Circular motion: the whirling bung sim
Whirl a rubber bung on a string: set its speed, radius and mass and read the pull on a newton-meter, let go and watch it leave along the tangent, then hang slotted masses on the string, whirl until the paper clip holds steady, time ten turns and plot F against v².
Forces and motion · covers the IGCSE, GCSE topic
What the sim shows
Model: the force towards the centre is F = mv² ÷ r, with g = 10 N/kg. Explore: the bung goes round a level circle of radius r at speed v; the string also holds up the bung's weight, so it slopes down a little and the newton-meter (0 to 20 N, read to 0.1 N) reads the whole pull, √(F² + (mg)²). Practical: the hanging masses (hanger 50 g plus 1 to 5 slotted masses of 50 g) pull with F = Mg; r is the string's length from the top of the tube to the bung, so the slope cancels exactly (Mg = mω²r). The clip looks steady within ±3 % of Mg (chosen). Each start or stop of the stopwatch is late by a reaction time of 0.15 to 0.30 s (seeded, so it repeats on every load); the watch reads to 0.01 s. Idealised: no air resistance, a smooth tube held still (in class it is held in the hand, and small circles of the hand keep the bung going), a string with no mass that does not stretch, the bung as a point, no zero error on the newton-meter, a perfectly steady whirl. Not to scale: the circle and the flight after letting go are drawn at a third of real size; the let-go is replayed at a twentieth of real speed.
- The rubber bung: the part to watch
- Glass tube in a clamp, string, newton-meter or hanger with slotted masses, paper clip, stopwatch
Use it in your lessons
- Try it here: anyone can unlock 2 figures free, and a free account unlocks up to 5. Runs on a laptop, a Chromebook or a phone; nothing to install.
- Present it and share student links with the teacher plan, from US$5 a month, billed yearly.
- Put it on your own website or LMS for US$9, paid once, or every sim from US$29 a month.
Free guide: 12 practicals as sims, and how to run one in a lesson →
Need a version for your exact set-up? Have one made: US$49 a figure in an order of 5.
More physics sims
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- Simple pendulum: timing, and finding g simSet the length, time 20 oscillations, fill the table and find g from the gradient of T² against l.
- Sound: vibrations, a medium and the oscilloscope simDrag the microphone and turn the oscilloscope's time-base and Y-gain to see louder = taller and higher pitch = more waves; read n waves over d divisions, work out T and f = 1 ÷ T and have each step checked; watch the air's compressions and rarefactions; pump the air out of a bell jar and graph the fading sound; and race a pulse through steel, water and air.
- Speed of sound by echo simClap in front of a wall, get in time with the echoes and time a run of them with a stopwatch, work out v = 2d ÷ t and see why one echo is too short to time by hand; then time a pulse between two microphones in air, water and steel with v = d ÷ t and compare the three speeds on one graph.
- Stability and toppling simTilt a hinged board with a model bus, a bottle of sand or a wooden cone on it and watch the weight line from G leave the base as it topples; then measure the toppling angle of six wooden blocks on the protractor and plot tan θ against (b ÷ 2) ÷ h.
- States of matter: the particle model simDrag a heater slider to take 50 g of ice to water to steam and back while a magnified box shows its particles go from a vibrating lattice, to sliding past each other, to flying apart and hitting the walls; record the heating curve with its flat parts, then squash sealed syringes of air, water and wax.