# DAV Solutions for Class 8 Science Chapter 4 Force and Pressure

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## Class- 8 DAV ScienceForce and Pressure Question and Answer

### A.Fill in the blanks .

1. For a force to come into play, the two concerned bodies must interact with each other.

2. To draw water from a well, we have to pull the rope.

3. An unbalanced force, acting on a moving object, may change its speed and/or its direction.

4. In the game of cricket the ‘bails’ fall when the ball strikes them. This is an example of a/an contact force.

5. The force, responsible for raising our body hair, when we try to take off a synthetic (terylene or polyster) shirt in dry weather conditions, is an electrical force.

6. The atmospheric pressure decreases as we move towards higher altitudes.

### C. Tick the correct option

1. When two forces, applied on an object. are equal and opposite, then these forces

may move the object.
may stop a moving object.
may move the object and also cause a change in its shape
do not move the object but may cause a change in its shape.

Ans- Do not move the object but may cause a change in its shape.

2. The following are the names of some of the well-known forces
A. gravitational force
B. muscular force
C. magnetic force
D. frictional force

Out of these, the ‘contact forces are the forces labelled as
A and B
B and C
B and D
C and D

Ans- b and d

3.Rohini Is pushing a box towards the west direction with a force of magnitude F. To help her move the box faster in the same direction, her friend should
push the box in the east direction with a force of magnitude
pull the box in the west direction with a force of magnitude ‘F’.
push the box in the west direction with a force of any magnitude.
push the box in the east direction with a force of any magnitude.

Ans- Push the box in the west direction with a force of any magnitude F.

4.A container is filled with water as shown in the figure below. Which of the following statement is correct about the pressure of water in this container?

pressure at A > pressure at B > pressure at C
pressure at C > pressure at B > pressure at A
pressure at C = pressure at B = pressure at A
pressure at A < pressure at B > pressure at C

Ans- Pressure at C>Pressure at B>Pressure at A

5.  An empty metal bottle has to be placed on a table so that it exerts the maximum pressure on the surface of the table?

For this, the bottle need to be placed in the position(s) labeled as

Ans- Position B

6. When we press the bulb of a dropper, with its nozzle kept underwater, air in the dropper is seen to escape in the form of bubbles. If we then release the pressure on the bulb, water gets filled in the dropper. The rise of water in the dropper is due to

Ans- atmospheric pressure

7. The length, breadth, and height, of a given rectangular box, are 100cm, 50cm, and 20cm, respectively. This box is kept on ground in three different ways, one by one. Which of the following statement is the correct statement about the pressure exerted, by this box, on the ground?

Ans- the pressure exerted is maximum when the ‘breadth’ and ‘height’ sides form the base.

### D. Answer the following questions in brief.

1. In the following situations, identify the agent exerting the force. Also, state the effect of the force in each case.

(a) Squeezing a piece of lemon between the fingers to extract its juice.

Answer 1(a): Agent of force is fingers & effect is the change in shape.

(b) A person diving into a swimming pool.

Answer(b): Agent of force is person muscles & effect changes in motion.

(c) A labourer moving a loaded cart.

Answer 1(c): Agent of force is labourer & effect is the change in motion of cart.

(d) A car coming to rest once its engine is switched off.

Answer 1(d): Agent of force is friction from road & effect is the change in motion.

(e) An athlete making a high jump to clear a bar kept at a certain height.

Answer 1(e): Agent of force is muscles & effect is the change in motion.

2. An object experiences a net non-zero force. Is it possible for the object to move with a constant speed along a straight line?

Answer 2: No, because a net non-zero force i.e., Unbalanced force is acting on the body, so the speed must be changing.

3. Distinguish between:

(a) balanced and unbalanced forces.

(b) contact and non-contact forces.

4. When we throw a ball upwards, what happens to its speed while it is going up? Give reason for your answer.

Answer 4: When we throw a ball upwards, its speed decreases because gravitational force is acting on the ball in a downward direction. Also, air resistance opposes the motion of the ball.

5. The figure shows a man with a parachute.

(a) Name the force which is responsible for his downward motion. Is it a contact force or a non-contact force?

Answer 5(a): Gravitational force is responsible for his downward motion; It is non-contract force.

(b) Will the man come down with the same speed without the parachute? Explain.

Answer 5(b): No, because the parachute experiences a very large air resistance which decreases the speed. So, without a parachute, the man will come with greater speed.

6. How would the pressure (exerted by a force on an area) change if-

(a) the area is doubled keeping the thrust constant?

Answer 6(a): If the area is doubled then pressure is reduced to half.

(b) thrust is doubled keeping the area constant?

Answer 6(b): If thrust is doubled then pressure becomes doubled.

7. When will a diver ‘experience a greater pressure’-100 cm below the surface of water or 200 cm below the surface of water? Justify your answer.

