work energy theorem,work, kinetic energy
Work:
Work is said to be done by a force when a body undergoes displacement parallel to the line of action of the force.
SI unit of work : joule ; CGS unit of work : erg; 1 Joule = 10 7 ergs : 1 KWh =3.6 x 10 6 J
If a force F acts on a body at an angle and displaces the body through a distance ‘S’ then,
Work done W = (F Cosθ) S.
Work is a scalar
Work done:
(i) When θ = 90 º work done is zero
(ii) When 0 < θ < 90 º work done is positive
(iii) When 90 º < θ < 180 º work done is negative
Conditions for no zero work:
(i) When force is zero
(ii)When displacement is zero
(iii) When angle between force and displacement is
Kinetic energy:
The energy possessed by a body by virtue of its motion is called Kinetic energy
Examples:
(1) A vehicle in motion
(2) Water flowing along a river
(3) A bullet fired from a gun
(4) A satellite going around the earth
→ A flying bird, moving aeroplane, freely falling body, a body moving on an inclined plane and oscillating pendulum posses both potential energy and kinetic energy.
→ The relation between K.E. and momentum is 
Example:
A drop of mass falling from a height of 1.00Km. What is the work done by the gravitational force? (Take g=10ms-2)
(A) Given, mass =1g=10-3Kg, height=1Km=103m
Work done by gravitational force 
Example:
In a ballistics demonstration a police officer fires a bullet of mass 50.0g with speed of 200ms-1 on soft plywood of thickness 2.00cm. The bullet emerges with only 10% of its initial kinetic energy. What is the emergent speed of bullet?
Example:
Find the total energy of a body of mass 5kg. Which is at a height of 10m from the earth and falling downward with a velocity of 20m/s (g=10ms-2)
(A) Given, m=5Kg,h=10m, V=20m/s, g=10ms-2
Total energy= Potential energy + Kinetic energy
Work energy theorem:
“The work done on a particle by the net force is equal to the change in its kinetic energy” is known as work energy theorem .
From work energy theorem, Work done
Proof:
If a body of mass ‘m’ moving with an initial velocity ‘U’ be acted on by a constant force ‘F’ and attains a final velocity ‘V’ after time ‘t’.
From equations of motion, 
Multiplying with 
on both sides,
Thus, Work-energy theorem is proved.