Simple Machines. Spring 2008 2 Definition of work Work carries a specific meaning in physics Simple...
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Transcript of Simple Machines. Spring 2008 2 Definition of work Work carries a specific meaning in physics Simple...
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