Power Physics Example 2

Follow the full solution, then compare it with the other examples linked below.

Example 2

medium
A 70 kg70 \text{ kg} person runs up a 5 m5 \text{ m} staircase in 4 s4 \text{ s}. What is the power output? Use g=9.8 m/s2g = 9.8 \text{ m/s}^2.

Solution

  1. 1
    First find the work done against gravity: W=mgh=70×9.8×5=3430 JW = mgh = 70 \times 9.8 \times 5 = 3430 \text{ J}.
  2. 2
    Use the power formula: P=WtP = \frac{W}{t}.
  3. 3
    P=34304=857.5 WP = \frac{3430}{4} = 857.5 \text{ W}

Answer

P≈857.5 WP \approx 857.5 \text{ W}
Power output increases with both the work done and the speed at which it is done. Running up stairs faster requires more power even though the total work is the same.

About Power

The rate at which work is done or energy is transferred, measured in watts (joules per second).

Learn more about Power →

More Power Examples

Example 1 easy

A motor does [formula] of work in [formula]. What is the power output?

Example 3 easy

A [formula] light bulb is on for [formula]. How much energy does it use?

Example 4 medium

Two cranes each lift a [formula] load [formula]. Crane A takes [formula] and Crane B takes [formula]

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