Question 7
An object is pushed along a horizontal surface at a constant velocity by an applied force, as shown.
Which arrow best shows the direction of the frictional force acting on the object?
That is it. Step through the hints below to see why. In 2025, 65.8% of Queensland students chose (A).
Not quite. (B) copies the 30° of the push and points partly into the floor. Friction acts along the contact surface, so on a horizontal floor it is horizontal. In 2025, 6.1% of Queensland students chose (B).
Not quite. (C) points to the right, the way the object is moving. Friction opposes the sliding, so it points against the motion. In 2025, 2.8% of Queensland students chose (C).
Not quite. (D) is the applied force turned around. Friction does not oppose the push as a whole. It opposes the sliding, along the surface, so it is horizontal. In 2025, 25.0% of Queensland students chose (D).
Arrows to scale for an illustrative 10 kg object with μ = 0.8, chosen so the push matches QCAA’s drawing at 30°. The question itself gives no sizes.
The floor pushes on the object in two ways. The part straight out of the surface is the normal force. The part along the surface is friction.
So friction lies along the surface. The floor is horizontal, so friction is horizontal. That rules out (B) and (D).
The object moves to the right at constant velocity, so it is sliding over the floor. Kinetic friction opposes the sliding.
Friction points to the left. That rules out (C).
The 30° changes how big friction is, never its direction. Split the push into its two parts.
$$\begin{aligned}F_N&=mg+F_A\sin30^\circ\\F_f&=F_A\cos30^\circ\end{aligned}$$
The downward part presses the object into the floor, which raises $F_N$ and so $F_f=\mu_k F_N$. Constant velocity means the horizontal part of the push exactly balances friction.
Friction acts along the contact surface and against the sliding. The surface is horizontal and the object slides to the right, so friction is horizontal and points to the left.
The angle of the push only changes the size of friction, through the normal force.
The most common wrong answer was (D), the push reversed. One in four students treated friction as the force that opposes the applied force. It opposes the sliding, and it always lies along the surface.
Only mark this done when you could do it without help. Reading the solution does not count.
Question wording and any figures 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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