1. Work
Yes you out there !

2. Key ideas
When a body moves a distance along the line of force work is done
Work done = force X distance moved
W = Fs
The amount of work done is measured in joules (J)

3. A brick, initially at rest, is raised by a force of 40N to a height 5m above the ground where it is left stationary. What is the work done?
Work done = force x distance moved
= 40 x 5
Work done = 200 J
A sledge, initially at rest, is pulled across smooth ice by a horizontal string in which there is a constant tension 40N. What is the work done after the sledge has travelled 18 m?
Work done = force x distance moved
= 40 x 18
Work done = 720 J

4. How to answer the questions
Draw a clear forces diagram
Resolve the forces in the direction of motion
Summarise any information
Apply W = Fs

5. Draw your forces diagram
Example 1
A mass of 2.5 kg is pulled along a smooth horizontal surface by a 25N force which acts horizontally. The work done by this force is 125 J. Determine the distance covered by the mass.
Draw your forces diagram R ?
25 N
2.5g
It should have looked like this !

6. A mass of 2.5 kg is pulled along a smooth horizontal surface by a 25N force which acts horizontally. The work done by this force is 125 J. Determine the distance covered by the mass.
Now summarise the information
W = 125 J F = 25 N s = ?
Apply W = Fs
W = Fs
125 = 25 x s
125 = 25s
S = 5 m
Got it ? See if you remember how to do the next one !

7. Summarise the information
A baby of mass 5 kg is lifted vertically 1.2 m from the ground by her mother. Find the work done against gravity.
Draw a forces diagram
baby
1.2 m
5 kg
baby
Summarise the information
W = ? m = 5 kg h = 1.2 m
Apply W = mgh
W = mgh
W = 5 x 9.8 x 1.2
W = 58.8 J

8. The vertical distance the mass is moved The work done against gravity
A mass of 10kg is pulled up a smooth inclined plane for a distance of 10m. The plane is inclined at sin to the horizontal. Calculate
The vertical distance the mass is moved
The work done against gravity
Draw a forces diagram
10 m h θ
10 m R F
10g θ

9. Use trigonometry to find vertical height h
h = 10sinθ
h = 10 x
h = 6 m
10 m h θ b.
Summarise the information
W = ? m = 10 kg h = 6 m g = 9.8 ms-2
Apply W=mgh
W = mgh
W = 10 x 9.8 x 6
W = 588 J

10. Other ways of looking at work done
Work done = change in KE + change in PE + work done against resistances

11. Try some yourself
Blue book pg 247, 248, enjoy
Mechanics 2 pg 78 exercise 4A
question 2, 4, 5, 10