An escalator motor has a total (input) power of 900 W. The motor does useful work raising passengers, delivering a useful output power of 630 W; the rest is dissipated as heat in the motor and its gears. What is the efficiency of the escalator motor?

A Level Physics 9702: Efficiency of an Escalator Motor (MCQ)

Step 1: Recall the definition of efficiency

Efficiency is the ratio of useful power output to total power input, usually expressed as a percentage: $\text{efficiency} = \frac{P_{\text{useful}}}{P_{\text{input}}}\times100\%$

Step 2: Substitute the values

$\text{efficiency} = \frac{630}{900}\times100\%$

Dividing first: $630/900=0.7$ . Multiplying by $100$ : $0.7\times100=70\%$ .

Check by recomputing differently: $630/900$ simplifies to $7/10$ (dividing both by $90$ ), which is exactly $0.7$ , confirming $70\%$ .

Why the other options are wrong

A ( $30\%$ ): this is the fraction of power that is wasted, $(900-630)/900\times100\%=270/900\times100\%=30\%$ , not the fraction usefully transferred.
B ( $0.7\%$ ): this comes from correctly finding the decimal fraction $0.7$ but forgetting to multiply by $100$ to convert it to a percentage.
D ( $143\%$ ): this comes from inverting the ratio, $900/630\times100\%\approx142.9\%$ , which is impossible since no real machine can output more power than it is supplied.

Final answer

The efficiency of the escalator motor is $\boxed{70}\%$ , option C.