On a rotor ride at a fair, riders stand against the inside of a spinning drum-shaped wall. As the ride spins, each rider moves in a circle. No cord holds her, and her shoes may even leave the floor.
What force points at the center of her circle?
Look at what touches a rider: the wall, against her back. A surface exerts a normal force on whatever presses against it, directed away from the surface. The rotor's wall curves around the ride's center. So away from the wall means toward the center.
The wall presses inward on the rider's back. That normal force points from the wall straight at the center of the circle, and it is the net inward force her circle requires.
A marble rolls around the inside of a circular rim on a level table. The rim's normal force on the marble points away from the rim, toward the center. The rim's normal force supplies the marble's centripetal force.
Now the rim is lifted away. The table still exerts a normal force on the marble, but that force points straight up, not toward the center. No force points at the center anymore, so the marble cannot keep moving in its circle.
The rim is set back down, and the inward press returns with it. The marble can circle again.
In a spinning space-station ring, the ring's floor presses on an astronaut standing inside it, toward the ring's hub. That normal force supplies the astronaut's centripetal force.
A motorcycle rides in a horizontal circle around the inside wall of a huge wooden cylinder. Which force supplies the motorcycle's centripetal force?
A toy train runs around a circular track at a steady speed, guided by the outer rail. Which force supplies the train's centripetal force?
The outer rail is a curved surface that the train's wheels press against.
The rail's normal force on the wheels points away from the rail, toward the center of the circular track.
The outer rail's normal force on the wheels supplies the train's centripetal force.
Minna stands with her back against the outer rail of a spinning carousel, circling with it. She says: 'The rail's normal force on my back supplies my centripetal force.' Is she correct?
A book rests on a level table. The table exerts a normal force on the book. Does that normal force supply a centripetal force?
The book does not move in a circle, so no centripetal force is required at all.
And the table's normal force points straight up, not toward the center of any circle.
No. The normal force supplies a centripetal force only when a surface presses the object toward a circle's center.
A toy car runs around the inside of a circular plastic wall on a level floor. The wall's normal force on the car points in which direction?
In which case does the normal force supply the centripetal force?
A rider circles inside a spinning carnival barrel, pressed against its wall. Which force supplies the rider's centripetal force?
When does a surface's normal force supply an object's centripetal force?