**Work**: Work is done when a force (push or pull) applied to an object causes a displacement of the object. **The SI unit of work is Joule.**

**Power**: Power is the work done per unit of time. The SI unit of power is the watt. One watt is equal to **one joule per second.**

**Energy**: Energy is the capacity to do work. It exists in various forms like potential, kinetic, chemical, thermal, nuclear, electrical energy, and so on. **The total amount of energy is always constant. **

## Table of Contents

## Quiz for Work, Power and Energy

#### Work done by the constant force

A rigid body of mass m is moving in a circle of radius r with a constant speed v. The force on the body
is mv^{2}/r and is directed towards the centre. What is the work done by this force in moving the body
over
half the cirumference of the circle.

- mv
^{2}/πr^{2} - Zero
- mv
^{2}/r^{2} - πr
^{2}/mv^{2}

If the unit of force and length each be increased by four times, then the unit of work is increased by

- 16 times
- 8 times
- 2 times
- 4 times

A man pushes wall and fails to displace it. He does

- Negative work
- Positive but not maximum work
- No work at all
- Maximum work

A rigid body moves a distance of 10 m along a straight line under the action of a force of 5 N. If the work done by this force on the body is 25 joules, the angle which the force makes with the direction of motion of the body is

- 0º
- 30º
- 60º
- 90º

A rigid body of mass m kg is lifted uniform velocity by a man to a height of one metre in 30 sec. Another man lifts the same mass with uniform velocity to the same height in 60 sec. The work done on the body against gravitation by them are in ratio

- 1 : 2
- 1 : 1
- 2 : 1
- 4 : 1

A particle moves from position

- 100 J
- 50 J
- 200 J
- 75 J

A ball is released from the top of a tower. The ratio of work done by force of gravity in first, second and third second of the motion of the ball is :

- 1 : 2 : 3
- 1 : 4 : 9
- 1 : 3 : 5
- 1 : 5 : 3

A block of mass m is suspended by a light thread from an elevator. The elevator is accelerating upward
with uniform acceleration a. The work done by tension on the block during t seconds is (u = 0) :

- m/2 (g + a) at
^{2} - m/2 (g – a) at
^{2} - m/2gat
^{2} - 0

Work done by force of kinetic friction on the system.

- must be zero
- must be positive
- must be negative
- None of these

#### Work done by A variable force

Two springs have their force constant as k_{1} and k_{2}(k_{1} > k_{2}). When
they are stretched individually by the
same constant force up to equilibrium –

- No work is done by this force in case of both the springs
- Equal work is done by this force in case of both the springs
- More work is done by this force in case of second spring
- More work is done by this force in case of first spring

A rigid body is acted upon by a horizontal variable force which is inversely proportional to the distance covered from its initial position ‘s’. The work done by this force will be proportional to :

- S
- S
^{2} - √s
- None of these

The work done by the frictional force on a surface in drawing a circle of radius r on the surface by a pencil of negligible mass with a normal pressing force N (coefficient of friction µk) is :

- 4πr
^{2}µK N - -2πr
^{2}µK N - -2πr µK N
- zero

#### Work Energy Theorem

The kinetic energy of a body of mass 2 kg and momentum of 2 Ns is

- 1 J
- 2 J
- 3 J
- 4 J

A particle of mass m at rest is acted upon by (only) force F for a time t. Its kinetic energy after an interval t is :

- F
^{2}t^{2}/m - F
^{2}t^{2}/2m - F
^{2}t^{2}/3m - F t/2m

A particle is projected horizontally from a height h. Taking g to be constant every where, kinetic energy E of the particle with respect to time t is correctly shown in (Neglect air resistance)

If v, p and E denote the megnitude of velocity, momentum and kinetic energy of the particle, then :

- p = dE/dv
- p = dE/dt
- p = dv/dt
- None of these

A body moving at 2 m/s can be stopped over a distance x. If its kinetic energy is doubled, how long will it go before coming to rest, if the retarding force remains unchanged ?

- X
- 2X
- 4X
- 8X

A retarding force is applied to stop a train. The train stops after 80 m. If the speed is doubled, then the distance travelled when same retarding force is applied is

- The same
- Doubled
- Halved
- Four times

A particle moves in a straight line with retardation proportional to its displacement. Its loss of kinetic energy for any displacement x is proportional to

- x
^{2} - e
^{x} - X
- log
_{e}X

A block weighing 10 N travles down a smooth curved track AB joined
to a rough horizontal surface (figure). The rough surface has a
friction coefficient of 0.20 with the block. If the block starts slipping on
the track from a point 1.0 m above the horizontal surface, the
distance it will move on the rough surface is

- 5.0 m
- 10.0 m
- 15.0 m
- 20.0 m

A small mass slides down an inclined plane of inclination θ with the horizontal. The co-efficient of
friction is μ = μ_{0}x where x is the distance through which the mass slides down and μ_{0} is a
constant.
Then the distance covered by the mass before it stops is:

A toy car of mass 5 kg starts from rest and moves up a ramp under the influence of force F (F is applied
in the direction of velocity) plotted against displacement x. The maximum height attained is given by
(g = 10 m/s^{2})

- y
_{max}= 20 m - y
_{max}= 15 m - y
_{max}= 11 m - y
_{max}= 5 m

A body of 5 kg mass is raised vertically to a height of 10 m by a force of 120 N. Find the final velocity of the body :

- √280 m/s
- √200 m/s
- 20 m/s
- None of these

The ratio of work done by the internal forces of a car in order to change its speed from 0 to V and from V to 2V is (Assume that the car moves on a horizontal road) :

- 1
- 1/2
- 1/3
- 1/4

A body of mass 4 kg moves under the action of a force where t is the time in second. The initial velocity of the particle is . If the force is applied for 1 s, work done is :

- 4J
- 8J
- 12J
- 16J