Why cars skid? Uncover the invisible grip between tyres and the road.

Multiple Choice Question

Question

A wet surface primarily lowers the

1
normal force on tyres
2
coefficient of friction
3
mass of the car
4
air resistance

Hint:

Think about how slippery ice feels.

Why slippage?

Friction

Friction is a surface force that resists motion because countless microscopic bumps on the two surfaces lock together.

A thin film of water acts like tiny ball bearings, loosening the lock-up and lowering friction, so wet floors feel slippery.

Source: Engineering Tribology texts

Static friction

Term

Static friction

Definition

Contact force that keeps a body at rest by adjusting to oppose impending motion, along the surface and opposite the applied push.

Maximum value

\(f_{s,\text{max}} = \mu_s N\)

Beyond this limit the object begins to move.

  • Self-adjusting
  • Opposes impending motion
  • Along surface
Block on table with arrows indicating applied force and static friction

Static vs Kinetic

Static Friction

Self-adjusts up to a limiting value.
Maximum \(F_{\text{max}} = \mu_s N\).
Usually larger than kinetic friction.

Kinetic Friction

Acts once surfaces slide.
Nearly constant: \(F_k = \mu_k N\).
\(\mu_k\) is smaller than \(\mu_s\).

Key Similarities

Both oppose relative motion between surfaces.
Direction is along contact and opposite to slide.

Key equations

\[f_{s,\text{max}}=\mu_s N \qquad f_k=\mu_k N\]

Both forces are proportional to the normal reaction \(N\).

Variable Definitions

\(f_{s,\text{max}}\) maximum static friction
\(f_k\) kinetic friction
\(\mu_s\) coefficient of static friction
\(\mu_k\) coefficient of kinetic friction
\(N\) normal reaction force

Applications

Predict slip onset

Compare applied force with \(f_{s,\text{max}}\) to decide if motion starts.

Calculate sliding force

Use \(f_k\) to find the net force once the object is moving.

Source: NCERT Section 5.4

Force vs Friction Graph

Interpret how friction responds as applied force increases.

Graph with rising line then step down to flat.
Friction first rises with force (\(F_s\)), then drops to a flat kinetic level (\(F_k\)).

Key observation

Notice the sudden drop at motion onset.

  • Static region: \(F_f = F_{applied}\) up to \(F_s^{\max}= \mu_s N\).
  • Corner point marks the instant the block starts to move.
  • Kinetic region: friction holds nearly constant at \(F_k = \mu_k N\).

Tip: Because \(F_s^{\max} > F_k\), the graph always drops from \(F_s\) to \(F_k\) when sliding begins.

Worked Example

Calculate friction on a 5 kg block pushed 15 N on a horizontal surface.

  1. 1

    List data

    \(m = 5\text{ kg},\; \mu_s = 0.4,\; \mu_k = 0.3,\; F_{\text{app}} = 15\text{ N}\).

  2. 2

    Find normal \(N\)

    Horizontal plane ⇒ \(N = mg = 5 \times 9.8 = 49\text{ N}\).

  3. 3

    Static limit

    \(f_{s,\max} = \mu_s N = 0.4 \times 49 = 19.6\text{ N}\).

  4. 4

    Compare & decide

    15 N < 19.6 N ⇒ block remains at rest; \(f_s = 15\text{ N}\) (use \(f_s\), not \(f_k\)).

Common pitfalls

Main Points

  1. 1 Friction resists relative motion, not the applied push.
  2. 2 Changing contact area rarely changes the coefficient \( \mu \).
  3. 3 Usually \( \mu_k < \mu_s \); force drops once sliding starts.

Key Highlights

  • Set friction arrow along intended slide, not along applied force.
  • Grip depends on normal force, so footprint size cancels out.
  • Expect a step down in required force once motion begins.

Multiple Choice Question

Question

Transfer check: A 2 kg puck slides on ice (μk = 0.05). Take g = 9.8 m/s². Select the kinetic friction force acting on the puck.

1
0.49 N
2
0.98 N
3
1.96 N
4
4.9 N

Hint:

Use \(f = \mu_k N\); here \(N\) equals the weight.

Key takeaways

Four friction facts to remember.

1

Static vs kinetic

Static blocks start; kinetic opposes sliding.

2

Proportional to \(N\)

More normal force → more friction.

3

Material, not area

\( \mu \) is set by surface pair, area irrelevant.

4

Angle of repose

Measure \( \mu_s \) using \( \tan\theta = \mu_s \).

Multiple Choice Question

Question

Which change increases the static friction limit?

1
Reduce surface roughness
2
Decrease normal force
3
Use stickier materials
4
Increase contact area

Hint:

Think \( \mu_s \).