Chapter 11. Force and Pressure

MULTIPLE CHOICE QUESTIONS

1. In Fig 11.1, two boys A and B are shown applying force on a block. If the block moves towards the right, which one of the following statements is correct?

   

                     Fig. 11.1
(a) Magnitude of force applied by A is greater than that of B.
(b) Magnitude of force applied by A is smaller than that of B.
(c) Net force on the block is towards A.
(d) Magnitude of force applied by A is equal to that of B.

Answer: (a) Magnitude of force applied by A is greater than that of B.
 [ Since the block moves towards the right , the force applied by Boy A much be greater than the force applied by Boy B .]

Q2. In the circuit shown in Fig.11.2, when the circuit is completed, the hammer strikes the gong. Which of the following force is responsible for the movement of hammer?

   

            Fig. 11.2

(a) gravitational force alone         (b) electrostatic force alone       (c) magnetic force alone             (d) frictional force alone

Answer: (c) magnetic force alone .

[ When the circuit completes, the electromagnet creates a magnetic force that pulls the hammer to strike the gong .]

3. During dry weather, while combing hair, sometimes we experience hair flying apart. The force responsible for this is
(a) force of gravity.        (b)  electrostatic force.      (c)  force of friction.   (d)  magnetic force.

Answer: (b) electrostatic force

[ During dry weather, friction between comb and hair creates electric charges. Like charges repel each other, so hair strands move apart due to electrostatic force.]

4. Fig.11.3 shows a container filled with water. Which of the following statements is correct about pressure of water?

             

                           Fig.11.3
(a) Pressure at A > Pressure at B > Pressure at C
(b) Pressure at A = Pressure at B = Pressure at C
(c) Pressure at A < Pressure at B > Pressure at C
(d) Pressure at A < Pressure at B < Pressure at C

Answer: (d) Pressure at A< Pressure at B<Pressure at C

[ Because liquid pressure increases with depth. Since C is at the greatest depth and A is the shallowest .]

5. Two objects repel each other. This repulsion could be due to
(a) frictional force only         (b) electrostatic force only
(c) magnetic force only       (d) either a magnetic or an electrostatic force

Answer: (d) either a magnetic or an electrostatic force.

[ Repulsion occurs only when like charges (electrostatic) or like poles (magnetic) interact. Friction alone cannot cause repulsion.]

6. Which one of the following forces is a contact force?
(a) force of gravity    (b) force of friction     (c) magnetic force     (d) electrostatic force

Answer:  (b) force of friction.

[ Friction acts only when two surfaces are in physical contact, unlike gravity, magnetism or electrostatic force, which can act from a distance.]

7. 

     

                 Fig. 11.4
A water tank has four taps fixed at points A, B, C, D as shown in Fig. 11.4. The water will flow out at the same pressure from taps at
(a) B and C           (b) A and B    (c) C and D     (d) A and C

Answer:  (a) B and C .

[ Water pressure depends on depth ; since B and C are at the same horizontal level, they experience the same pressure . ]         

8. 

     

                               Fig. 11.5
A brick is kept in three different ways on a table as shown in Fig.11.5. The pressure exerted by the brick on the table will be
(a) maximum in position A       (b) maximum in position B       (c) maximum in position C    (d) equal in all cases.

Answer: (a) maximum in position A .  

[ Pressure is maximum at position A because it has the smallest contact area. A smaller area results in higher pressure for the same weight. ]

VERY SHORT ANSWER QUESTIONS

9. A ball of dough is rolled into a flat chapatti. Name the force exerted to change the shape of the dough.

Answer: The force exerted is muscular force. When we roll the dough, our muscles apply a contact force that pushes and flattens the ball, overcoming its elasticity and changing its shape into a flat chapatti.

10. Where do we apply a force while walking?

Answer: While walking, we apply force by pushing the ground backward with our feet. The ground pushes us forward with an equal and opposite force (friction), which helps us move.

11. A girl is pushing a box towards east direction. In which direction should her friend push the box so that it moves faster in the same direction ?

Answer: Her friend should also push the box towards the east direction, in the same direction as the girl. This increases the net force and makes the box move faster.

12.

          

                            Fig. 11.6

In the circuit shown in Fig.11.6, when the key is closed, the compass needle placed in the match box deflects. Name the force which causes this deflection.

Answer: The force responsible for the deflection is the magnetic force . When the key is closed, the electric current flowing through the wire creates a magnetic field that moves the needle.

13. During dry weather, clothes made of synthetic fibre often stick to the skin. Which type of force is responsible for this phenomenon?

Answer: The force responsible for this phenomenon is electrostatic force.

14. While sieving grains, small pieces fall down. Which force pulls them down?

Answer: The force of gravity pulls them down. It is a non-contact force that acts on all objects, attracting them toward the centre of the Earth.

