Force and Pressure - Exemplar Solutions 4
NCERT Exemplar Solutions
CHAPTER – 11
Force and Pressure
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.
Ans: The archer stretches (pulls) the string of the bow by applying a muscular force. In the process, the shape of the bow changes. The string is elongated when a muscular force is applied by the archer. When the string is released by the archer, the string tries to regain its original position. This provides the initial force (push) to set the arrow in motion and hence, the arrow moves horizontally.
The arrow ultimately falls down after moving some distance because the force of gravity is acting on the arrow in the downward direction. The arrow also experiences a "drag" while moving through the air and slows down.
Explanation: The force that results due to the action of the muscles is known as the muscular force. Muscular force is a "contact force". The force of friction acts on a moving object like the arrow. The direction of force of friction (a contact force) is always opposite to the direction of motion. The force of gravity (a non-contact force) is exerted on the arrow by the Earth.
26. It is difficult to cut cloth using a pair of scissors with blunt blades. Explain.
Ans: Blunt blades have a larger area as compared to the area of sharp-edged blades. Thus, the force applied on blunt blades produces a lower pressure. The low pressure of the blunt blades makes it difficult to cut the cloth.
Explanation: The area over which the muscular force is applied has an important role in determining the relative difficulty or ease of the task which makes use of the muscular force. The smaller the area of contact, the larger will be the pressure on the surface for the same force, and hence, the less difficult will be the task.
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?
Ans: Rod B will go deeper as it has a smaller area of contact with the sand. The same force (same weight of the two rods) produces more pressure when the area of contact is smaller. In case of rod A, the same force produces lesser pressure since the area of contact is greater.
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.
Ans: The woman wearing "sandals with flat soles" will feel more comfortable while walking on the sandy beach.
Since the two women are of the same weight, they will apply the same amount of force on the sand. The area of contact of "sandals with flat soles" will be larger as compared to the area of contact of "sandals with pointed heels". The pressure exerted by the pointed heels will be more than that exerted by "sandals having flat soles". As a result, the pointed heels will sink in the sand and offer higher friction to walking.
29. It is much easier to burst an inflated balloon with a needle than by a finger. Explain.
Ans: When we prick the surface of an inflated balloon with a needle, the needle exerts a larger pressure because it has a smaller area of contact compared to that of the finger. The large pressure exerted by a needle pierces the surface of the inflated balloon 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.
Ans: Vessels in the order of decreasing pressure at the base of the vessel will be as follows:-
Vessel D, Vessel B, Vessel A, Vessel C
The pressure exerted at the bottom of the vessel will be the highest in vessel D.
The pressure exerted at the bottom of the vessel will be the lowest in vessel C.
The pressure exerted by a liquid column at the bottom of the vessel depends upon the height of the liquid column and not on the volume of the liquid.
