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Force And Pressure

Class 8th Science CBSE Solution
Exercises
  1. Give two examples each of situations in which you push or pull to change the state of…
  2. Give two examples of situations in which applied force causes a change in the shape of an…
  3. Fill in the blanks in the following statements. (a) To draw water from a well we have to…
  4. An archer stretches her bow while taking aim at the target. She then releases the arrow,…
  5. In the following situations identify the agent exerting the force and the object on which…
  6. A blacksmith hammers a hot piece of iron while making a tool. How does the force due to…
  7. An inflated balloon was pressed against a wall after it has been rubbed with a piece of…
  8. Name the forces acting on a plastic bucket containing water held above ground level in…
  9. A rocket has been fired upwards to launch a satellite into its orbit. Name the two forces…
  10. When we press the bulb of a dropper with its nozzle kept in water, the air in the dropper…
Exercises
Question 1.

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

Two examples of push force follow:-

1) A chair at rest is pushed to move it from one room to another. This changes the state of motion of the chair.

2) A player hits a ball using his cricket bat. This changes the state of motion of the ball.

Two examples of pull force follow:-

1) The bucket attached to the rope is pulled to draw water from a well. This changes the state of motion of water in the bucket.

2) A drawer of a desk is pulled to open it. This changes the state of motion of drawer.
Question 2.

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

Two examples of forces that cause a change in the shape of an object are as follows:

1) Making of bread from dough.


2) Deformation of clay by pressing it between the hands.
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 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 or 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 it 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)


(b)



(c)



(d)
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, he uses his muscular force through hammer. This muscular force changes the shape of the iron, iron being metal has the property of malleability and ductility. So, it can be given the desired shape the blacksmith wants.
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:

Electrostatic forces are responsible for the attraction between the balloon and the wall. The charges are produced on rubbing the balloon against the wall and the attraction is due to the positive and negative charges.
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:

In this case, muscular force is acting in an upward direction and the force of gravity is acting in a downward direction.


Both forces are acting in opposite directions to each other and hence nullify the effect of each other. Due to this, there is no change in the state of motion of the bucket.
Question 9.

A rocket has been fired upwards to launch a satellite into its orbit. Name the two forces acting on the rocket immediately after leaving the launching pad.
Answer:

The two forces acting on the rocket are the force of gravity, which pulls the rocket towards the ground, and the force of friction due to earth's atmosphere, which opposes its motion.
Question 10.

When we press the bulb of a dropper with its nozzle kept in water, the 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 the pressure of water.

a) the gravity of the earth.

b) the shape of rubber bulb

c) atmospheric pressure
Answer:

The correct answer is option(d) i.e. atmospheric pressure

When we press the nozzle the air escapes out due to pressure and the when we release the nozzle the liquid flows into to balance the atmospheric pressure.