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(d) both (b) and (c)

Given :

Pressure = \(\frac{F}{A}\)

F = 20 N

A = 4 cm² = 4 x 10^-2 m

P = \(\frac{F}{A}\) = \(\frac{20}{40 \times 10^{-2}}\)

= 5 x 10²

= 500 Nm^-2 (or) 500 pack

If the area is doubled, keeping the force the same, the pressure will become half the initial pressure.

True A liquid exerts pressure in all directions.

Buoyancy is the property of a liquid due to which it exerts an upward force on an object immersed in it.

Yes, both the liquids and gases also exerts pressure.

Keeping the surface area constant, if the applied force is doubled, the pressure becomes double.

1. When air is filled in a balloon, it acquires its characteristic shape such as round or oval. 

2. When a bicycle tube is filled with air, it acquires its characteristic (tube-like) shape throughout. This shows that air exerts equal pressure in all directions.

A fluid is a substance which can flow. 

Examples: 

1. Water (liquid) 

2. Air (gas)

[Note: Liquids and gases together are called fluids. Gases have very low viscosity compared to liquids. A liquid with low viscosity flows easily. A liquid with high viscosity does not flow easily.]

The tip of a pointed nail has an extremely small area, while that of a blunt nail has a comparatively large area. A given force creates a large pressure on the pointed nail and it can be easily hammered into the wood, while a very less pressure is created on the blunt nail and it cannot be easily hammered into the wood.

This shows that pressure varies inversely as the area of the surface on which the force is applied, if the force remains constant. If the same force is applied to surfaces having different areas, the pressure is more on the surface having a smaller area. 

[Note: The bottom surface of a camel’s feet is broad. Hence, the camel’s weight acts on a large surface area. This reduces the pressure on the sand. Hence, the camel’s feet do not penetrate deep into the sand. Therefore, it becomes easier for the camel to walk on sand.]

(1) A magnet attracts the iron bar. 

As a result it is easier to lift the heavy iron bar using magnetic force.

(2) When an inflated balloon is rubbed against a silk cloth, it develops electrostatic force due to friction. 

This force pulls the hair on our skin.

(3) Wet or oily floor reduces the friction between our feet and surface. 

Hence we are likely to slip on wet or oily floor.

Force : “Is an external agent which changes or tends to change the state of rest or of uniform motion of a body in straight line or bends or deforms a body.” 

Or 

Force : “Is the which produces or tries to produce a change in the state of rest or of uniform motion or shape of the body.”

1. The stone is moving with uniform speed as given in the question.

 2. Uniform acceleration. It acts at right angles to the direction of motion of stone. 

3. The force which acts on the hand is the Centripetal force. It is directed towards the hand along the string. 

Streamlined shapes experience minimum drag (fluid friction) while travelling through air and water. Hence, boats and ships are given streamlined shape.