Power, Efficiency

Jonathan is rushing to attend a lecture he is late for. He runs with a stack of books at a speed of 5 m/s. Jonathan exerts a force of 10 N to carry the books.

He runs for 50 m across the canteen to the lecture hall.

What is the power dissipated by Jonathan on the stack of books?

• 0 W

• 50 W

• 100 W

• 500 W

Bob pushes a 10 kg box with 50 N of force across 12 m in 1 minute.

What is the power dissipated by Bob on the box?

• 0 W

• 10 W

• 20 W

• 600 W

The diagram shows a motor dragging a box up a smooth slope.

The motor takes 10 minutes to drag up the 54 kg box.

What is the power dissipated by the motor?

• 4.5 W

• 10.8 W

• 11.7 W

• 270 W

The diagram shows a motor dragging a 60 kg box up a rough slope.

There is a constant frictional force of 100 N acting on the box as it travels up the slope.

Find the efficiency of the motor.

• 43.3 %

• 69.8 %

• 71.4 %

• 85.7 %

A LED is able to generate 2400 J of light energy in 1 minute.

Assuming it has an efficiency of 40%, what is the power rating of the LED?

• 16 W

• 40 W

• 100 W

• 6000 W