Work Done and Power

Question 1

Give the word equation used to calculate work done on an object moved by a force.

Answer 1

Work done = force x distance

Question 2

One newton-metre is equal to how many joules?

Answer 2

1 newton-metre = 1 joule

Question 3

A resultant force of 80N moves an orange 250cm. What is the work done in joules?

Answer 3

200 J

E = F x d and 250cm = 2.5m, so the work done is 80 x 2.5 = 200 Nm = 200 J

Question 4

A toaster takes 200s to toast a slice of bread. In that time, the toaster transfers 180000J of energy. What is the power of the toaster?

Answer 4

900W

P = E / t = 180000 / 200 = 900W

Question 5

How much work is done by a 150kW powerboat operating at full power for 10s

Answer 5

1.5MJ
P = E / t, so E = P / t.
P = 150kW = 150000 W
E = 150000 x 10 = 1500000 J or 1.5MJ

Question 6

A 2kW electric kettle transfers 330 kJ to the thermal energy store of the heating element to bring the water to boil. How long does it take for the water to boil?

Answer 6

165s
P = E / t, so t= E/P
P = 2kW = 2000W
E = 330kJ = 330000J
t = 330000 / 2000 = 165s

Question 7

Derek is entering a 100m lawnmower race. One of his lawnmowers has an engine power of 9000W, and his other lawnmower has a power of 7000W. The two lawnmowers are otherwise identical. Which lawnmower should he choose for the race? Why?

Answer 7

The 9000W lawnmower would be the better choice for the race as it has a more powerful engine. This means it can do the amount of work needed to reach the end of the course in a shorter time