Answer the following question.

Question 15 Marks

How is work calculated if the direction of force and the displacement are inclined to each other?

Answer

If the direction of force and the displacement are inclined to each other then, we must convert the applied force into the force acting along the direction of displacement.If θ is angle between force and displacement, then force (F1) in direction of displacement ishttps://pg-data.sgp1.digitaloceanspaces.com/chapter_wise/15898/Q12.png" alt="Image" width="180" height="" />

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Question 25 Marks

Explain the relation between, the commercial and SI unit of energy.

Answer

The commercial unit of energy is a kilowatt-hour (kWh) while the SI unit of energy is the joule. Their relation is
1 kWh = 1kW x 1hr
= 1000 Wx 3600 s
= 3600000J
(Watt x Sec = Joule)
1 kWh = 3.6 x 106 J.

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Question 35 Marks

When can you say that the work done is either positive, negative or zero?

Answer

When the force and the displacement are in the same direction, the work done by the force is positive.
When the force and displacement are in the opposite directions, the work done by the force is negative.
When the applied force does not cause any displacement or when the force and the displacement are perpendicular to each other, the work done by the force is zero.

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Question 45 Marks

Derive the expression for potential energy.

Answer

(i) To carry an object of mass ‘m’ to a height ‘h’ above the earth’s surface, a force equal to ‘mg’ has to be used against the direction of the gravitational force.

(ii) The amount of work done can be calculated as follows:
Work = force x displacement
∴ W = mg x h
∴ W = mgh

(iii) The amount of potential energy stored in the object because of its displacement.
PE = mgh (W = P.E)

(iv) Displacement to height h causes energy equal to mgh to be stored in the object.

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Question 55 Marks

Write effects of the following : Force

Answer

A force can move a stationary object. The force of engine makes a stationery car to move.
A force can stop a moving object. The force of brakes can stop a moving car.
A force can change the speed of a moving object. When a hockey player hits a moving ball, the speed of ball increases.
A force can change the direction of a moving object. In the game of carrom ,when we take a rebound then the direction of striker changes because the edge of the carrom board exerts a force on the strike.
A force can change the shape and size of an object. The shape of kneaded wet clay changes when a potter converts it into pots of different shapes and sizes because the p otter applies force on the kneaded wet clay.

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Question 65 Marks

Explain the statement “Potential Energy is relative”.

Answer

The potential energy of an object is determined and calculated according to a height of the object with respect to the observer.
So, the person staying on 6th floor more potential energy than those staying on the 3rd floor.
But, the person on the 6th floor will have lesser potential energy than on the 8th floor. Hence potential energy is relative.

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Question 75 Marks

When a body is dropped on the ground from some height its P.E is converted into K.E but when it strikes the ground and it stops, what happens to the K.E?

Answer

When a body is dropped on the ground, its K.E appears in the form of:

Heat (collision between the body and the ground).
Sound (collision of the body with the ground).
The potential energy of change in state of the body and the ground.
Kinetic energy is also utilized to do work i.e., the ball bounces to a certain height and moves to a certain distance vertically and horizontally till Kinetic energy becomes zero.
The process in which the kinetic energy of a freely falling body is lost in an unproductive chain of energy is called the dissipation of energy.

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