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 s-1, what power is developed?
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
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?