1. Simple Machines

2. Definition of work Work carries a specific meaning in physics
4/23/2008
Definition of work
Work carries a specific meaning in physics
Simple form: work = force  distance
W = F x d
Work is a measure of energy used
You get tired doing work
Spring 2008 2
Lecture 9 2

3. Doing work
Work done = Force x distance moved (in direction of the force)
In this case the effort move the same distance as the load
The effort force is equal to the load force (weight of load)
Load
Effort

4. Making doing work easier
Apply less force over larger distance for same work
Machines were invented to make work easy (ramps, levers, etc. are simple machines)

5. 4/23/2008
Ramps
A ramp is a simple machine. A smaller force over a larger distance to achieve the same amount of work done (height raised)
Same amount of work done in lifting the object
Larger Force Small Force
Short Distance Long Distance 5
Lecture 9 5

6. 4/23/2008
Ramp Example
How much work is needed to lift a 200 Kg mass vertically by 2 metres?
The force needed is 200 x 9.81 (gravitational field strength)
1962 Newtons
The work done is 1962 x 2 (metres)
= 3924 Joules
2 m 6
Lecture 9 6

7. Ramp Example
The work done pushing the block up a 10 metre ramp is also 3924 Joules (ignoring friction)
The force needed is therefore 3924 ÷ 10 = Newtons
Which is 5 times less the lifting the block up vertically
2 m

8. 4/23/2008
Work Examples
How much work does it take to lift a 22 kg suitcase onto the table, 2meter high?
(g =9.8 m/s2)
How much work is done in pushing a crate 15 m across a floor with a force of 400N?
Spring 2008 8
Lecture 9 8

9. Work Examples - Answers
4/23/2008
Work Examples - Answers
How much work does it take to lift a 22 kg suitcase onto the table, 2 meter high?
W = (22 kg)  (9.81 m/s2)  (2 m) = J
Pushing a crate 15 m across a floor with a force of 400 N requires 6,000 J (6 kJ) of work
Spring 2008 9
Lecture 9 9

10. Force multipliers Simple Machines
Work done = Force x distance moved (in direction of the force)
Work done by effort = work done on load
F x d (effort)= F x d (Load)
The further the distance moved by the effort force compared to the distance moved by the load means that a smaller effort force can move a larger load force

11. Simple Machines Effort Load
In the case of a lever the effort force moves further than the load so the less effort can lift a larger load

12. Simple Machines Diameter 400mm Diameter 200mm Driven pulley
Driver pulley
20 RPM
Driven pulley
10 RPM

13. Simple Machines
A wheel and axle assembly used as a hoist the effort force again moves further than the load force
Load
Effort
100mm
400mm

14. Work is Exchange of Energy
4/23/2008
Work is Exchange of Energy
There are two main categories of energy Useful energy and Potential energy (stored)
Spring 2008 14
Lecture 9 14

15. Stored energy (Potential Energy)
Gravitational potential energy (due to height)
Mechanical potential energy (like in compressed spring)
Chemical potential energy (stored in bonds)
Nuclear potential energy (in nuclear bonds)

16. There are 4 types of useful energy
Sound
Light
Heat
Movement (kinetic)

17. Heat energy (specific heat capacity)
Heat is not the same as temperature
Specific heat capacity is defined as how much heat is required to raise the temperature of 1 Kg of a material through 1o C
For example , it takes 4200 joules of energy to raise 1 Kg of water by 1 degree.
The specific heat capacity of water
Is 4,200 J/KgCo

18. Linear coefficient of expansion
Heating a material such as a metal cause it to expand
If a 10,000 meter steel railroad track with a coefficient of linear expansion of 12 x 10-6 per degree Celsius changes temperature from 18°C to 38°C. it will expand by 2.4 metres

19. Linear coefficient of expansion
The coefficient of linear expansion (α) defines how much a material will increase in length when heated
α= ∆L/Lo x ∆T
∆L = change in length
Lo = original length
∆T = change in temperature
∆L = α x Lo x ∆T

20. Coefficient of friction
This allows us to calculate the force needed to move one material over another
μ = Force needed to move the object ÷ the weight of the object (in Newtons)

21. Coefficient of friction
F = μ x wt
) Example
Find the force required to move a 25 kg crate South across the floor (µ = 0.45) at a uniform speed.

22. Coefficient of friction
F = μ x wt
25Kg = newtons
F =0.45 x
=110.37newtons

23. Power is the rate of using energy in joules per second
Power, defined
Power is the rate of using energy in joules per second
1 watt = 1j/s