Class-8 Science Chapter-11 Force and Pressure

 

Class 8th Science Solutions 

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 objects.
Answer:Examples of Pull:

• Pulling of a suitcase
• Pulling of a cow by a man.

Examples of Push:

• Pushing of a car to move
• Pushing of an almirah by a man.

Question 2.Give two examples of situations in which applied force causes a change in the shape of an object.
Answer:Examples of change in shape of an object when force is applied on it:

1. Pressing a ball of dough when rolled to make a chapati.
2. Pressing an inflated balloon between two palms.

Question 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. - pull/push
(d) The north pole of a magnet the north pole of another magnet. - repels.

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:

(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.
(a) In this situation, the fingers are the agents exerting a force. Lemon is the object on which force acts. It involves muscular force.

(b) Taking out paste from a toothpaste tube.
(b) In this situation, the fingers are the agents exerting a force. Toothpaste is the object on which force acts. It involves muscular force.

(c) A load suspended from a spring while its other end is on a hood fixed to a wall.
(c) In this situation, the load is the agent exerting force. Spring is the object on which force acts. It involves gravitational force.

(d) An athlete making a high jump to clear the bar at a certain height.
(d) In this situation, the athlete is the agent exercising force, bar is the object to be cleared. It involves a non-contact gravitational force.

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: The force due to. hammering causes the changes in the shape of the iron and iron can be moulded in the shape of the required tool.

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: This is an electrostatic force.

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:Forces acting on the plastic bucket are the muscular force and gravitational force. These forces do not bring the change in its state of motion because they are acting in opposite direction with equal magnitude. The effect of the gravitational force will pull it down if the muscular force will grow weak. The body will feel the stretch of gravitational force and will have to bend to cancel the magnitude of gravitational force.

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:Gravitational force and frictional force.

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.