NCERT Class VIII Science Solutions: Chapter 11-Force and Pressure Part 5

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Question 24:

As showing in fig is a a car sticking to an electromagnet.

A Car Sticking to an Electromagnet.

As showing in fig is a a car sticking to an electromagnet.

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

Answer:

Magnetic force (in the upward direction) and force of gravity or the weight of the car (downward). Magnetic force is larger than the force of gravity

LONG ANSWER QUESTIONS

Question 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 archer stretches the string of the bow by applying muscular force. In the process the shape of the bow changes. When the string is released, it regains its original position that provides the initial force to set the arrow in motion. The force of gravity that acts on the arrow in the downward direction brings it to the ground

Question 26:

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

Answer:

Blunt blades have larger area compared to the sharp-edged blades. Thus, the applied force produces a lower pressure in case of blunt blades, which makes it difficult to cut the cloth

Question 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. Which one of them will sink move? Why?

As showing in fig is a Two rods of the same weight and equal length have different thickness.

Rods of Same Weight and Length

As showing in fig is a Two rods of the same weight and equal length have different thickness.

Answer:

Rod B will go deeper as it has a smaller area of contact, therefore the same force (weight of the rod) produces more pressure. In case of rod A the same force produces less pressure

Question 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 will feel more comfortable while walking on the sandy beach. The flat soles have larger area compared to the sandals with pointed heels. Since the two women are of the same weight, they will apply same force on the ground. Therefore, the pressure exerted by the pointed heels will be more compared to that with sandals having flat soles. As a result the pointed heel sandals will sink more in the sand than the flat sole sandals. Hence, walking with flat sole sandals will be more comfortable

Question 29:

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

Answer:

When we prick the surface of an inflated balloon with a needle it exerts a larger pressure because it has a smaller area of contact compared to the finger. The large pressure pierces the surface of the balloon easily

Question 30:

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

As showing in fig is a Volume of water

Volume of Water

As showing in fig is a Volume of water

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:

B, D, A, C. Because pressure of a liquid column depends upon the height of the liquid column and not on volume of the liquid

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