A car rounds an unbanked highway curve at a steady speed. Nothing is tied to the car. No wall touches it. Yet the car's path bends around the curve, so a net force must be acting on it, toward the curve's center. Which force is it?
Look at what touches the car: only the road, at the tires. A road exerts two forces on a tire: the normal force, straight up from the surface, and friction, along the surface. The normal force points up, so it cannot point at the center. That leaves friction.
On the flat road, friction from the road acts on the tires, along the road surface, toward the center of the turn.
That inward friction is the net inward force the circle requires. For a vehicle turning on a flat road, friction supplies the centripetal force.
Now the same car meets the same curve coated with ice. Almost no friction acts between the tires and the ice, so no force points toward the center. With no net inward force, the car cannot hold its circular path.
Back on dry pavement, the friction returns and the car can round the curve again.
A runner leans through the flat curve of a track. Friction between her shoes and the track points toward the center of the curve and supplies her centripetal force.
The pattern: when a vehicle or a runner turns on a flat surface, friction between the surface and the tires, shoes, or load is the force that points toward the center.
A delivery van rounds a flat corner at a steady speed. Which force supplies the van's centripetal force?
A pickup truck circles a flat roundabout at a steady speed. Which force supplies the pickup's centripetal force?
Only the road touches the pickup, at the tires.
The road's normal force points straight up, so it cannot point at the center.
Friction from the road on the tires acts along the surface, toward the center of the roundabout.
Friction between the tires and the road supplies the pickup's centripetal force.
A phone rests on a car's dashboard as the car rounds a corner, and the phone circles with the car. Which force supplies the phone's centripetal force?
Only the dashboard touches the phone.
The dashboard's normal force points straight up, so it cannot point at the center.
Friction between the phone and the dashboard acts along the surface, toward the center of the turn.
Friction between the phone and the dashboard supplies the phone's centripetal force.
A coin rests on a spinning turntable and circles with it. Odile says: 'Friction between the coin and the turntable supplies the coin's centripetal force.' Is she correct?
A city bus rounds a flat corner at a steady speed. The friction from the road on the bus's tires points in which direction?
In which case does friction supply the centripetal force?
A crate rides on the flat bed of a truck as the truck rounds a corner, and the crate circles with the truck. Which force supplies the crate's centripetal force?
A taxi enters a flat curve that is coated with ice and cannot hold its circular path. Why not?