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A.2.7—Frictional force

Syllabus
First assessment 2025
Objective
—
Level
SL

Model Static and Dynamic Friction

Friction follows the contact

Friction acts parallel to the contact surface and opposes relative motion or the tendency of surfaces to move relative to each other.

Static friction adapts

Before slipping, static friction has whatever value is needed up to a maximum:

Ff≤μsNF_f\leq\mu_sN

It is not automatically equal to μsN\mu_sN; that value occurs at impending motion.

Dynamic friction during sliding

Once surfaces slide, the model gives

Ff=μdNF_f=\mu_dN

Use the normal force for the actual contact and combine friction with the other forces along the surface.

Common trap

Do not use the dynamic coefficient before motion begins, or assume static friction is always at its maximum.

A.2.7 Exam Analysis

Assessment in practice

2–4 marks
How it is assessed

The evidence asks for the minimum force needed to start a box or move a stacked-block system, so the key decision is the static-friction threshold and the correct normal force.

Command terms

Show / Calculate / Determine

What earns marks

Decide whether the object is just about to move or is already sliding. At impending motion use the maximum static friction \(\mu_sN\); during sliding use \(\mu_dN\). Resolve the applied force and calculate the normal force for the actual contact.

Watch for

Using μd for a minimum-starting-force question or using the total weight as the normal force without checking which surfaces are in contact.

Representative question

Question 1

[Maximum number: 2]

Show that the minimum force needed to accelerate the box is about 4 N .