NCERT Solutions class 8 Science 11. Force And Pressure Text Book Exercise

Text Book Exercise 

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 squeezing a piece of lemon, we make use of muscular force to extract its juice. This muscular force is exerted on the lemon by our fingers. As a result, the shape of the lemon gets changed

Concept insight: Muscular force is a type of contact force

(b) We use our muscular force to take out paste from a toothpaste tube. The muscular force is exerted on the toothpaste tube by our fingers. As a result, the shape of the tube gets change

(c) Here, the suspended load exerts a force on the spring and pushes the spring downwards. As a result, the spring gets stretched. Hence, its shape gets changed.

(d) An athlete pushes the ground with his feet. His feet exert a muscular force on the ground. This allows him to jump over the bar. As a result, his state of motion gets changed.

Concept insight: Every action has an equal and opposite reaction

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:

While hammering a hot piece of iron blacksmith changes the shape of iron using his muscular force.

Concept insight: Force brings change in the shape of an object.

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 force is responsible for the attraction between the balloon and the wall. The balloon rubbed with a synthetic cloth gets electrically charged and sticks to the wall. This is because a charged object attracts uncharged object.

Concept insight: Electrical force is a type of non contact force. A charged object attracts an uncharged object

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 bucket filled with water held above the ground experiences two types of forces:

1. Muscular force (acting upwards)

2. Gravity (acting downwards)

These two forces are balanced because they are equal in magnitude but act in opposite directions. So, the net force on the bucket is zero and hence the state of motion of the bucket remains unchanged.

Concept insight: Non-zero net force is required to change the state of motion of a body.

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 two forces acting on the rocket immediately after leaving the launching pad are:

 (i) Upward force applied by the rocket engine.

(ii) Downward gravitational force applied by the Earth

 (Ignoring frictional force due to air resistance)

 The upward force due to rocket engine exceeds the downward gravitational force which makes the rocket move upward.

Concept insight: Unbalanced forces can change the state of motion of a body.

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