Describe solid friction as the force between two surfaces that may impede motion and produce heating

1.5.1 Effects of Forces – Solid Friction

Learning Objective

Describe solid friction as the force between two surfaces that may impede motion and produce heating.

What is Solid Friction?

Solid friction is the resistive force that arises when two solid surfaces are in contact. It acts opposite to the direction of relative motion (or the tendency for motion) and can:

• Prevent an object from starting to move (static friction).
• Oppose the motion of an object that is already sliding (kinetic friction).
• Convert mechanical energy into thermal energy, causing heating of the surfaces.

Types of Solid Friction

1. Static friction (\$f_s\$) – acts when the surfaces are at rest relative to each other. It increases up to a maximum value to prevent motion.
2. Kinetic (sliding) friction (\$f_k\$) – acts when the surfaces are sliding past each other. Its magnitude is usually less than the maximum static friction.

Mathematical Description

The magnitude of frictional force is proportional to the normal reaction \$R\$ between the surfaces:

\$f = \mu R\$

where:

• \$f\$ = frictional force (N)
• \$\mu\$ = coefficient of friction (dimensionless)
• \$R\$ = normal reaction (N)

Two different coefficients are used:

• \$\mu_s\$ – coefficient of static friction
• \$\mu_k\$ – coefficient of kinetic friction

Factors Influencing Friction

• Nature of the surfaces: Rougher surfaces generally have larger \$\mu\$ values.
• Normal reaction (\$R\$): Greater \$R\$ increases the frictional force.
• Area of contact: For solid friction, the area of contact has little effect on \$f\$ (assuming the surfaces are rigid).
• Speed of sliding: Kinetic friction may change slightly with speed, but for IGCSE it is treated as constant.

Typical Coefficients of Friction

Heating Effect of Friction

When friction does work, mechanical energy is transformed into internal energy, raising the temperature of the contacting surfaces.

The work done by friction over a distance \$d\$ is:

\$W_f = f \, d\$

Assuming all this work becomes heat \$Q\$, the temperature rise \$\Delta T\$ of a body of mass \$m\$ and specific heat capacity \$c\$ is:

\$Q = mc\Delta T \quad \Rightarrow \quad \Delta T = \frac{f d}{mc}\$

This explains why brakes become hot when a car slows down – the brake pads and discs experience kinetic friction.

Practical Examples

• Sliding a book across a table – kinetic friction opposes motion.
• Pushing a heavy box – static friction must be overcome before it starts moving.
• Car tyres on a road – static friction provides the grip needed for acceleration and turning.
• Braking a bicycle – kinetic friction between brake pads and rim converts kinetic energy into heat.

Key Points to Remember

1. Friction always acts opposite to the direction of relative motion or the tendency for motion.
2. Static friction adjusts up to a maximum value \$f{s,\text{max}} = \mus R\$; kinetic friction has a constant value \$fk = \muk R\$.
3. The heating effect is a direct consequence of the work done by friction.
4. For most solid surfaces, the area of contact does not significantly affect the magnitude of friction.