Work, Energy and Power (M3)

Question 1

Efficiency of a motor

Answer 1

Efficiency = mgh/input X 100
Measure m with balance
Measure h with meter rule
Measure input energy with joulemeter

Question 2

Work done

Answer 2

Force X distance moved in the direction of the force
Measured in Joules (J)

Question 3

Power

Answer 3

Work done / time
Measured in Watts (W)
W = Js^-1

Question 4

Efficiency

Answer 4

Efficiency = (useful power output / total power input) X 100
Often expressed as a percentage or decimal

Question 5

Gravitational energy

Answer 5

GPE = mgh
Measured in Joules (J)

Question 6

Kinetic energy

Answer 6

KE = 1/2 mv^2
Measured in Joules (J)

Question 7

Principle of the conservation of energy

Answer 7

The total energy of a closed system remains constant - energy cannot be created or destroyed

Question 8

Kinetic energy equation from first principles

Answer 8

v^2 = u^2 + 2as
s = (v^2 - u^2) / 2a
u = 0
s = v^2 / 2a
Work done is entirely transferred to KE so:
Work done = Ek = Fs
Ek = ma X s = ma X v^2/2a
Ek = 1/2 mv^2

Question 9

Gravitational potential energy from first principles

Answer 9

GPE = work done = force X distance
GPE = mg X h = mgh