DAV Solutions for Class 8 Science Chapter 4 Force and Pressure

DAV Solutions for Class 8 Science Chapter 4

DAV Solutions for Class 8 Science Chapter 4 Force and Pressure of The Living World Book is provided here to help students to follow the concepts in an easy way. The topic of Force and Pressure is one of the most important concepts in Science. It is very important to grasp every bit of this Chapter as they are basics for all your future endeavours. These DAV Solutions for Class 8 Science helps you to attain perfection on the chapter by providing answers to a variety of questions such as fill in the blanks, true or false, tick the correct option, answer the following questions in brief and long descriptive answer types question.

In order to score good marks in the Class 8 Science examination and to grasp the concepts involved in the chapter, students are advised to refer to the DAV Solutions for Class 8 Science thoroughly. Answers provided here will help you to get the knowledge on the concept of Force, magnitude and direction of force, a change in its shape or change in its state of motion, definition of pressure, the exertion of pressure on gases and liquid.

Class 8 Science Chapter 4 Question Answer

DAV Solutions for Class 8 Science Chapter 4 Force and Pressure is given below. Here DAV Class 8 Science Chapter 4 Question Answer is provided with great explanation. Chapter 4 of class 8 science deals with force and pressure. In this chapter, we shall study about force, effects of force, balanced and unbalanced forces, types of forces, pressure in details.

Highlights

• Fill in the blanks.
• True or False.
• Tick the correct option.
• Answer the following questions in brief.
• Answer the following questions.

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A. Fill in the blanks.

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

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

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

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

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

Answer: (1) interact (2) pull (3) speed, direction (4) contact (5) electric (6) decreases

B. State True or False for the following statements.

1. The effect of a force may change by changing the direction of the applied force.

2. A force has to be specified both in terms of its magnitude as well as its direction.

3. Frictional force is an example of a non-contact force.

4. A gas filled balloon moves up. The upward force acting on it is smaller than the force of gravity, i.e. (the downward acting force).

5. We can reduce the pressure of a given force by increasing the area over which the force acts.

6. Inside a bottle filled with water, pressure is least at the bottom and the greatest at the surface of water.

Answer: (1) True (2) True (3) False (4) False (5) True (6) False

C. Tick the correct option.

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

Answer: 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-

Answer: 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-

Answer: pull the box in the west direction with a force of 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?

Answer: pressure at C > pressure at B > pressure at A

Answer: position B

6. When we press the bulb of a dropper, with its nozzle kept under water, 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-

Answer: atmospheric pressure

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

Answer: 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: Agent of force is fingers & effect is change in shape.

(b) A person diving into a swimming pool.

Answer: Agent of force is person muscles & effect is change in motion.

(c) A labourer moving a loaded cart.

Answer: Agent of force is labourer & effect is change in motion of cart.

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

Answer: Agent of force is friction from road & effect is change in motion.

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

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

3. Distinguish between:

(a) balanced and unbalanced forces.

Answer:

Balanced Forces	Unbalanced Forces
When resultant of two or more forces acting on a body is zero then these forces are called balanced forces.	When resultant of two or more forces acting on a body is not zero, then these forces are called unbalanced forces.
Balanced forces do not change the state of rest or motion of an object. They may change in the size and shape of the object.	Unbalanced forces may change in state of rest or motion as well as in size and shape of an object.

(b) contact and non-contact forces.

Answer:

Contact Forces	Non-contact Forces
The forces which act only when objects are in direct physical contact with each other.	The forces which act when objects are not in direct physical contact with each other.
Examples-Frictional force, muscular force.	Examples – Gravitational force, Electric Force, Magnetic Force.

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

Answer: When we throw a ball upwards, it’s speed decreases because gravitational force is acting on the ball in 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: 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: No, because parachute experiences a very large air resistance which decreases the speed. So, without 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: If area is doubled then pressure reduced to half.

(b) thrust is doubled keeping the area constant?

Answer: 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: At 200 cm a diver will experiences greater pressure because 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: Force is a push or pull which 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 nail even from a distance.

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

Answer: 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: 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: 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: 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: 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: 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: 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: 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: 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: 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: 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: 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.