simple machines. spring 2008 2 definition of work work carries a specific meaning in physics simple...
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Simple Machines
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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
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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)
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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)
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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
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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
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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
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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?
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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
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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
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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
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Simple Machines
Driver pulley20 RPM
Driven pulley10 RPM
Diameter 200mm Diameter 400mm
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Simple Machines
Load Effort
A wheel and axle assembly used as a hoist the effort
force again moves further than the
load force400mm
100mm
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Spring 2008 14
Work is Exchange of Energy
There are two main categories of energy
Useful energy and
Potential energy (stored)
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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)
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Useful energy
•There are 4 types of useful energy•Sound •Light •Heat
• Movement (kinetic)
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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
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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
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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
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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)
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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.
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Coefficient of friction•
•F = μ x wt•25Kg = 245.25 newtons
•F =0.45 x 245.25•=110.37newtons
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Power, defined
•Power is the rate of using energy in joules per second
•1 watt = 1j/s