In this article
Take a sharp turn in a car and you feel pushed toward the door. On a spinning playground ride, you have to hold on or be flung off. It feels like a force is pulling you outward. Yet many physics teachers say centrifugal force is “not real”.
The honest answer is that it depends on where you are watching from. Once you see the two points of view, the confusion disappears.
Key facts
- Centripetal force pulls an object toward the centre of a circle. It is a real push or pull, such as friction, tension or gravity.
- Centrifugal force is the outward push you seem to feel when you are the one going round. It appears only from a spinning point of view.
- Let go and you fly off in a straight line, along the edge of the circle, not straight outward.
- Earth’s spin makes you weigh about 0.3 percent less at the equator than you would if Earth did not rotate.
View 1: watching from the roadside
Imagine standing on the pavement, watching a car take a curve. Objects naturally keep moving in a straight line unless something pushes them, which is Newton’s first law. The car turns because friction between its tyres and the road pulls it toward the centre of the curve.
That inward pull is the centripetal force. The word means “centre-seeking”. Without it, the car would carry straight on. The force needed is larger for heavier objects, faster speeds and tighter turns.
| Car | Speed | Curve radius | Inward force needed |
|---|---|---|---|
| 1,200 kg | 15 m/s (54 km/h) | 50 m | about 5,400 newtons |
| 1,200 kg | 30 m/s (108 km/h) | 50 m | about 21,600 newtons |
From the roadside, your body in the passenger seat is just trying to keep going straight. The car turns underneath you, and the door swings in to meet your shoulder. Nothing is pushing you outward. The door is pushing you inward.
View 2: sitting inside the car
Now look from the passenger seat. From where you sit, the car is not moving; you are at rest inside it. Yet you are pressed against the door. To make sense of this from inside, it is natural to describe an outward force acting on you. That is the centrifugal force.
Physicists call it a fictitious or inertial force. It is not caused by any object pushing on you. It appears because your point of view, called a reference frame, is itself turning. The OpenStax physics textbook describes it as a force that merely seems to arise because the observer is rotating, and notes that what people call centrifugal force is really their own inertia. Inside a spinning frame, using it still gives the right answers, which is why engineers often do.
The test that settles it: let go
Swing a ball on a string in a circle, then let go. If a real outward force were acting on it, the ball would fly straight out from the centre. It does not. It flies off in a straight line along the edge of the circle, in the direction it was moving at that instant. That is exactly what you expect if the only real force was the inward pull of the string.
Where centrifugal effects show up
- Washing machines. In the spin cycle the drum pushes the clothes inward, but water can escape through the holes, so it carries straight on and leaves the clothes.
- Playground rides. On a merry-go-round, your hands must pull you inward. The faster it spins, the harder you hold on.
- Earth itself. Earth spins once a day. At the equator, this reduces your apparent weight by roughly 0.3 percent. Using the radius and rotation in NASA’s Earth fact sheet, the effect works out to about 0.034 m/s².
- Space stations of the future. A ring-shaped station 100 metres in radius would need to spin about three times a minute to give people Earth-like weight on its outer floor.
So, is it real?
Both answers are useful. From the outside, there is no outward force: only an inward one, keeping things on a curved path. From the inside of something spinning, the outward centrifugal force is a convenient and accurate way to describe what you feel. The physics is the same; only the point of view changes.
Frequently asked questions
What is the difference between centripetal and centrifugal force?
Centripetal force is the real inward pull that keeps something moving in a circle. Centrifugal force is the outward push that seems to act when you are the one rotating.
Why do I slide outward in a turning car?
Your body tends to keep moving in a straight line. The car turns underneath you, so you slide toward the outside until the door or seat belt pushes you inward.
Would you weigh less if Earth spun faster?
Yes. At the equator you would feel lighter. If Earth spun about 17 times faster, objects at the equator would feel weightless.
Related: why guns recoil and rockets work in space, or browse our Physics section.
