Force and Pressure Chapter 4 of D.A.V school Science book

 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 distance.

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 electric force.

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

B. State true or false for the following statements.

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

Ans- True

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

Ans- True

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

Ans- False

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).

Ans- False

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

Ans- True

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

Ans-False

C. Tick the correct option.

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

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 force' are the force labelled as----

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----

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

4. A container is filled with water as shown in the figure given below.

Which of the following statement is correct about the pressure of water in this container?

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) labelled 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.

Ans- Agent- Fingers, Effects of force- Change in shape.

(b) A person diving into a swimming pool.

Ans- Agent- Water, Effect of force- Change in state of motion.

(c) A labourer moving a loaded cart.

Ans- Agent- Labourer, Effect of force- Change in state of motion.

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

Ans- Agent- Friction from road, Effect of force- Change in motion.

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

Ans-  Agent- Muscles, Effect of force- 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?

Ans- Yes, it is possible for the object to move within constant speed along a straight line as the value of opposing force is not experienced by the object.

3. Distinguish between:

(a) balanced and unbalanced force.

Ans- Balanced force- A force is said to be balanced if the resultant force acting on the object is 0. 

        Unbalanced force- A force is said to be unbalanced if the resultant force acting on the object is not 0. 

(b) contact and non-contact force

Ans- Contact force- Force is said to be contact if bodies are directly in physical contact with each other. 

Example- Muscular force and frictional force.

        Non-contact force- Force are said to be non-contact if bodies are not physically in contact with each other.

Example- Gravitational force, electrical force, etc.

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

Ans- When we throw a ball upwards, its speed gradually decreases because the earth produces a gravitational force that brings down everything.                                      

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?

Ans- Gravitational force. It is a non-contact force.

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

Ans- The man will not come with the same speed without the parachute because the parachute has a large area that helps it to trap air below it and friction is generated.

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

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

Ans- The pressure will be halved.

(b) thrust is doubled keeping the area constant?

Ans- The pressure will also be doubled.

 

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

Ans- At 200cm diver will experience greater pressure because pressure increases with an increase in depth.

E. Answer the following question.

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

Ans- Force is a push or pull which is due to the interaction that changes or tends to change in the object.

                                                                          It is not necessary for the bodies to be in direct contact for a force to exists between them as there are non-contact forces too like a magnetic force, gravitational force, etc.

2. State three effects forces. Give suitable examples.

Ans- i) Change in state of the object.

Example- Force applied to move a chair, etc.

         ii) Change in the shape and size of the object.

Example- During making a dough.

         iii) Change in direction of the object.

Example- When we hit a ball with a bat, it changes its direction.

3. Give reasons for the following statements----

a) 'Snowshoes' are more effective than ordinary shoes for walking on snow.

Ans- 'Snowshoes' are more effective than ordinary shoes for walking on snow because snowshoes apply less pressure than ordinary shoes.

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

Ans-  Porters have a large round piece of cloth on their heads when they carry heavy loads because the increase in the area reduces the pressure. 

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

Ans- A sharp knife cuts vegetables and fruits more effectively than a blunt knife because the sharp knife has a less surface area than the blunt knife so, it applies more pressure and it is easy to cut vegetables and fruits with a sharp knife.

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

Ans- School bags are provided with wide straps to carry them because it reduces the pressure of books book and notebooks.

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

Ans- It is much easier to burst an inflated balloon with a needle than (directly) with the finger because a needle has a lower surface area as compared to finger.

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

Ans- Mountaineers may suffer from nose bleeding at high altitudes because, at high altitude, the atmospheric pressure decreases. Since, the pressure of the blood inside the mountaineer increases.

g) Air-planes have pressurised cabins for passenger safety while flying high in the atmosphere.

Ans- Air-planes have pressurised cabins for passenger safety while flying high in the atmosphere because at very high altitude humans can't breathe well.

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

.

Ans- Rod 'B' will sink less because it has more surface area and less pressure.

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

Ans- We will observe that the water from the hole A goes the farthest, water from hole B goes at a middle or average but water from hole C travels the minimum distance. This happens because the pressure exerted by liquid increases with depth.

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

Ans- Observation- The water will pass in an equal amount through the holes.

         Conclusion- The water pressure is the same at that level.

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

Ans- As we move upwards through the atmosphere, the height of the air column would decrease. So, the pressure will decrease with higher altitude.

                                                       Yes, change in temperature also cause a change in atmospheric pressure.