Question 13
An object on an inclined plane is subjected only to its own weight. Describe static friction and kinetic friction in terms of the angle of repose. Draw two force diagrams to support your response.
Static friction holds the object until the slope passes the angle of repose, then kinetic friction takes over
The plank tilts slowly. Compare the orange friction arrow with the dashed pull down the slope, and watch what happens when the plank passes the dashed angle of repose line. Arrows are drawn to scale for surfaces with a 25° angle of repose.
At or below the angle of repose, static friction matches the pull down the slope and the object stays still. Above it, the pull beats the most static friction can give, the object slides, and the smaller kinetic friction acts up the slope against the motion.
Build both diagrams and the description, and see which marks they earn
Choose a diagram, then add and label its forces. Below, pick one sentence for each description mark. The checklist marks your response against the 2025 marking guide as you go.
Work through the solution one mark at a time
Try each step yourself before you reveal it. Two marks are for the diagrams and four are for the description.
Static friction diagram. Draw the slope with the object at rest. Show the weight straight down, the normal force perpendicular out of the slope, and static friction up the slope. Label every arrow and give each one an arrowhead.
Static friction points up the slope because it opposes the slide that is about to start. Writing $\theta\le$ angle of repose on the diagram, as the QCAA sample does, links it to your description.
QCAA’s notes for both diagrams:
Allow the diagram to include, normal force, frictional force and weight without components labelled or normal, frictional force and both weight components labelled.
Accept force diagram to support the description, in ‘incorrect’ box if it is drawn correctly and clearly labelled.
Marker: provides a correctly labelled force diagram to support the description of static friction
Kinetic friction diagram. Draw a steeper slope with the object sliding down. The forces are the same three, but friction is now kinetic friction, and it is smaller than the pull down the slope.
QCAA’s sample labels only $mg\sin\theta$ as a component, while its own note allows no components or both. Draw both components dashed, or leave them off, and you meet the note either way.
Marker: provides a correctly labelled force diagram to support the description of kinetic friction
Say what static friction is. QCAA’s sample wording:
The coefficient of static friction $\mu_s$ is the amount of friction between two surfaces that are not moving in relation to each other.
Strictly, $\mu_s$ is a ratio, $F_f\div F_N$, so it is safer to describe the force itself: static friction acts between two surfaces that are not moving relative to each other, and it can grow up to a maximum of $\mu_s F_N$.
Marker: correctly describes static friction
Now tie static friction to the angle of repose. QCAA’s sample wording:
When an object on an inclined plane is not moving, the angle of the incline θ must be less than or equal to the angle of repose, the angle at which an object on an inclined plane is just about to slide.
The comparison, $\theta\le$ angle of repose, is the mark. At the angle of repose static friction is at its maximum and $\mu_s=\tan\theta$.
Marker: correctly describes static friction in terms of the angle of repose
Say what kinetic friction is. QCAA’s sample wording:
The coefficient of kinetic friction $\mu_k$ is the amount of friction between two surfaces when there is movement between them.
As in Step 3, describe the force: kinetic friction acts while the surfaces slide over each other, $F_f=\mu_k F_N$, and it is usually smaller than the maximum static friction.
Marker: correctly describes kinetic friction
Tie kinetic friction to the angle of repose. QCAA’s sample wording:
For an object to start to move or slide down the inclined plane, the angle of the incline θ must be greater than the angle of repose.
Put the four sentences together and the description is complete:
Static friction acts between two surfaces that are not moving relative to each other. It matches the pull of the weight down the slope, up to a maximum of $\mu_s F_N$, so while the angle of the incline is less than or equal to the angle of repose the object stays at rest. Kinetic friction acts while the surfaces slide over each other, with $F_f=\mu_k F_N$, and it is smaller than the maximum static friction. When the angle of the incline is greater than the angle of repose, $mg\sin\theta$ is bigger than the maximum static friction, so the object slides down the slope and kinetic friction acts up the slope, against the motion.
Marker: correctly describes kinetic friction in terms of the angle of repose
Each phrase of the question gave you something
The 2025 subject report says the effective responses gave a correctly labelled force diagram for each kind of friction, and used both the angle of repose and the angle of the incline in each description. Its advice to teachers for 2026 asks for exactly this skill: explain the difference between static and kinetic friction, and label force vectors clearly, with arrows.
QCAA did not publish a band for this question. It reads as simple familiar because it is bookwork: two standard force diagrams and two definitions, each tied to one comparison between the incline angle and the angle of repose. The 2025 syllabus trimmed the definitions of the coefficient of friction, the normal force and the angle of repose, but you still have to distinguish static from kinetic friction and use $\mu_s=\tan\theta$, so this question shape is live for 2026. It sits at the top of my 2026 watchlist.
If I had to guess where you might lose marks, it would be the diagrams. The subject report asked for clearly labelled force vectors with arrows, and the guide gives a separate mark for each diagram, so one diagram labelled for both cases puts a mark at risk. In the writing, the trap is defining the angle of repose without comparing the incline angle with it. “In terms of the angle of repose” is worth two marks on its own, one for each kind of friction.
Only mark this done when you could do it without help. Reading the solution does not count.
Question wording, any figures and marking-guide steps are from the 2025 QCAA Engineering external assessment, © State of Queensland (QCAA) 2025, licensed under CC BY 4.0, and have been adapted, except where a third-party source is named. Tangent Tuition is not affiliated with the QCAA.
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