Question:
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.
Answer:
Given, mass of the object (m) = 20kg
initial velocity of the object (u) = 5m/s
final velocity of the object (v) = 2 m/s
According to Work Energy theorem, W = ½ mv2 - ½ mu2
∴ W = ½ × 20g × ( 22- 52) = 10 × (4 - 25) = 10 × -21 = -210J
The negative sign shows that the force is acting in the direction opposite to the motion of the object.
Work and Energy
Q 1.
Fill in the blanks :
Work is a ______ (scalar/vector) quantity.
Q 2.
Fill in the blanks :
Work done on an object is _____ if displacement is zero.
Q 3.
A force of 10 N moves a body with a constant speed of 2 m/s. Calculate the power of the body.
Q 4.
A pair of bullocks exerts a force of 140 N on a plough. The field being ploughed is 15 m long. How much work is done in ploughing the length of the field?
Q 5.
Calculate potential energyof a person having 60 kg mass on the summit of Mt. Everest. Height of Mt. Everest is 8848 m from sea level. (g = 9.8 m/s
2)
Q 6.
Fill in the blanks :
Work done by gravity on a falling body is available as ______ (kinetic/potential) energy on reaching the ground.
Q 7.
Fill in the blanks :
Work done is defined as the product of _____ and ______ in the direction of force.
Q 8.
Fill in the blanks :
Work done is negative when the angle between force and displacement is ____ (acute/obtuse)
Q 9.
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 10.
An electric bulb of 60 W is used for 6 h per day.Calculate the ‘units’ of energy consumed in one day by the bulb
Q 11.
A man exerts a force of 200 N in pulling a cart at a constant speed of 16 m/s.Calculate the power spent by the man.
Q 12.
What do you understand by the term energy ? Give its scientific conception.
Q 13.
An object of mass 40 kg is raised to a height of 5 m above the ground. What is its potential energy? If the object is allowed to fall, find its kinetic energy when it is half-way down.
Q 14.
What is the work done by the force of gravity on a satellite moving round the earth? Justify your answer.
Q 15.
A bullet of mass 20 g is found to pass two points 30 m apart in 4 s ? Assuming the -speed to be constant,find its kinetic energy.
Q 16.
Is it possible to havfc a situation where mechanical energy (E) - potential energy (
) Is negative,i.e.,(E-
)< 0.
Q 17.
Define 1 J of work.
Q 18.
A battery lights a bulb.Describe the energy changes involved in the process
Q 19.
How do you differentiate between energy and power ?
Q 20.
Many people debate about whether there is a God,or what God is.Is God an individual personality,sitting up in heaven listening to and granting our requests,or is God simply all the energy that exists in the Universe,or is God simply nature or the laws of nature ?
Q 21.
Fill in the blanks :
Work done against a force is stored as _____.
Q 22.
Write an expression for the kinetic energy of an object.
Q 23.
What is mechanical energy? What are its types?
Q 24.
What are other forms of energies apart from mechanical energy? Does Law of Conservation of Energy apply to all of them?
Q 25.
A certain household has consumed 250 units of energy during a month. How much energy is this in joules?
Q 26.
A man does 200 J of work in 10 s and a boy does 100 J of work in 4 s.(a) Who is delivering more power ?
(b) Find the ratio of the power delivered by the man to that delivered by the boy.
Q 27.
A mass of 10 kg is at a point A on a table.It is moved to a point B.If the line joining A and B is horizontal,what is the work done on the objjgft by the gravitational force ? Explain your answer.
Q 28.
What are the various energy transformations that occur when you are riding a bicycle ?
Q 29.
What are the units of work and energy ?
Q 30.
What is the difference between potential energy and kinetic energy ?
Q 31.
Define kinetic energy.Give some examples of bodies possessing kinetic energy.
Q 32.
Two bodies of masses m
1 and m
2 have equal kinetic energies. If p
1 and p
2 are their respective momentum, then ratio p
1 : p
2 is ___?
Q 33.
Two bodies of equal masses move with uniform velocities v and 3 v respectively.Find the ratio of their kinetic energies.
Q 34.
What do you mean by the law of conservation of energy ?
Q 35.
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 36.
Can there be displacement of an object in the absence of any force acting on it?
Q 37.
Is energy a vector quantity ?
Q 38.
Give an example where potential energy is acquired by a body due to change of its shape.
Q 39.
What is meant by the term potential energy ? Give its two examples.
Q 40.
What do you mean by work ?Give its scientific conception.
Q 41.
Is it possible that a force acts on a body still the work done is zero? Explain with an example.
Q 42.
Fill in the blanks :
SI Unit of work is _______.
Q 43.
What do you mean by kinetic energy ? Derive an expression for the kinetic energy of an object of mass m moving with velocity,v.
Q 44.
A boy of mass 45 kg climbs up 20 steps in 20 second. If each step is 25 cm high, calculate the power used in climbing
Q 45.
Fill in the blanks :
Work done against gravity in lifting a mass is stored as ______. Numerically, it is equal to m ×_ ×__.
Q 46.
Does the work done on a load depend on the time taken to raise the load?
Q 47.
Identify the kind of energy possessed by a running athlete.
Q 48.
A bullet of mass 0.03 kg moving with a speed of 400 m/s penetrates 12 cm into fixed block of wood.Calculate the average force exerted by the wood on the bullet.
Q 50.
An electric heater is rated 1500 W.How much energy does it use in 10 hours ?
