QCE Physics - Unit 3 - Gravity and motion
Inclined Planes and Circular Motion | QCE Physics
Learn inclined plane force diagrams and uniform circular motion for QCE Physics, including normal force, components, period, centripetal acceleration and centripetal force.
Updated 2026-06-15 - 4 min read
QCAA official coverage - Physics 2025 v1.3
Exact syllabus points covered
- Solve problems involving force due to gravity (weight) and mass using $F_g = mg$.
- Describe the concept of normal force.
- Describe the forces acting on an object on an inclined plane, e.g. force due to gravity, normal force, tension, frictional force and applied force, through the use of free-body diagrams.
- Determine the net force acting on an object on an inclined plane using vector analysis.
- Describe the concept of uniform circular motion.
- Describe the concepts of average speed and period.
- Solve problems involving objects undergoing uniform circular motion at a constant speed using $v = \frac{2\pi r}{T}$ and $a_c = \frac{v^2}{r}$.
- Describe the concepts of centripetal acceleration and centripetal force.
- Solve problems involving forces acting on objects in uniform circular motion using $F_c = F_{net} = \frac{mv^2}{r}$.
Inclined planes and circular motion look different, but the same principle controls both: draw the forces carefully, choose useful directions and find the net force in the direction that matters.
Original Sylligence diagram for physics inclined circular force map.
Weight, normal force and ramps
Weight is the force due to gravity:
$ F_g = mg $
It acts vertically downward, no matter how the surface is tilted. The normal force is the contact force from a surface acting perpendicular to that surface. On a flat horizontal surface with no other vertical forces, $F_N$ often equals $mg$. On an inclined plane, $F_N$ is usually smaller than $mg$ because only part of the weight acts perpendicular to the plane.
For a ramp at angle $\theta$, it is useful to split weight into components parallel and perpendicular to the plane:
$ F_{g\parallel} = mg\sin\theta $
$ F_{g\perp} = mg\cos\theta $
If there is no acceleration perpendicular to the plane, the normal force balances the perpendicular component:
$ F_N = mg\cos\theta $
The parallel component tends to pull the object down the slope. Friction, tension or an applied force may oppose or assist that motion depending on the context.
Free-body diagrams on inclined planes
A useful ramp diagram includes only real forces, not components as extra forces. Draw:
- weight $F_g$ vertically downward
- normal force $F_N$ perpendicular to the surface
- friction $F_f$ along the surface opposing relative motion or likely motion
- tension $T$ along the string if a string is attached
- applied force in the direction it is applied
Then resolve forces into axes parallel and perpendicular to the ramp. For motion along the ramp:
$ F_{net,\parallel} = ma $
For example, if a block slides down a frictionless slope:
$ F_{net,\parallel} = mg\sin\theta $
so:
$ a = g\sin\theta $
Uniform circular motion
Uniform circular motion means the object moves around a circle at constant speed. The speed is constant, but the velocity is not constant because the direction keeps changing. Any change in velocity means acceleration.
The average speed around one complete circle is circumference divided by period:
$ v = \frac{2\pi r}{T} $
where $r$ is radius and $T$ is the period, or time for one revolution.
Centripetal acceleration points toward the centre:
$ a_c = \frac{v^2}{r} $
The net force toward the centre is:
$ F_c = F_{net} = \frac{mv^2}{r} $
Centripetal force is not a new type of force. It is the name for the inward net force causing circular motion. The real force could be tension, gravity, friction, normal force or a combination.
What supplies the centripetal force?
Always ask: what actual force points toward the centre?
For a ball on a string, tension supplies the centripetal force. For a satellite, gravity supplies the centripetal force. For a car turning on a flat road, friction between tyre and road supplies the centripetal force. If the inward force is too small, the object does not maintain that circular path.
This is why the direction matters. A force tangent to the circle changes the speed. A force toward the centre changes the direction. Uniform circular motion requires inward acceleration without changing speed.
Worked example
Linking ramps and circles
In both sections, vector analysis is the real skill. On a ramp, you resolve weight into components parallel and perpendicular to the surface. In circular motion, you resolve forces into radial and tangential directions. The equation $F_{net}=ma$ still applies, but the relevant direction changes.
For QCE data questions, identify whether the measured variable changes speed, period, radius or force. The strongest relationship is often the square relationship in $F_c=mv^2/r$: doubling speed requires four times the inward net force if mass and radius stay fixed.