Section 1 of 6Three ways to change shape
Stretching
- Stretching Two forces pull in opposite directions, away from the object.
- Examplea mass hung on the end of a spring, with weight pulling down and tension in the spring pulling up.
Compressing
- Compressing Two forces push in opposite directions, towards the object.
- Examplea mass resting on a spring, with the weight pushing down and the reaction force from the surface pushing up.
Bending
- Bending Forces act at different points on the object and in opposing directions.
- Examplea diving board, with the swimmer's weight at the free end and a reaction force at the fixed support.
Section 2 of 6Why one force is not enough
1Single force
- Single force One force on a free object makes it accelerate or move rather than change shape.
2Object held in place
- Object held in place If the object is stationary, a second force must balance the first.
3Pair of forces
- Pair of forces The two forces squeeze, stretch or bend the object.
4Shape changes
- Shape changes The object changes shape until the internal forces balance the applied forces.
Section 3 of 6Elastic and inelastic deformation
| Feature | Elastic deformation | Inelastic deformation |
|---|---|---|
| After the force is removed | Returns to original shape and size | Stays permanently changed |
| Examples of materials | Steel spring, rubber band, fabric | Plastic, clay, glass-like materials |
| Called | Elastic | Inelastic (or plastic) |
| Energy | Stored as elastic potential energy while stretched | Mostly transferred to other stores as the shape changes |
Section 4 of 6Checking the type of change
1
Apply a deforming force to the object, for example by hanging a load on it.
2
Measure the new length or record how its shape has changed.
3
Remove the force completely.
4
Measure the length or shape again.
5
If the original length is recovered, the deformation was elastic; if not, it was inelastic.
Section 5 of 6Key terms
| Term | Meaning |
|---|---|
| Deformation | A change in the shape or size of an object caused by forces. |
| Elastic deformation | A change of shape that is reversed when the deforming force is removed. |
| Inelastic deformation | A change of shape that is not fully reversed when the force is removed. |
| Compression | Squashing an object with forces directed towards each other. |
| Tension | A stretching force along a spring, rope or wire. |
Section 6 of 6Exam technique
1
Saying 'elastic' means stretchy; it means the object returns to its original shape afterwards.
2
Writing that a single force can stretch or compress a stationary object; at least two forces are needed.
3
Calling a diving board bent by two forces in line with each other; bending needs forces acting at different points.
4
Forgetting to name both forces, for example weight and the reaction force or tension.