Work and Energy Worksheet7

Explain by an example that a body may possess energy even when it is not in motion.

Give two examples where a body possesses both, kinetic energy as well as potential energy.

How much is the mass of a man if he has to do 2500 joules of work in climbing a tree 5 m tall ?
(g = 10 m s^{2})

If the work done by a force in moving an object through a distance of 20 cm is 24.2 J, what is the magnitude of the force?

A boy weighing 40 kg makes a high jump of 1.5 m.
(i) What is his kinetic energy at the highest point?
(ii) What is his potential energy at the highest point? (g = 10 m/s^{2}).

What type of energy is possessed :
By the piece of stone which is thrown away on releasing the stretched rubber strings of catapult?

(a) Define the term 'work'. Write the formula for the work done on a body when a force acts on the body in the direction of its displacement. Give the meaning of each symbol which occurs in the formula.

When do we say that work is done? Write the formula for the work done by a body in moving up against gravity. Give the meaning of each symbol which occurs in it.

What happens to the work done when the displacement of a body is at right angles to the direction of force acting on it? Explain your answer.

(a) Define the term 'energy' of a body.
(b) What is the SI unit of energy.
(c) What are the various forms of energy?
(d) Two bodies having equal masses are moving with uniform speeds of v and 2v respectively. Find the ratio of their kinetic energies.

(a) What do you understand by the kinetic energy of a body ?
(b) A body is thrown vertically upwards. Its velocity goes on decreasing. What happens to its kinetic energy as its velocity becomes zero ?
(c) A horse and a dog are running with the same speed. If the weight of the horse is ten times that of the dog, what is the ratio of their kinetic energies ?

Explain by an example what is meant by potential energy. Write down the expression for gravitational potential energy of a body of mass m placed at a height h above the surface of the earth. What is the difference between potential energy and kinetic energy?

What is the difference between gravitational potential energy and elastic potential energy? Give one example of a body having gravitational potential energy and another having elastic potential energy.

A boy tries to push a truck parked on the roadside. The truck does not move at all. Another boy pushes a bicycle. The bicycle moves through a certain distance. In which case was the work done more: on the truck or on the bicycle? Give a reason to support your answer.

The work done by a force acting obliquely is given by the formula : W = F cos θ × s. What will happen to the work done if angle θ between the direction of force and motion of the body is increased gradually? Will it increase, decrease or remain constant?

What should be the angle between the direction of force and the direction of motion of a body so that the work done is zero?

In which of the following case the work done by a force will be maximum: when the angle between the direction of force and direction of motion is 0° or 90°?

How much work is done by the gravitational force of earth acting on a satellite moving around it in a circular path ? Give reason for your answer.

A man is instructed to carry a package from the base camp at B to summit A of a hill at a height of 1200 meters. The man weighs 800 N and the package weighs 200 N. If g = 10 m/s^{2},
Calculate the work the man does against gravity ?
Also calculate what is the potential energy of the package at A if it is assumed to be zero at B ?

When a ball is thrown vertically upwards, its velocity goes on decreasing. What happens to its potential energy as its velocity becomes zero ?
Answer:

A ball thrown in air then at the top most point its final velocity is zero but it has potential energy.
A stationary box put at the top of a house has potential energy.

A flying bird, a man climbing a hill.

50 Kg
PE = mgh
2500 = m × 10 × 5
m = 50 kg

121 N

Kinetic energy = (1/2)mv^{2} & PE = mgh
At highest point v = 0 so KE will be zero.
(i) Zero (ii) 600 J

Both potential energy and Kinetic energy

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(d) 1 : 4 Using Kinetic energy = (1/2)mv^{2}

The energy of a body due to its motion is called Kinetic energy.
Kinetic energy = (1/2)mv^{2}
(b) Kinetic energy becomes zero
(c) 10 : 1

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Work done = Force X Displacement
More work is done on the bicycle. This is because the bicycle moves through a certain distance on applying force ; No work is done on the truck because it does not move at all on applying force.

At θ = 0 work done will be maximum. When θ increases work done will be reduced and at θ = 90 degree work done will be zero.
Decrease 73.90°

θ = 90 degree

When the angle between the direction of force and direction of motion is 0°
W = F cos θ × s

W = F cos θ × s Here in this case work done will be Zero ; Because the gravitational force acts along the radius of circular path, at right angles (90°) to the motion of satellite

12 × 105 J , 2.4 × 105 J

Potential energy becomes the maximum