Everything Science Grade 12

8.4 CHAPTER 8. WORK, ENERGY AND POWER

W

F

Step 3 : Determine how to approach the problem

From the force diagram,we see that the weight of the box is acting

at right angles to the direction of motion. Theweight does not

contribute to the work done and does not contribute to the power

calculation. We can therefore calculate power from: P = F· v.

Step 4 : Calculate the power required

P = F· v

= (10 N)(1 m· s−^1 )

= 10 W

Step 5 : Write the final answer

10 W of power are required for a force of 10 Nto move a 10 kg

box at a speed of 1 ms over a frictionless surface.

Machines are designedand built to do work onobjects. All machines usually have a
power rating. The power rating indicates the rate at which that machinecan do work
upon other objects.

A car engine is an example of a machine whichis given a power rating.The power
rating relates to how rapidly the car can accelerate. Suppose that a 50 kWengine could
accelerate the car from 0 km· hr−^1 to 60km· hr−^1 in 16 s. Then a car withfour times
the power rating (i.e. 200 kW) could do the same amount of work in a quarter of the
time. That is, a 200 kWengine could acceleratethe same car from 0 km· hr−^1 to
60 km· hr−^1 in 4 s.

Example 8: Power Calculation 2

QUESTION

A forklift lifts a crate ofmass 100 kg at a constant velocity to a height of 8 m

over a time of 4 s. The forklift then holds the crate in place for 20 s. Calculate

how much power the forklift exerts in lifting thecrate? How much power does