1.7.4 Power (3)
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1.
Define power in terms of work done and time, and in terms of energy transferred and time. State the relevant equations.
Definition: Power is the rate at which work is done, or the rate at which energy is transferred.
Work done per unit time: Power (P) = Work done (W) / Time (t) or P = W / t
Energy transferred per unit time: Power (P) = Energy transferred (ΔE) / Time (t) or P = ΔE / t
2.
A motor transfers 120 J of energy in 5 seconds. Calculate the power of the motor. State the equation you used.
Equation used: P = ΔE / t
Given:
- Energy transferred (ΔE) = 120 J
- Time (t) = 5 s
Calculate the power of the motor:
Power (P) = ΔE / t = 120 J / 5 s = 24 W
Answer: The power of the motor is 24 W.
3.
A student lifts a 5kg weight vertically by 1m in 2 seconds. Calculate the power output of the student. Show your working.
Given:
- Mass (m) = 5 kg
- Height (h) = 1 m
- Time (t) = 2 s
Calculate the potential energy gained by the weight:
Potential Energy (PE) = mgh = 5 kg * 9.8 m/s² * 1 m = 49 J
Calculate the work done by the student:
Work done (W) = Force (F) * Distance (d)
The force required to lift the weight is equal to the weight of the object: F = mg = 5 kg * 9.8 m/s² = 49 N
Work done (W) = 49 N * 1 m = 49 J
Calculate the power output of the student:
Power (P) = Work done (W) / Time (t) = 49 J / 2 s = 24.5 W
Answer: The power output of the student is 24.5 W.