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Unit summary

Revise!

1. To what height must a 400 g book be lifted to 7. An 8 kW crane engine lifts a 2-tonne load to a
increase its potential energy by 2 J? height of 10 m. Determine the time needed to lift
the load if the efficiency of the crane is 80%.
textbooks nis edu kz
2. An airplane has 400 MJ of kinetic energy at
take-off, and its mass is 20 tonnes. Determine the 8. Students constructed a launching device and
speed of the plane at take-off. launched a 200 g stone vertically upwards at an
initial speed of 10 m/s. (Disregard air resistance to
3. A 10 kg stone falling from height has 625 J of
answer these questions.)
kinetic energy at the moment it strikes the
ground. What is the stone’s speed just before it а) What is the kinetic energy of the stone?
hits the ground?
b) What is the potential energy of the stone at
4. The movement of energy in an electric power maximum height?
plant is shown in the following diagram:
c) To what maximum height will the stone rise?
‘Waste’ heat energy d) What is the speed of the stone at half
this height?
70%
e) What is the mechanical energy of the stone?
Primary fuel
(coal) Electrical
energy 9. We tire more easily when we walk on loose
100% 30% sand or through snow than when we walk on a
hard road. Explain this phenomenon in terms of
energy and work.

а) Determine the efficiency of the plant.

b) We derive 72 MJ of energy from coal
combustion. How much energy will a
consumer get in electric energy?
5. A 20 dm copper ball is suspended on a thread at
3
a height of 3 m. Determine its potential energy if
the density of copper is 8960 kg/m .
3
6. A crane lifts a 100 kg load applying 3 kN force.

а) What is the potential energy of the load at a
height of 10 m?
b) What work is done by the crane’s engine?

c) What is the kinetic energy of the load at
that height?

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