15. Does force of gravity act on dust particles?

Answer: Yes, the force of gravity acts on dust particles.

16. A gas filled balloon moves up. Is the upward force acting on it larger or smaller than the force of gravity?

Answer: The upward force acting on the balloon is larger than the force of gravity.

17. Does the force of gravitation exist between two astronauts in space?

Answer: Yes, the force of gravitation exists between two astronauts in space.

SHORT ANSWER QUESTIONS

18. A chapati maker is a machine which converts balls of dough into chapati’s. What effect of force comes into play in this process?

Answer: The effect of force coming into play is change in shape of the dough.

19.

      

                    Fig. 11.7

Fig.11.7 shows a man with a parachute. Name the force which is responsible for his downward motion. Will he come down with the same speed without the parachute?

Answer: The force is gravity . He would fall faster without a parachute because there is much less air resistance to slow his downward motion.

20. Two persons are applying forces on two opposite sides of a moving cart. The cart still moves with the same speed in the same direction. What do you infer about the magnitudes and direction of the forces applied.

Answer: The two forces are equal in magnitude but opposite in direction. They cancel each other out, so no change occurs in the cart's speed or direction. The net force on the cart is zero.

21. Two thermocol balls held close to each other move away from each other. When they are released, name the force which might be responsible for this phenomenon. Explain.

Answer: The force responsible is electrostatic force.

When two thermocol balls are rubbed, they acquire same type of charge (both positive or both negative). Like charges repel each other. Due to this electrostatic repulsion, the balls move away from each other when released.

22. Fruits detached from a tree fall down due to force of gravity. We know that a force arises due to interaction between two objects. Name the objects interacting in this case.

Answer:  The two objects interacting are the fruit and the Earth.

The Earth pulls the fruit towards itself due to gravitational force, so the fruit falls down

23. A man is pushing a cart down a slope. Suddenly the cart starts moving faster and he wants to slow it down. What should he do?

Answer: He should apply force in the opposite direction of motion. By pulling the cart backward or reducing the push, he can oppose its motion and slow it down.

24.

    

Fig. 11.8 shows a car sticking to an electromagnet. Name the forces acting on the car? Which one of them is larger?

Answer: The two forces acting on the car are magnetic force (acting upwards) and gravitational force (acting downwards). The magnetic force is larger because it overcomes the car’s weight, allowing the electromagnet to lift and hold the car in the air.

LONG ANSWER QUESTIONS

25. An archer shoots an arrow in the air horizontally. However, after moving some distance, the arrow falls to the ground. Name the initial force that sets the arrow in motion. Explain why the arrow ultimately falls down.

Answer: The initial force is muscular force (applied by the archer by stretching the bowstring). The arrow ultimately falls down due to the force of gravity, which pulls it toward the Earth's surface. Air resistance also slows it down, but gravity causes the downward fall.

26. It is difficult to cut cloth using a pair of scissors with blunt blades. Explain.

Answer: A pair of scissors with blunt blades increases the force of friction between the blades and the cloth. This makes cutting difficult because more muscular force is required to overcome friction. Sharp blades reduce friction and cut easily by applying force at a smaller area.

27. Two rods of the same weight and equal length have different thickness. They are held vertically on the surface of sand as shown in Fig.11.9. Which one of them will sink move? Why?

       

                            Fig. 11.9

Answer: Rod B will sink more. Both rods exert the same force (weight) , but Rod B has a smaller contact area. Since pressure is force divided by area, the smaller area of Rod B creates higher pressure on the sand, causing it to sink deeper.

28. Two women are of the same weight. One wears sandals with pointed heels while the other wears sandals with flat soles. Which one would feel more comfortable while walking on a sandy beach? Give reasons for your answer.

Answer: The woman wearing sandals with flat soles would feel more comfortable.

Flat soles have a larger surface area in contact with the sand. This reduces the pressure (force per unit area) on the sand, so her feet do not sink much. Pointed heels have a small area, so they exert more pressure and sink deeper into the sand, making walking difficult and uncomfortable.

29. It is much easier to burst an inflated balloon with a needle than by a finger. Explain.

Answer: A needle has a very small pointed surface area, so it applies force on a very small area, creating high pressure and easily bursting the balloon. A finger spreads the force over a larger area, producing less pressure, so the balloon does not burst easily.

30. Observe the vessels A, B, C and D shown in Fig.11.10 carefully.

  

Volume of water taken in each vessel is as shown. Arrange them in the order of decreasing pressure at the base of each vessel. Explain.

Answer: The pressure at the base depends only on the height of the liquid column, not the volume.

The decreasing order of pressure is : B > D > A > C .

In vessel B, the water reaches the highest level, while in vessel C, it is the lowest . Greater height leads to greater pressure at the base.