Work and Energy
Joule - The force of 1 Newton being applied over 1 metre
Energy - The ability to do work or cause change
Energy - The ability to do work or cause change
Energy
All energy is the same as it is measured in joules it just acts in different ways
Energy can't be destroyed or created it is conserved or transferred
Efficiency
The percentage of useful energy produced
useful energy x 100
input energy
useful energy x 100
input energy
Work Done
(Work done J or Nm) W = Fd (Force N x Distance m)
Work will only have been done if a change in distance has occurred. If 20 J were applied on either side of a box applied in opposite directions the box won't move and so no work will have been done even though energy will have been applied
Power
The rate of energy transfer in a unit time. Measured in Watts
Power (W or J/s) P = W Work Done (J)
t Time (s)
t Time (s)
Elasticity and Plasticity
Elastic behaviour / deformation is when the material returns to it's original shape
Plastic behaviour / deformation is when the material is permanently distorted
This is returned to further down the page
Plastic behaviour / deformation is when the material is permanently distorted
This is returned to further down the page
Hooke's Law
The extension of a spring is proportional to the load applied, provided that the elastic limit is not exceeded.
(Force N) F = kx (spring constant N/m x extension m)
(Force N) F = kx (spring constant N/m x extension m)
Elastic Potential Energy
The work done is the area under the graph of tension versus extension.
The elastic potential energy is the energy stored which the material can withstand when under tension or compression.
This means the potential energy is:
(Energy J) E = 1/2 kx^2 (spring constant N/m x extension squared m^2)
or
(Energy J) E = 1/2 Fx (force applied x extension)
The elastic potential energy is the energy stored which the material can withstand when under tension or compression.
This means the potential energy is:
(Energy J) E = 1/2 kx^2 (spring constant N/m x extension squared m^2)
or
(Energy J) E = 1/2 Fx (force applied x extension)
Young's Modulus
Stress (Pa) = Tension (force) Stress - Force per unit cross-sectional area
cross-sectional area
Strain = extension Strain - extension per unit length
length
Young's Modulus is the ratio of stress to strain
Y = Tension / Area = Tension x length or stress
Extension / length Extension x Area strain
cross-sectional area
Strain = extension Strain - extension per unit length
length
Young's Modulus is the ratio of stress to strain
Y = Tension / Area = Tension x length or stress
Extension / length Extension x Area strain
Young's Modulus on Different Materials
Ductile Material
Ductile materials are both elastic and plastic.
Up to a point of stress they will return to their original form but past this point it becomes plastic.
The top of the graph shows the ultimate tensile stress of a material.
After this point even if the stress is reduced the material will continue extending until it breaks
Plastic Bags or Copper are a good example of this
Up to a point of stress they will return to their original form but past this point it becomes plastic.
The top of the graph shows the ultimate tensile stress of a material.
After this point even if the stress is reduced the material will continue extending until it breaks
Plastic Bags or Copper are a good example of this
Brittle Material
Brittle Materials are plastic
Brittle materials have very little elastic potential energy
A high amount of strain needs to be applied to distort the material even slightly
Once distorted very slightly the ultimate tensile stress is where the material breaks or fractures
Concrete is a good example
Brittle materials have very little elastic potential energy
A high amount of strain needs to be applied to distort the material even slightly
Once distorted very slightly the ultimate tensile stress is where the material breaks or fractures
Concrete is a good example
Polymeric Materials
Polymeric Materials are Elastic
They are easy to stretch up to a point where they become harder to stretch
They have lots of elastic potential and break suddenly
Rubber is a good example of this
They are easy to stretch up to a point where they become harder to stretch
They have lots of elastic potential and break suddenly
Rubber is a good example of this
Rubber Band Test
When testing with a rubber band the extension will be more when the elastic band is unloaded than when it's loaded