Know that forces may produce changes in the size and shape of an object

Cambridge IGCSE Physics 0625 – Effects of Forces

Objective

Understand that forces can cause changes in the size and shape of an object.

Key Concepts

• Force: A push or pull that can change the motion or shape of an object.
• Deformation: The alteration of an object's shape or size when a force is applied.
• Elastic deformation: Temporary change; the object returns to its original shape when the force is removed.
• Plastic deformation: Permanent change; the object does not return to its original shape after the force is removed.

Types of Deformation Produced by Forces

Quantifying Deformation

The amount of deformation is described by stress and strain:

\$\$

\text{Stress} \; (\sigma) = \frac{F}{A}

\$\$

where \$F\$ is the applied force and \$A\$ is the cross‑sectional area.

\$\$

\text{Strain} \; (\varepsilon) = \frac{\Delta L}{L_0}

\$\$

where \$\Delta L\$ is the change in length and \$L_0\$ is the original length.

Elastic vs. Plastic Behaviour

1. When the applied stress is below the material’s elastic limit, deformation is elastic.
2. Beyond the elastic limit, the material yields and deformation becomes plastic.
3. Materials have a characteristic Young’s modulus \$E\$, defined by \$E = \sigma / \varepsilon\$ in the elastic region.

Practical Examples

• Compressing a sponge – the sponge returns to its original shape (elastic).
• Stretching a piece of soft clay – it retains the new shape (plastic).
• Applying a torque to a metal rod – it may twist elastically at low torque, but will permanently deform if the torque exceeds the yield point.

Important Points for Examination

• Identify the type of force and the resulting deformation.
• Distinguish between elastic and plastic deformation.
• Use the stress‑strain relationship to calculate deformation when required.
• Remember that not all forces change motion; some primarily change shape or size.