Grade 12 Physics Knowledge area Mechanics Work energy

Grade 12 Physics Knowledge area: Mechanics

Work, energy and power 3. 2 Power or rate of work (P) Definition Power is the rate at which work is done. Symbol form: power (W) Unit: watt (W) W = J⋅s-1 work (J) W P = Δt time (s)

Work, energy and power One watt is the power when one joule of work is done in one second. Examples Masenyani, mass 60 kg, runs up a staircase in 10 s. If the vertical height of the staircase is 4 m, calculate the work rate/power. Masenyani did work because he applied a force with the same magnitude as the gravitational force, but in the opposite direction. In the process of doing so, he gained potential energy. Definition

Work, energy and power P = = W Δt mgΔy cosθ Δt 60 × 9, 8 × 4 10 235, 2 W

Work, energy and power Another formula: w Paverage = Δt FΔx = Δt Δx = F Δt = Fv therefore P = Fappliedvave In short: Paverage = Fvave

Work, energy and power Wnc = ΔEK + ΔEP Examples An electric pump is bought to pump water to a higher dam. It pumps 100 kg of water in 2 minutes to a height of 10 m. The pump is marked 120 W. Calculate the efficiency of the pump. It can be assumed that the water is at rest at the bottom and the top dam.

Work, energy and power P = = = Wnc Δt ΔEK + ΔEP Δt (0 - 0) + (mghtop/at 10 m - 0) Δt 100 × 9, 8 × 10 2 × 60 81, 67 W

Work, energy and power The power of the pump is 120 W, but only 81, 67 W is used to pump the water. The efficiency is: 81, 67 120 × 100 = 68, 06%