A steel ball circles a magnet standing at the center of a smooth table. No cord is tied to the ball. No wall presses on it. The table is too slick for friction to matter, and gravity points down, not inward. None of the four suppliers you have met is doing the job. Yet the ball circles.
What holds it in?
The magnet does. The magnetic pull on the steel ball points from the ball toward the magnet, at the circle's center. That force is called the magnetic force, and here it is filling a role you have only seen other forces fill.
Work through what acts on the object. A taut cord running to the center means tension. A flat surface under a turning vehicle means friction. An orbit means the central body's gravitational pull. A wall or track curving around the center means its normal force.
And when the scenario is new, the same question still decides it. For the steel ball, the magnetic pull points at the center, so the magnetic force supplies the centripetal force.
Only a real force can hold the job. And the job has exactly one test: the force must point at the center.
One habit catches most slips: name the object exerting the force. The cord, the road, Earth, the wall, the magnet. If you cannot name the exerting object, you have not found the supplier.
A jogger rounds the flat curve of a park path at a steady speed. Which force supplies the jogger's centripetal force?
What acts on the jogger? Her weight, down; the path's normal force, up; and friction from the path on her shoes.
Weight and the normal force are vertical, so neither points at the curve's center.
Friction acts along the surface and can point toward the center. It is the only candidate left.
Friction between her shoes and the path supplies the jogger's centripetal force.
A steel ball circles a magnet at the center of a smooth table. Which force supplies the ball's centripetal force?
What acts on the ball? Its weight, down; the table's normal force, up; and the magnet's pull on the steel.
Weight and the normal force are vertical, so neither points at the center.
The magnetic pull points from the ball toward the magnet, at the circle's center.
The magnetic force supplies the ball's centripetal force.
A camper whirls a water bottle in a horizontal circle on a cord. Which force supplies the bottle's centripetal force?
A minivan rounds a flat traffic circle at a steady speed. Which force supplies the minivan's centripetal force?
A laundry bag circles with the spinning drum of a dryer, pressed against the drum's wall. Which force supplies the bag's centripetal force?
A dwarf planet circles the Sun on a near-circular orbit. Which force supplies the dwarf planet's centripetal force?
A paperback book lies on the back seat as a car rounds a corner, circling with the car. Which force supplies the book's centripetal force?
A steel washer slides in a circle around a strong magnet standing at the center of a smooth glass table. Which force supplies the washer's centripetal force?
A racehorse rounds the flat turn of a dirt track at a steady speed. Which force supplies the horse's centripetal force?
In which scenario does gravity supply the centripetal force?
Odile lists the forces on a mail truck rounding a flat corner at a steady speed: its weight, the road's normal force, and friction from the road. Which listed force supplies the truck's centripetal force?