Question:
A body of mass 120 g is taken vertically upwards to reach the height of 5m. Calculate work done. (Take g = 10 m/s2)
Answer:
Given,
a = g = - 10 m/s2
m = 120g = 0.120 kg
s = 5m
Work W = F × s
= m × g × s = 0.12 × -10 × 5 = -6 J
Since Force and displacement act in opposite directions, work is negative. Here work is considered to be done by the body against the applied force.
Work and Energy
Q 1.
Name a machine that transforms muscular energy into useful mechanical work.
Q 2.
A body is thrown vertically upwards. Its velocity goes on decreasing. Write the change in kinetic energy when its velocity becomes zero.
Q 3.
A 60 kg person climbs stairs of total height 20 m in 2 min.Calculate the power delivered.
Q 4.
Flowing water can rotate a turbine.What type of energy is used up by the turbine ?
Q 5.
Define potential energy.Derive an expression for the potential energy of a body of mass m,at a height h above the surface of the Earth.
Q 6.
A freely falling object eventually stops on reaching the ground.What happens to its kinetic energy ?
Q 7.
What do the machines need for their working ?
Q 8.
Two girls,each of weight 400 N,climb up a rope through a height of 8 m.We name one of the girls A and the other B.Girl A takes 20 s while B takes 50 s to accomplish this task.What is the power expended by each girl ?
Q 9.
How does the wind move the blades of a wind-mill ?
Q 10.
An object of mass 15 kg is moving with a uniform velocity of 4 m/s.What is the kinetic energy possessed by the object ?
Q 11.
Name the device which converts electrical energy into mechanical energy.
Q 12.
Does the transfer of energy take place when you push a huge rock with all your might and fail to move it? Where is the energy you spend going?
Q 13.
The work done by the heart is I J per beat.Calculate the power of the heart if it beats 72 times/min.
Q 14.
A force of 7 N acts on an object. The displacement is, say 8 m, in the direction of the force. Let us take it that the force acts on the object through the displacement. What is the work done in this case?
Q 15.
Name two forms of mechanical energy.
Q 16.
Fill in the blanks :
Ability to do work is called _______.
Q 17.
What is the form of energy possessed by a running car?
Q 18.
Why do some engines require fuels like petrol and diesel ?
Q 19.
Fill in the blanks :
A stretched rubber band acquires _________(potential/kinetic) energy.
Q 20.
A porter lifts a luggage of 15 kg from the ground and puts it on his head 1.5 m above the ground.Calculate the work done by him on the luggage.
Q 21.
Write an expression for the kinetic energy of an object in terms (i) velocity (
v) of an object (ii) in terms of momentum (
p) of an object.
Q 22.
A boy of mass 55 kg runs up a flight of 40 stairs,each measuring 0.15 m.Calculate the work done by the boy
Q 23.
Is energy a vector quantity ?
Q 24.
How is energy stored in a clock ?
Q 26.
A spring is stretched, will the work done by the stretching force positive or negative or zero.
Q 27.
The mass of a ball A is double the mass of another ball B.The ball A moves at half the speed of the ball B.Calculate the ratio of the kinetic energy of A to the kinetic energy of B.
Q 28.
A force of 7 N acts on an object.The displacement is,say 8 m,in the direction of the force,[in Fig.].Let us take it that the force acts on the object throughout the displacement What is the work done in this case ?
Q 29.
The velocity of a body moving in a straight line is increased by applying a constant force F,for some distance in the direction of the motion. Prove that the increase in the kinetic energy of the body is equal to the work done by the force on the body.
Q 30.
What type of energy is possessed by a cricket ball just before being caught by a fielder ?
Q 31.
What is meant by the term potential energy ? Give its two examples.
Q 32.
What is the work done by a constant force ?When is it positive,negative or zero ?
Q 33.
Can a body have energy without having momentum ?
Q 34.
The global climate has always fluctuated.Millions of years ago,some parts of the world that are now quite warm, were covered with ice,and over recent centuries,average temperatures have risen and fallen in cycles.What is new,however,is that current and future climate change will be caused not just by natural events but also by activities of human beings.Suggest three simple ways to help save the climate of our planet.
Q 35.
Fill in the blanks :
Ability of a body to do work due to its position or configuration is known as _________ of the body.
Q 36.
A stone of mass 2Kg is tied with a string and is whirled with a centripetal force of 10N in a circular path. How much work is done for making one revolution?
Q 37.
Fill in the blanks :
Work done against gravity in lifting a mass is stored as ______. Numerically, it is equal to m ×_ ×__.
Q 38.
Calculate the power of an engine required to lift 105 kg of coal per hour from a mine 360 m deep.
Q 39.
Define average power.
Q 40.
Is a person doing any work by holding a suitcase ?
Q 42.
Fill in the blanks :
1 Joule = ____ × ____.
Q 43.
Fill in the blanks :
Work done by gravity on a falling body is available as ______ (kinetic/potential) energy on reaching the ground.
Q 44.
Certain force acting on a 20 kg mass changes its velocity from 5 m/s to 2 m/s. Calculate the work done by the force.
Q 45.
If the momentum of a body is increased
n times, its kinetic energy increases (a) n times (b) 2n times (c) √n times (d) n
2 times
Q 46.
Name the type of energy possessed by a raised hammer?
Q 47.
Name the type of energy possessed by the following? (i) Stretched slinky (ii) Speeding car
Q 49.
What is a horsepower?
Q 50.
A body of mass 15 kg undergoes downward displacement of 40m under the effect of gravitational force. How much work is done? (take g = 10 m/s
2)
