Paper 1 Q3 — Which way does friction point?

Statics on an incline — friction direction and drawing forces to scale

What the examiners said

“The most common mistake was to draw the friction up the slope… the arrows were not drawn to appropriate lengths… Some normal force arrows were not perpendicular to the slope. Candidates found this challenging. Some realised that friction must be acting down the slope.”

Try the question (reconstruction)

A block is pushed by an external force up an incline and is on the verge of sliding up. In which direction does the friction force on the block act?

Examiner report: “The common mistake drew friction up the slope; here it must act down the slope.”

Explore — free-body diagram on the incline

Choose whether the block is about to slide up or down. Watch the friction arrow flip to oppose the impending motion, and see the normal force stay perpendicular to the slope.

Impending motion: up the slope Slope angle (°): 30

Friction is reactive: it points opposite to the way the block tends to move. Flip the impending direction and the arrow flips with it.

Original reconstruction inspired by the misconceptions described in the Singapore-Cambridge GCE A-Level H3 Physics 9814 November 2023 Examiner Report. Not the actual examination question. For classroom exploration.

Made by lookang, using Claude Fable 5. More resources: iwant2study.org