Centripetal force and centrifugal force

In short: For something to move in a circle, a force must constantly pull it towards the centre: the centripetal force. The centrifugal force you feel on a roundabout, for example, is the inertia of your own body, which would rather carry on in a straight line.

In more detail: According to the law of inertia, a body with no force acting on it keeps moving in a straight line. On a curve its direction has to be changed all the time. The force needed for this grows with the mass and with the square of the speed, and it is larger the tighter the curve.

In detail

• Where the force comes from: For a car on a bend, the friction of the tyres provides the centripetal force; for a ball swung on a string, the tension in the string; for the Moon, the Earth’s gravity.
• Centrifugal force as an apparent force: If you are riding along, you feel pushed outwards. Seen from outside, your body simply wants to keep going straight while the vehicle beneath it turns.
• Double the speed: Driving through the same bend twice as fast needs four times the force. That is why taking bends too fast is so dangerous, especially in the wet or on ice.
• When the force is missing: If the string snaps or the friction is not enough, the body moves on tangentially, straight ahead in its direction of travel, not directly outwards.
• Spin cycle: The drum of a washing machine keeps the laundry on its circular path, but the water can escape straight through the holes. That is how the laundry is spun dry.
• Centrifuge: In laboratories, centrifuges separate substances of different density, such as blood components, because heavier particles move outwards more strongly.
• Banked curves: Race tracks, velodromes and railway tracks are tilted on bends. Part of the weight force then helps to supply the force towards the centre.
• Satellites: A satellite is constantly falling towards the Earth but is so fast that it keeps falling past it. Here gravity acts as the centripetal force.
• Connections: In everyday life, gravity and friction often provide the necessary force (schwerkraft-und-freier-fall, reibung-im-alltag, satelliten-und-iss).