A student draws the free-body diagram of a ball whirling on a string: tension toward the center, plus a second arrow pointing outward, labeled 'centrifugal force'. The drawing looks balanced and tidy. It is wrong, and your job in this lesson is to catch arrows like that one.
The test: every arrow on a correct free-body diagram names a real force exerted by an identifiable object. That one test catches both of the classic added arrows.
Start with the whirled ball done right. In the horizontal plane, one force acts on the ball: the string's tension, toward the center. The net force points at the center.
Now the same diagram with a second inward arrow added, labeled 'centripetal force'. Ask what object exerts it. No object does. 'Centripetal force' is the role the tension already fills, so the added arrow counts the same force twice. Wrong.
Now the same diagram with an outward arrow added instead, labeled 'centrifugal force'. Ask what object exerts it. No object does. No real outward force acts on the ball at all. Wrong.
The outward arrow tempts people because the diagram then looks balanced. But a circling object must not have balanced forces: it needs an unbalanced net force, toward the center, or it would move in a straight line.
Strip the bogus arrow, and the diagram is correct again: real forces only, net force at the center.
Odile's top-view diagram of a whistle whirling on its cord shows the cord's tension toward the center and an outward arrow labeled 'centrifugal force'. Which arrow does not belong?
Nestor's free-body diagram of a scooter rounding a flat corner shows weight down, the normal force up, friction toward the center, and a fourth arrow toward the center labeled 'centripetal force'. Which arrow is wrong, and why?
Test each arrow: what object exerts it? Earth exerts the weight, the road exerts the normal force and the friction.
No object exerts the fourth arrow's force. 'Centripetal force' is the role the friction already fills.
The added arrow counts the friction's job twice.
The 'centripetal force' arrow is wrong: it is not a real force, and the friction already supplies the inward net force.
Nestor adds an extra arrow toward the center, labeled 'centripetal force', to the diagram of a minibus rounding a flat corner, beside the friction arrow. What is the error?
Pavel's top-view diagram of a sled whirling on a rope shows the rope's tension toward the center and an arrow of equal length pointing outward, labeled 'centrifugal force'. Which arrow is wrong, and why?
Test each arrow: the rope exerts the tension. No object exerts the outward force.
And check the net force: with the two arrows balanced, the net force would be zero, and the sled would move in a straight line, not a circle.
The circle requires an unbalanced net force toward the center.
The outward 'centrifugal force' arrow is wrong: no real outward force acts on the sled.
Odile says: 'A correct free-body diagram of a satellite circling Mars shows exactly one arrow: Mars's gravitational pull.' Is she correct?
Four students drew free-body diagrams. Which diagram is flawed?
Minna's top-view diagram of a tennis ball circling a center post on a cord shows the cord's tension toward the post and an outward arrow labeled 'centrifugal force'. What single change makes the diagram correct?
Which statement about a correct free-body diagram of an object in uniform circular motion is true?