o Average Power : Ratio of work done ( W ) in a total time interval of t .
Pav = W / t o
Instantaneous Power : When the time interval , t , approaches zero the limiting value of average power becomes instantaneous power . P = dW / dt
o We can write W = F . dr , P = F . dr / dt P = F . v , where v is instantaneous velocity . o
Power is a scalar quanitity o SI unit of power – Watt ( W ) o
1 hp = 746 W
Example : A pump on the ground floor of a building can pump up water to fill a tank of volume 30 m 3 in 15 min . If the tank is 40 m above the ground , and the efficiency of the pump is 30 %, how much electric power is consumed by the pump ?
Solution : P = W / t = mgh / t mass of water to be moved = ρV = 30000 kg g = 9.8 h = 40 m t = 15x60 = 900 s P = 13066.67 W = 13.067 kW Now η = P / PACTUAL => PACTUAL = 13.067 / 0.3 = 43.6 kW
o
Collosion
A collision is an event in which two or more bodies exert forces on each other for a relatively short time .
o In all collisions the total linear momentum is conserved .
o The total impulse on the first object is equal and opposite to that on the second , if two bodies collide .
o Elastic collision is when the initial Kinetic energy is equal to the final kinetic energy .
o Inelastic collision is when some of the kinetic energy is lost after collision .
o Completely inelastic collision is when the bodies after collision move together .