Mechanics
Circular motion
The ball orbits at a constant radius: the rod's tension is the centripetal force (F = m·v²/r), always pointing toward the centre. Drop a Timer on the ball to read its orbital period.
Circular motion — interactive Mechanics simulation. The ball orbits at a constant radius: the rod's tension is the centripetal force (F = m·v²/r), always pointing toward the centre. Drop a Timer on the ball to read its orbital period.
Uniform circular motion
An object in a circle at steady speed still accelerates toward the center because velocity direction changes continuously.
Centripetal acceleration points inward even when speed is constant. For a ball on a rod, the rod supplies the centripetal force that bends the path into a circle.
- a_c = v²/r
- F_c = mv²/r
Investigation brief
Plan the question before you open the lab
The brief mirrors the prerendered page: driving question, competing predictions, variable roles, governing laws, setup, analysis and extension prompts remain visible and in this order.
Driving question
A ball on a rod orbits a fixed central pivot at constant speed. If you increase the ball's orbital speed while keeping the radius the same, does the time for one full lap (the period) get longer, shorter, or stay the same?
Predictions to weigh
- The period gets longer (it takes more time per lap).
- The period stays the same (speed doesn't affect the period).
- The period gets shorter (it takes less time per lap).
Variable roles
What you set:
- Mass m (kg)
- Radius r (m)
- Orbital speed v (m/s)
What you measure:
- Centripetal force F = mv²/r (N)
- Period T (1 lap) (s)
How the investigation runs
- Open the Circular Motion preset. Note the rod's length (the orbit radius) and the ball's mass, and confirm gravity is off — the rod alone supplies the force that curves the path.
- Confirm the speed probe is attached to the ball, and drop a Timer on the ball so you can time one full revolution.
- For each trial below, set the ball's mass, the rod length (radius), and its initial tangential speed, then press Play and time one complete lap.
Governing equation
Centripetal Force — F = m·v² / r
The inward force the rod must supply to keep the ball moving in a circle instead of a straight line. It depends on the ball's mass, its (constant) orbital speed, and the radius of the circle — this is the tension you'd feel in the rod at each trial's settings.
What the printable worksheet asks students to work out
- For one trial, show your work computing the period from the radius and speed you set (T = 2πr/v) and compare it to the Timer's reading in the sim.
- Across your three trials, how does increasing the speed at a fixed radius change the period? How does increasing the radius at a fixed speed change the period? Explain both using T = 2πr/v.
Where this shows up beyond the lab
- This preset turns gravity off so the motion is genuinely uniform. If gravity were switched back on and the rod still traced a horizontal circle, would the ball's speed stay constant through the whole lap? Why or why not?
- A real orbiting object (like a ball on a string swung by hand) loses a little energy to air resistance and to the pivot each lap. Would you expect its measured period to stay perfectly constant over many laps, or to drift — and in which direction?
- AP Physics 1 — Unit 6: Energy and Momentum of Rotating Systems
- AP Physics C: Mechanics — Unit 2: Force and Translational Dynamics
- IB Physics — A.4 Rigid body mechanics
- General High School Physics — Rotation, gravitation & orbits
- NGSS High School Physics — Gravitational and electrostatic forces
- Welcome to Circular Motion
- Select the ball
- Press Play
- Always turning
- Open the data
- You did it!
Open the interactive simulation to build the scene, press Play, and explore with live measurements and a guided tutorial.