 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.

Answer: Squeezing a piece of lemon between the fingers to extract its juice requires its muscular force. This changes shape of the lemon.

(b) Taking out paste from a toothpaste tube.

Answer: Muscular force is applied to take out toothpaste from the tube. This changes the shape of the tube.

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

Answer: Load exerts force on the spring and pushes it downward. This causes stretching of spring. Thus, shape of spring changes.

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

Answer: To make a high jump an athlete pushes ground with feet. This requires muscular force. Thus, it causes change its 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?

Answer: Blacksmith applies muscular force while hammering a piece of iron. This force provides desired shape to 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?

Answer: When balloon is rubbed with piece of synthetic cloth it gets charges. A charged body always attracts uncharged body, thus balloon stick to the wall. This force is known as electrostatic force.

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.

Answer: A muscular force is applied to hold bucket of water using hands. Also, gravitational force is applied on bucket which pulls it downward. Muscular force and gravitational force acting on bucket are equal in magnitude but in opposite direction. Thus, total force acting on bucket is zero, therefore, it does not bring any 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.

Answer: Forces acting on rocket will be force of gravity and force of friction. Force of gravity pulls the rocket downward. Force of friction is exerted by earth’s atmosphere which opposes its motion.

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.

Answer: (d) atmospheric pressure

Check other lessons NCERT Solutions for Class 8

Download free NCERT textbook class 8 Science – Chapter 8 Cell Structure and Functions