PPT Work, Energy and Power - Physics for JEE Main & Advanced PDF Download

 Page 1


Work, Energy and 
Power
Page 2


Work, Energy and 
Power
What is Work?
In daily life “work? = any effort.
In physics, work is done only if a force causes displacement of its 
point of application in the direction of the force.
Definition (scalar): W = F ? · s? = F ? s? cos ?
Example (holding a weight at height h): the weight doesn't move 
? displacement = 0 
? work done on the weight = 0 J (even though you feel tired).
Page 3


Work, Energy and 
Power
What is Work?
In daily life “work? = any effort.
In physics, work is done only if a force causes displacement of its 
point of application in the direction of the force.
Definition (scalar): W = F ? · s? = F ? s? cos ?
Example (holding a weight at height h): the weight doesn't move 
? displacement = 0 
? work done on the weight = 0 J (even though you feel tired).
Work Done by a Constant Force
There are mainly two methods of finding work done by a force:
(i) Work done by a constant force.
(ii) Work done by a variable force.
Formula
Say, if a constant force F 
displaces a body through 
displacement "d", then the work 
done, W is given by
W = F d cos? = F.d
Where
d is the magnitude of 
displacement
? is the angle between force 
and displacement
SI Unit
The SI unit of work is joule (J).
Page 4


Work, Energy and 
Power
What is Work?
In daily life “work? = any effort.
In physics, work is done only if a force causes displacement of its 
point of application in the direction of the force.
Definition (scalar): W = F ? · s? = F ? s? cos ?
Example (holding a weight at height h): the weight doesn't move 
? displacement = 0 
? work done on the weight = 0 J (even though you feel tired).
Work Done by a Constant Force
There are mainly two methods of finding work done by a force:
(i) Work done by a constant force.
(ii) Work done by a variable force.
Formula
Say, if a constant force F 
displaces a body through 
displacement "d", then the work 
done, W is given by
W = F d cos? = F.d
Where
d is the magnitude of 
displacement
? is the angle between force 
and displacement
SI Unit
The SI unit of work is joule (J).
Unit of Work
S.I. System
In the S.I. system, the unit of work done is Joule. 
Another name for joule is newton-meter.
CGS System
In the CGS system, the unit of work is erg.
One erg of work is said to be done when a force of one 
dyne displaces a body through one centimeter in its 
direction.
Relation between joule and erg:
1 joule = 107 erg
1 erg = 10?7 joule
Dimensions of Work: [Work] = [Force] [Distance] = [MLT?²] [L] = [ML²T?²]
Page 5


Work, Energy and 
Power
What is Work?
In daily life “work? = any effort.
In physics, work is done only if a force causes displacement of its 
point of application in the direction of the force.
Definition (scalar): W = F ? · s? = F ? s? cos ?
Example (holding a weight at height h): the weight doesn't move 
? displacement = 0 
? work done on the weight = 0 J (even though you feel tired).
Work Done by a Constant Force
There are mainly two methods of finding work done by a force:
(i) Work done by a constant force.
(ii) Work done by a variable force.
Formula
Say, if a constant force F 
displaces a body through 
displacement "d", then the work 
done, W is given by
W = F d cos? = F.d
Where
d is the magnitude of 
displacement
? is the angle between force 
and displacement
SI Unit
The SI unit of work is joule (J).
Unit of Work
S.I. System
In the S.I. system, the unit of work done is Joule. 
Another name for joule is newton-meter.
CGS System
In the CGS system, the unit of work is erg.
One erg of work is said to be done when a force of one 
dyne displaces a body through one centimeter in its 
direction.
Relation between joule and erg:
1 joule = 107 erg
1 erg = 10?7 joule
Dimensions of Work: [Work] = [Force] [Distance] = [MLT?²] [L] = [ML²T?²]
Nature of Work
Looking at this equation W = F d cos?, we can understand three different scenarios for the work done:
1. Positive Work
When the force and the direction of movement 
are at an acute angle (? < 90º), cos? is positive. 
Hence, the work done is positive.
2. Zero Work
If the force applied and the direction of 
movement are at right angles (perpendicular), 
i.e., ?=90º, cos 90º = 0, the work done is zero.
3. Negative Work
When the force and the direction of movement 
form an obtuse angle (? >90º), cos ? is 
negative, and hence work done is negative.