1For stationary objects, to change the shape
More than one force must be applied
2How can a stationary obiect's shape change
Stretching - forces in opposite directions away from the object
Bending - forces that distort the object
Compressing - forces in opposite directions towards the object
3A change of shape can be
Elastic or inelastic
4Elastic distortion occurs when
Obiects return to their original shape when the stretching force is removed
5Examples of elastic distortion
Rubber band
Fabric
Steel spring
6Inelastic distortion occurs
When objects remain stretched and do not return completely to their original shape even when the stretching force is removed
7Examples of inelastic distortion
Plastic
Clay
Glass
8Hooke's law states that
The extension of an elastic object is directly proportional to the force applied, up to the limit of proportionality
9The limit of proportionality is where
If more force is added, the object may extend but will not return to its original shape when the force is removed
10Hooke's law equation
F = kx
Force = spring constant(N/m) * extension (m)
11Beyond the limit of proportionality
There is a non linear relationship between force and extension
12Linear is where
The graph is straight but does not necessarily pass through the origin
13When a spring is stretched or compressed by a force ... is done ....
Work is done by the spring( ie energy is transferred to the EPE store of the spring)
14Within the limit of proportionality, work done and elastic potential energy...
Are equal
15Work done when compresssing or extending a spring equation
E = 1/2 * k * x^2
16Core practical on force and extension variables
Independant - force
Dependant - extension
Control - spring constant k
17Equipment list for core practical on Hooke's law ( view notes for diagram)
Clamp and stand (hold spring and masses)
Ruler (measure extension)
Spring (measure extension of this)
5 * 100g masses ( to apply a force to the spring)
100g mass hanger ( to hold additional masses)
Fiducial marker - to accurately read extension from ruler
18Method for core practical on Hooke's law
1. Set up apparatus (view diagram in notes)
2. Align marker to a value on the ruler, recording this initial spring length
3. Add the 100g mass hanger onto the spring and record the position now that the spring has extended
4. Add another. 100g and record the new mass and position
5. Repeat until all masses are added
6. Remove masses and repeat three times to calculate an average length
19Weight is calculated as
W = mg