NCERT Class 8 Science Chapter 11 - Force and Pressure

Chapter 11 - Force and Pressure

Question 1: Give two examples each of situations in which you push or pull to change the state of motion of object.
Answer:
Pull
• Open drawer
• Drawing water from well

Push
• A cricket ball hit by a batsman
• Moving loaded cart

Question 2: Grive two examples of situations in which applied force causes a change in the shape of an object.
Answer:
• A lump of dough on plate pressed by hand
• Hammering a hot piece of iron while making tool

Question 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.
Answer:
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/pull it.
d) The north pole of a magnet repels the north pole of another magnet. 

Question 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.
Answer:
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 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.

Question 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.
Answer:
a) In this situation, the agents are fingers, object is lemon, the effect of force changes the shape of lemon and extraction of juice. In this we apply muscular force.

b) In this situation, the agents are fingers of a person pressing the tube, object is toothpaste tube and the effect of force is changing the shape of tube and toothpaste comes out. In this we have to apply muscular force.

c) In this situation, the agents is the load suspended, object is spring and effects can be seen in the form of elongation of spring on suspension of spring.

d) In this situation, agents is muscle of athlete, the object is athlete and the effects of the force changes the state of motion of the athlete.

Question 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: A blacksmith uses his muscular force while hammering a hot piece of iron. The muscular force changes the shape of the iron so that it can be molded into different shapes.

Question 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 the inflated balloon is rubbed with synthetic cloth, it becomes a charged body. And when the balloon is put besides wall its sticks because the balloon is a charged body while wall is a uncharged body which applies electrostatic force. Electrostatic force is a force exerted by a charged body on another uncharged or charged body. 

Question 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: For holding bucket, we must apply a muscular force which is exerted by our muscles. This muscular force acts against force of gravity that pulls the bucket towards the ground. The force have the same magnitude but they act in opposite direction So, the net force becomes zero. Therefore there is no change in state of motion.

Question 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: The 2 forces acting on the rocket immediately after leaving the lauching pad are
• Gravitational force (downwards)
• Frictional force (upwards)

Question 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.

Question 11: Define:
a) Atmospheric pressure
b) Friction
c) Pressure
Answer:
a) The pressure exerted by the air around us is known as atmospheric pressure.

b) It is a force which opposes the motion of an object over a surface.

c) Pressure is the force acting on unit area of a surface.