Chapter 17 - Work and Power

Question 1

State the equation linking gravitational potential energy, mass and height

Answer 1

gpe = height x mass x gravity

Question 2

A man of mass 78kg steps onto an escalator.

The escalator lifts him a height of 5.0m

Show that the GPE gained by the man is about 4000J

Answer 2

78 x 10 x 5 = 3900J

Question 3

State the equation linking efficency, useful energy output and total energy input (1)

Answer 3

Efficency = useful energy output

total energy input

Question 4

Escalator lifts 30 people each minute

Each person has a mass of 78kg

Input is 7,5kw

Output is 3900J

What is the efficency (3)

Answer 4

In one second =

• useful energy out = (30 x 3900) / 60

efficency = 1950/7500

0.26/26%

Question 5

Electrical energy = 100J

3/4 goes to Light Energy

The rest goes to heat energy

Draw a sankey diagram to represent this information

Answer 5

Question 6

State the equation linking power, voltage and current [1 mark]

Answer 6

power = voltage x current

Question 7

State one advantage of transmitting electricity at high voltages [1 mark]

Answer 7

Higher voltages = lower current

= less power losses

Question 8

State the equation linking kinetic energy, mass and speed (1)

Answer 8

KE = 1/2 x mass x speed²

Question 9

A car has a mass of 900kg, with max speed of 15 m/s, calculate the maximum KE of the car (2)

Answer 9

KE = 1/2 x mass x speed²

KE = 1/2 x 900 x 15²

KE = 101250 J