Energy exercises 
Energy Units and Conversions
In the
International System, energy is measured in joules
Joule
(J) is the unit of energy, equal to the force of one Newton acting
through one meter.
1J
= 1N*1m
There is another energy unit, the calorie
1
calorie of heat is the amount needed to raise 1 gram of water 1
degree Centigrade.
1 calorie
(cal) = 4.184 J
1Ί Calculate how many calories
are:
a)
2005 joules
b)
3566 joules
Calculate
how many joules are
A)
34545 calories
B)
5664 calories
2Ί Find the
work (energy) done when a horse exert a horizontal force of 400 N on
a cart and pulls it 20 m in the direction of the force
Power is the rate at which work is done.
Power
= work done / time taken.
The
measured of power
is watt ( W)
So
if time is measured in seconds and work in Jules:
1W
= 1J/1s
3Ί A car engine provides a forward
force of 1000 N. If the car is travelling at 20 m s1, what power is
developed?
Remember
that:
work
= Force multiplied by distance or
W = F*D
4Ί A man raises a object of mass
40 kg through a vertical height of 20 m in 4 seconds. Find the
power. Note: The potential energy is measured by the following
equation:
Pe
= m x g x h
Where:
M
= mass , g = 9,8 , h = height
Solution:
1960 W
5Ί A 60 kg women run up a flight of 120
stairs in 30 seconds. If the height of each stair is 15 cm , find the
power
6Ί ) A 100 watt electric heater is
used to heat water and bring it to the boil. If 5000 j are needed to
bring the water to the boil, for how long is the heater operating?
7Ί ) How much energy is given out
by a 60W bulb if it is running 5 hours?
8Ί I'm making hot chocolate for a few friends, so I put one liter of milk in a teakettle. I only want hot milk, not boiling water, so I want to raise the temperature of the milk from 25 degrees C to 85 degrees C. a) How much heat energy will the stove have to add to the milk to reach this goal? b) How much it cost if we pay 20 cents per kwh (One liter is 1000 cm^3, or about four cups, and weighs one kg.) We need a calorie per gram to raise a degree Celsius 9Ί A meteorite was travelling
about 30 km/sec when it hit Earth 65 million ago. Its size is
estimated to be a sphere about 10 km across ( v= 4/3 * Pi * r^3).
The meteorite was probably made of iron, which has a density of 7.9
g/cm^3 How much kinetic energy did it have when it hit Earth?
