Conical Pendulums: Bringing Centripetal Force to Life

If you’ve ever been to a fairground, you’ll know the dizzy thrill of the chair-o-planes. Riders swing outwards as the carousel spins, each seat tracing out a neat circle in the air. What you’re really watching is a perfect example of a conical pendulum — and a brilliant way to understand centripetal force in action.

In physics lessons, we don’t usually have a giant chair-o-plane handy, so we use a simpler setup: a rubber bung on a string. By swinging it round the head, the bung travels in a horizontal circle while the string makes an angle to the vertical. The forces are easy to model:

• The weight of the bung acts downwards.

• The tension in the string pulls at an angle.

• The centripetal force that keeps the bung moving in a circle comes from the horizontal component of that tension.

With a bit of care (and enough courage to avoid letting go of the bung!), we can measure the angle of the string, the length of the string, and the time taken for each rotation. From these, students can calculate the centripetal force and see how it depends on speed, mass, and radius.

To make it even clearer, we’ve filmed the motion in slow motion, capturing the bung’s path and the steady angle of the string. The measurements match beautifully with the predictions, showing how theory and experiment line up.

Centripetal Motion – The Hoop and Ball Experiment

One of the simplest ways to explore circular motion is with a ball and a hoop. Place the ball inside the hoop and roll it around the inside edge. As long as the hoop is flat on the table, the ball follows a circular path. But lift the hoop away and the ball immediately shoots off in a straight line, tangential to the circle.

This shows two key ideas:

1. Centripetal force is required to keep an object moving in a circle. In this case, the force comes from the hoop pushing the ball inwards.

2. When that inward force is removed, the ball doesn’t “fly out” — it simply continues in a straight line, exactly as Newton’s First Law predicts.

Many students mistakenly believe in a so-called “centrifugal force” pushing the ball outwards. In reality, there is no such force acting on the ball. What we see is the ball’s natural tendency to move in a straight line, which only appears like an outward force when we are inside a rotating system.

How to Try It

• Take a lightweight hoop (like foam tubing joined into a circle) and a small ball.

• Gently set the ball rolling inside the hoop so it circles smoothly.

• While it’s moving, quickly lift the hoop.

• Observe how the ball immediately leaves the circular path and continues straight ahead.

What Students Learn

• Circular motion always requires an inward (centripetal) force.

• Without that inward force, the object’s velocity is unchanged — it just stops turning.

• The idea of “centrifugal force” is a misinterpretation of inertia, not a real force.

This is a brilliant low-cost classroom demo that makes Newton’s laws and the concept of centripetal force both visual and memorable. 

At Hemel Private Tuition we love using experiments like this to make physics come alive. A fairground ride may seem like just fun, but with the right approach, it’s also an unforgettable lesson in the laws of motion. We use simple experiments like this to help students connect the theory with what they can actually see happening.