1. Energy
Mechanical

2. Facts There are two types of mechanical energy
Potential energy and kinetic energy.
Potential energy is the energy a body has due to its position
PE = mgh
Kinetic energy is the amount of energy a particle possesses because of its motion
KE = ½mv2
Both are measured in joules J

3. How to answer the questions
Draw a clear forces diagram
Label zero potential energy
Think what is happening to the energies

4. And now for the examples
A mass of 100kg accelerates from 2ms-1 to 4ms-1. Find it’s gain in kinetic energy
Draw the diagram a
4ms-1
2ms-1
100kg before
100kg after
KEafter = ½mv²
= ½ x 100 x 4²
= 800 J
KEbefore = ½mv²
= ½ x 100 x 2²
= 200 J
KEgain = KE after – KE before
= 800 – 200
KEgain = 600J

5. Summarise the information to see what you’ve got
An apple of mass 0.1 kg falls from a height of 2m. Taking g = 10ms-2 , find the loss of potential energy
Draw a diagram
0.1g 2m
Zero PE level
Summarise the information to see what you’ve got
PE = ? m = 0.1 kg g = 10 ms h = 2m
PE = mgh
PE = 0.1 x 10 x 2
PE = 2 J

6. Summarise the information to see what you’ve got
A roller coaster train, of mass 8 tonnes, is winched up a slope inclined at 20º to the horizontal. The slope is 80 m long. Determine the increase in potential energy of the roller coaster train from the bottom of the slope.
80 m h
8000g
20º
Zero PE level
8000g
Summarise the information to see what you’ve got
PE = ? m = 8000kg g = 9.8 ms h = ?
We need to find h before we can find PE
Using trigonometry we have
h = 80 x sin 20º
h = m

7. h PE = ? m = 8000 kg g = 9.8 ms-1 h = 27.36 m PE = mgh
20º
Zero PE level
8000g
PE = ? m = 8000 kg g = 9.8 ms h = m
PE = mgh
PE = 8000 x 9.8 x 27.36
PE =
PE = J (3 s.f.)
PE = 2150 kJ (3 s.f.)

8. Your turn
Blue book pg 254, 255
Mechanics 2 pg 78
question 1, 3, 6, 7, 8, 9, 10