Chapter 11 Work and Energy Solutions
Question - 1 : - 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?
Answer - 1 : -
When a force F acts on an object to move it in its direction through a distance S, the work is done
The work on the body is done by force
Work done = Force × Displacement
W = F × S
Where,
F = 7 N S = 8 m
So, work done,
W = 7 × 8
W = 56 Nm
W = 56 J
Question - 2 : - When do we say that work is done?
Answer - 2 : -
Work is completed whenever the given conditions are satisfied:
(i) A force acts on the body.
(ii) There’s a displacement of the body caused by the applied force on the direction of the applied force.
Question - 3 : - Write an expression for the work done when a force is acting on an object in the direction of its displacement.
Answer - 3 : -
When a force F displaces a body through a distance S within the direction of the applied force, then the work done W on the body is given by the expression:
W = F × S
Question - 4 : - Define 1 J of work.
Answer - 4 : - 1 J is that the quantity of labor done by a force of one N on associate degree object that displaces it through a distance of one m within the direction of the applied force.
Question - 5 : - 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?
Answer - 5 : -
Work done by the bullocks is given by the expression:
W= F × d
Where,
Applied force, F = 140 N
Displacement, d = 15 m
W= a hundred and forty × fifteen = 2100 J
Hence, 2100 J of labour is finished in tilling the length of the sector.
Question - 6 : - What is the kinetic energy of an object?
Answer - 6 : - The energy possessed by a body by the virtue of its motion is termed mechanical energy or kinetic energy. Every moving object possesses mechanical energy. A body uses mechanical energy to try to work. Kinetic energy of the hammer is employed in driving a nail into a log of wood, mechanical energy of air is employed to run wind mills, etc.
Question - 7 : - Write an expression for the kinetic energy of an object.
Answer - 7 : -
If a body of mass m is moving with a speed v, then its K.E. E k is given by the expression,
Ek = 1/2 m v2
Its SI unit is Joule (J).
Question - 8 : - The kinetic energy of an object of mass, m moving with a velocity of 5 ms-1 is 25 J. What will be its kinetic energy when its velocity is doubled? What will be its kinetic energy when its velocity is increased three times?
Answer - 8 : - 
Question - 9 : - What is power?
Answer - 9 : - Power is that the rate of doing work or the speed of transfer of energy. If W is that the quantity of work wiped out time t, then power is given by the expression,
P = W/T
It is expressed in watt (W).
Question - 10 : - Define 1 watt of power.
Answer - 10 : -
A body is claimed to possess power of one watt if it will work on the speed of 1 joule in 1 s.
That is,
One W = 1 J/1 S