Simple Machines

Spring 2008 2

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

Doing work

Load

Effort

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)

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

RampsA ramp is a simple machine. A smaller force over a larger

distance to achieve the same amount of work done (height raised)

Larger Force Small ForceShort Distance Long Distance

Same amount of work done

in lifting the object

6

Ramp ExampleHow 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 NewtonsThe work done is 1962 x 2 (metres)

= 3924 Joules

2 m2 m

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 = 392. 4Newtons

Which is 5 times less the lifting the block up vertically

2 m2 m

Spring 2008 8

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 9

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) = 431.6 J

• Pushing a crate 15 m across a floor with a force of 400 N requires 6,000 J (6 kJ) of work

Simple Machines

•Force multipliers• 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

Simple Machines

Load

Effort

In the case of a lever the effort force moves further than the load so the less effort can

lift a larger load

Simple Machines

Driver pulley20 RPM

Driven pulley10 RPM

Diameter 200mm Diameter 400mm

Simple Machines

Load Effort

A wheel and axle assembly used as a hoist the effort

force again moves further than the

load force400mm

100mm

Spring 2008 14

Work is Exchange of Energy

There are two main categories of energy

Useful energy and

Potential energy (stored)

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)

Useful energy

•There are 4 types of useful energy•Sound •Light •Heat

• Movement (kinetic)

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

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

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

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)

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.

Coefficient of friction•

•F = μ x wt•25Kg = 245.25 newtons

•F =0.45 x 245.25•=110.37newtons

Power, defined

•Power is the rate of using energy in joules per second

•1 watt = 1j/s