Work and Energy Worksheet-7
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Explain by an example that a body may possess energy even when it is not in motion.
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Give two examples where a body possesses both, kinetic energy as well as potential energy.
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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)
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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?
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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/s2).
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What type of energy is possessed :
By the piece of stone which is thrown away on releasing the stretched rubber strings of catapult?
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(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.
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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.
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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.
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(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.
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(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 ?
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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?
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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.
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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.
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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?
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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?
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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°?
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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.
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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/s2,
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 ?
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When a ball is thrown vertically upwards, its velocity goes on decreasing. What happens to its potential energy as its velocity becomes zero ?
Answer:
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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.
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A flying bird, a man climbing a hill.
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50 Kg
PE = mgh
2500 = m × 10 × 5
m = 50 kg
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121 N
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Kinetic energy = (1/2)mv2 & PE = mgh
At highest point v = 0 so KE will be zero.
(i) Zero (ii) 600 J
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Both potential energy and Kinetic energy
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(d) 1 : 4 Using Kinetic energy = (1/2)mv2
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The energy of a body due to its motion is called Kinetic energy.
Kinetic energy = (1/2)mv2
(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.
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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°
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θ = 90 degree
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When the angle between the direction of force and direction of motion is 0°
W = F cos θ × s
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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
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12 × 105 J , 2.4 × 105 J
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Potential energy becomes the maximum