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.