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QLDPhysicsQuick questions
Unit 3: Gravity and electromagnetism
Quick questions on Uniform circular motion, centripetal force and banked curves (QCE Physics Unit 3)
12short Q&A pairs drawn directly from our worked dot-point answer. For full context and worked exam questions, read the parent dot-point page.
What is uniform circular motion in one paragraph?Show answer
An object moving at constant speed $v$ around a circle of radius $r$ has changing velocity (because direction is changing). The acceleration is directed toward the centre of the circle (centripetal):
What is period and frequency?Show answer
For one complete revolution the object travels a distance $2 \pi r$ at speed $v$, so the period is:
What is banked curves?Show answer
On a curve banked at angle $\theta$ to the horizontal, the normal force is perpendicular to the road surface. Resolving:
What is conical pendulum?Show answer
A mass swung in a horizontal circle on a string at angle $\theta$ from the vertical has tension components:
What is worked example?Show answer
A 900 kg car rounds a flat bend of radius 30 m at 12 m/s. The required centripetal force is $F_c = 900 \times 12^2 / 30 = 4320$ N, supplied entirely by static friction.
What is iA1 data test?Show answer
Expect a banked-curve diagram with the bank angle and a speed, asked to determine whether the vehicle stays on the curve, to find the design speed, or to extract the friction coefficient required at a non-design speed. Alternatively, a conical pendulum stimulus with multiple period measurements at different angles.
What is iA2 student experiment?Show answer
Common designs: a conical pendulum with mass, period and angle measured to test $T = 2 \pi \sqrt{L \cos\theta / g}$; a rotating-mass setup on a horizontal turntable with friction varied; a ball-in-a-cone or marble-in-a-bowl rolling at different heights. Strong reports linearise the relationship before fitting (e.g. $T^2$ vs $\cos\theta$ slopes give $4\pi^2 L / g$) and propagate uncertainty cleanly.
What is treating centripetal force as a new force?Show answer
It is the inward-pointing role played by an existing real force. Always state which real force (tension, friction, gravity, normal-component) supplies it.
What is pointing $F_c$ tangentially or outward?Show answer
Centripetal force is always directed toward the centre of the circle. The "centrifugal" force students sometimes invoke is a pseudo-force that appears only in the rotating frame; not used in QCAA solutions.
What is mixing up $v$, $\omega$ and $f$?Show answer
$v = \omega r$ and $\omega = 2 \pi f$. Substitute consistently.
What is banking with mass dependence?Show answer
The design angle $\tan\theta = v^2 / (r g)$ does not depend on $m$. If $m$ appears in your final answer for the bank angle, you have made an error.
What is treating uniform circular motion as zero acceleration?Show answer
Constant speed does not mean constant velocity. The acceleration is non-zero because the direction is changing.