Answer 7: At 200 cm a diver will experience greater pressure because the pressure inside the liquid increases with depth.

### E. Answer the following questions.

1. Define force. Is it necessary for the two bodies to be in direct contact for a force to exist between them? Explain with examples.

Answer 1: Force is a push or pull that an object experiences when it interacts with another object. No, it is not necessary for the two bodies to be in direct contact for a force to exist between them. For example, A magnet can pull iron nails even from a distance.

2. State three effects of a force. Give suitable examples.

Answer 2: Three effects of force are as follows:

• Force can change the state of motion. Example: When you kick a football, the football comes into motion.
• Force can change the direction of motion. Example: When a batsman hits a ball, direction of motion of the ball changes.
• Force can change shape. Example: Change the shape of an inflated balloon by gently pressing it between our palms.

3. Give reasons for the following statements-

(a) ‘Snow shoes’ are more effective than ordinary shoes for walking on snow.

Answer 3(a): Snowshoes are more effective than ordinary shoes for walking on snow, because the snow shoes are comparatively broader than the ordinary shoes. As we know that pressure is inversely proportional to the surface area which means that if area increases then pressure will decrease. So, the body weight will be distributed over a larger surface area of snow shoes and helps to walk effectively on snow.

(b) Porters place a large round piece of cloth on their heads when they carry heavy loads.

Answer 3(b): Porters place a large round piece of cloth on their heads when they carry heavy loads because by doing this, they increase the area of contact of the load with their heads and as we know that the pressure is inversely proportional to surface area so the pressure on their heads gets reduced.

(c) A sharp knife cuts vegetables and fruits more effectively than a blunt knife.

Answer 3(c): As the area of the sharp knife is less than that of the blunt knife thereby it increases the pressure while cutting vegetables and fruits, because pressure is inversely proportional to area. So, a sharp knife cuts vegetables and fruits more effectively than blunt knife.

(d) School bags are provided with wide straps to carry them.

Answer 3(d): School bags are provided with wide straps to carry them, because as we know that the pressure is inversely proportional to the contact area that is if the contact area increases pressure decreases. So wide straps will distribute the weight of the box over a larger area thereby decreasing pressure on our Shoulders.

(e) It is much easier to burst an inflated balloon with a needle than (directly) with the finger.

Answer 3(e): As needle has a smaller surface area than the finger, so the force applied by needle at almost a single point leading to a much larger pressure on the balloon. So, it is easier to burst an inflated balloon with needle than finger.

(f) Mountaineers may suffer from nose bleeding at high altitudes.

Answer 3(f): Atmospheric pressure on our body is balanced by our blood pressure. At Higher altitudes the air pressure/Atmospheric pressure reduces and it becomes lower than our blood pressure so delicate blood vessels in our nose burst sometimes resulting in the nose bleeding.

(g) Airplanes have pressurised cabins for passenger safety while flying high in the atmosphere.

Answer 3(g): As we know that at high altitudes, the atmospheric pressure decreases, so the low pressure can be highly uncomfortable for travellers. Due to this pressure in the aircraft cabin is increased to a sufficient value so as to safeguard the passengers.

4. Two rods, ‘A’ and ‘B’, of the same weight and equal length, have different thicknesses. A boy, while playing with them on a beach, held them vertically on the surface of sand as shown in the figure. Which one of them will sink less? Explain.

Answer 4: We know that the pressure varies directly with the thrust and inversely with the area on which the force is applied. For equal amount of force, a smaller area would result greater pressure hence the rod ‘B’ will sink less than that of the rod ‘A’. It is because rod ‘B’ has more surface area.

5. Three holes, A, B and C, are made in an empty can at different levels, one over the other, as shown in the figure. They are (temporarily) closed with an adhesive tape. The can is now filled with water. What will you observe, when the tape (on all of them) is pulled out? Justify your answer.

Answer 5: Water will start coming out of these three holes. Water from the hole ‘A’ will go to the maximum distance because pressure is highest at bottom. Water from the hole ‘C’ will go to the least distance because of least pressure.

6. Make two tiny holes, of the same size, at equal heights from the bottom in a plastic bottle. Cover them and fill the bottle with water. Let the ‘covers’ be now removed. State the ‘observations’, and the resulting ‘conclusions’, based on this activity.

Answer 6: As the holes are at the same height, so the water will come out and reach to same distance, because of the same pressure on both the walls of the plastic bottle.

7. Discuss, in brief, the variation in atmospheric pressure with altitude. Do changes in temperature also cause a change in atmospheric pressure?

Answer 7: With the increase in altitude, the atmospheric pressure decreases because the air column also decreases.

Yes, the change in temperature also causes a change in atmospheric pressure which is mainly due to the uneven heating of the Earth surface which causes winds to flow.

### 4 thoughts on “DAV Solutions for Class 8 Science Chapter 4 Force and Pressure”

1. I want question answer of this chapter 🥺