Class 8 Science Chapter 8 Question Answers - Force and Pressure

Question1: Give two examples each of situations in which you push or pull to change the state of motion of objects.
Answer: 
1. Opening or shutting a door

2. A cricket ball hit by a batsman

A Batsman

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

1. Rolling a ball of flour dough for making a chapati at home

2. Bursting of an inflated balloon by pressing between the palms. 

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 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.

Squeezing a Lemon.

(b) Taking out paste from a toothpaste tube.

Toothpaste from Tube. 

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

Fig: Loaded Spring.

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

 Answer: 

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: When  A blacksmith hammers a hot piece of iron while making a tool, its shape gets changed into a desired shape of 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: When we rub, an inflated balloon with a piece of synthetic cloth, its surface gets charged due to electrostatic force. On pressing it against a wall, the charged surface of balloon, due to electrostatic force, attracts uncharged surface of the wall and the balloon sticks to the wall. 

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: The forces acting on the plastic bucket containing water, held above the Ground Level are: 

Muscular Force which is being exerted to pull (lift) the weight of filled bucket, caused by force of gravity.

The force of Gravity, in the form of weight of bucket, acting in downward and opposite direction of muscular force. The forces acting on the bucket do not change its state of motion, as muscular force causing upward pull is cancelled out by downward pull of gravity.

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: When a rocket is fired upwards to launch a satellite, two forces acting on the rocket immediately after leaving the launching pad are : 

Combined weight of the rocket and payload (Satellit), caused by the force of gravity, acting in the downward direction, towards the center of the earth

• The force of thrust, caused by the burning of propellant fuel in the rocket. The force of thrust which exceeds force of gravity, acts in the opposite direction of gravity, and makes the rocket move upward. Rocket launching.

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

Dropper.

Answer: (c) shape of rubber bulb and (d) atmospheric pressure
On releasing the pressure on the bulb of dropper, it tries to regain its original form with greater volume earlier, inside. As the opening of dropper tube is dipped in water, water rises up to fill empty inside of dropper due to atmospheric pressure on the outer surface of water.