Power 1 - Intro

POWER is defined as the rate at which work is done. In other words, POWER is the amount of work done per unit time.

Equation:

• The S.I. unit for power is watts (W)
• When we speak of a 60W lightbulb, we mean that the lightbulb give out 60 joules of energy every one second
• The unit of power, watts, can also be called joules per second: J.s-1

Question 1
Study this diagram which shows a force F = 10N being applied to a box, and the box is thus moved 8m in 2s.

1.1. Calculate the WORK done by this force on the box.

1.2.Calculate the POWER involved.

Question 2
The applied force of 20N is brought about by an electric motor. The friction force is 5N because the ground is rough. The box is moved a distance of 12m in 10s.

2.1.Calculate the total work done by the motor.

2.2.Calculate the total power of the motor.

2.3.Calculate the effective work done by the motor.

2.4. Calculate the motor's effective power.

2.5. How much of the motor's power was lost in overcoming friction?
Ploss = 24 - 18 = 6W

Question 3
A 40W motor is used to move a trolley a distance of 20m rightwards in 5s. Ignore friction.
(obviously a force is involved)

3.1. Calculate the work done by the motor.

3.2. Calculate the magnitude of the force used.