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.
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.
What do the machines need for their working ?
Q 4.
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 5.
How does the wind move the blades of a wind-mill ?
Q 6.
Flowing water can rotate a turbine.What type of energy is used up by the turbine ?
Q 7.
When is work done by a force zero ?
Q 8.
What type of energy is possessed by a cricket ball just before being caught by a fielder ?
Q 9.
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 10.
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 11.
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 12.
A freely falling object eventually stops on reaching the ground.What happens to its kinetic energy ?
Q 13.
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 14.
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 15.
A 60 kg person climbs stairs of total height 20 m in 2 min.Calculate the power delivered.
Q 16.
What is the form of energy possessed by a running car?
Q 17.
Name two forms of mechanical energy.
Q 18.
The potential energy of a freely falling object decreases progressively. Does this violate the law of conservation of energy? Why?
Q 19.
Can a body have energy without having momentum ?
Q 20.
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 21.
A coolie carries a load of 50 kg on his head and walks on a level road upto 100 m.What is the work done by him ?
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.
Name the device which converts electrical energy into mechanical energy.
Q 24.
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 25.
Define average power.
Q 26.
Is a person doing any work by holding a suitcase ?
Q 27.
Name the type of energy possessed by a raised hammer?
Q 28.
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 29.
When an arrow is shot from a bow,it has kinetic energy.From where does it get the kinetic energy ?
Q 30.
What is the work done by a constant force ?When is it positive,negative or zero ?
Q 31.
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 32.
Fill in the blanks :
A form of energy an object has because of its motion is called ________.
Q 33.
From where the falling raindrops gains kinetic energy?
Q 34.
What is a conservative forces? Give an example.
Q 35.
Why do some engines require fuels like petrol and diesel ?
Q 36.
When a player hits a football it moves along the curved path and then falls to the ground. Calculate the work done by the force of gravity on the football.
Q 37.
A body of mass 120 g is taken vertically upwards to reach the height of 5m. Calculate work done. (Take g = 10 m/s
2)
Q 38.
A boy of mass 50 kg runs up a staircase of 45 steps in 9 s.If the height of each step is 15 cm,find his power.Take g =
.
Q 39.
Write an expression for kinetic energy of an object
Q 40.
What do you mean by the law of conservation of energy ?
Q 41.
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 42.
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 43.
A heavy stone is lowered to the ground with the help of rope. Is the work done by the applied force positive or negative?
Q 44.
What is the relationship between kinetic energy and work?
Q 45.
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 46.
A motor pump does same amount of water by lifting underground water in shorter time than a hand pump. What do you infer from this statement?
Q 47.
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 48.
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 49.
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 50.
What are the various energy transformations that occur when you are riding a bicycle ?
