Ch#6 : Work and Energy

Question 1

What is work? , What happens when force and displacement are not parallel?
What is the unit of work?

Answer 1

Work : is done when a force acting on a body displaces it in the direction of the force.
W = F S

Sometimes force and displacement arent parallel. The force F makes an angle 0 with the displacement.
Fx = F cos 0
Fy = F sin 0
In this case , only the x component parallel to displacement causes the body to move and not the y component Fy.
W = Fx S
W = F S cos 0
Scalar Quantity.
Si Unit : Joule
The amount of work done is one joule when a force of one newton displaces a body through one metre in the direction of the force.
1J = 1N x 1m
J is a small unit of work , kJ and MJ are its bigger units.

Question 2

What is energy? , state its forms.

Answer 2

Energy is an important and fundamental concept in physics. It explains almost all natural phenomena.
A body possesses energy if it is capable of doing work.
Example : Water running down a strean has the ability to do work. The energy of running can be used to run water mills.

Types : mechanical energy , light energy , sound energy , heat energy , electrical energy , nuclear energy and chemical energy.

Question 3

State and derive kinetic energy.

Answer 3

Definition : The energy possesses by a body due to its motion is called kinetic energy.

Explanation : Consider a body of mass moving with velocity v. The body stops moving after covering distance S due to some opposing force such as the force of friction.The body possesses kinetic energy and is capable of doing work against the opposing force F until all of its kinetic energy is used up.

K.E = Work done = FS
Vi = v
Vf = 0
F = ma
a = -F/m
Since motion is opposed , a is negative.
Using 3rd equation of motion , 
2aS = Vf² - Vi²
2(-F/m)S = 0 - v²
FS = 1/2mv²
K.E = 1/2mv²

Question 4

Define and derive potential energy. What are elastic potential energy and gravitational potential energy?

Answer 4

Definition : The energy possessed by a body due to its position is called potential energy.

Explanation : Consider a body of mass m raised up to height h. The body will aquire potential energy equal to the work done in lifting it to height h.

P.E = F x h
(F = w = mg)
P.E = mgh

The energy present in a stretched bow is called elastic potential energy.

The energy possessed by a body due to its height is called gravitational potential energy.

Question 5

What is efficiency? Give its formula.

Answer 5

Efficiency of a system is the ratio of required form of energy obtained from a system to the total energy given as input.

Efficiency = required form of energy/total input energy

Efficiency = required form of energy/total input energy x 100

Question 6

Why does a ideal system with 100% efficiency not exist?

Answer 6

An ideal system would be one that gives the required form of energy equal to the total input energy. In other words , the efficiency is 100%. People have tried to design such a system with 100%.But practically such a system doesnt exist.Every system meets energy losses due to friction which causes heat ,noise , etc. These arent useful forms of energy and thus they go to waste.This means we cant utilize all the energy given to a working system. The required form of energy is always less than the total input energy.

Question 7

What is power? , What is the unit of power and 1hp = ?

Answer 7

Power is defined as the rate of doing work.

P = W/t
Since work is scalar , power is also a scalar quantity. Si Unit : Watt(W)

Watt : The power of a body is one watt if it does work at the rate of one joule per second.

1hp = 746W