Concept Overview
- Inertia
- Net Vertical Force of Zero vs. Non-zero Horizontal Force
Lecturer Procedure
Steps to Follow
- Hold base of pedestal firmly.
- Pull back spring.
- Release spring.
Expected Result
Spring will cause card to move from beneath the ball and the ball will fall into the opening at the top of the pedestal.
Historical References; Taken from original work of Percy H. Carr, 1970
“Please do not give the all too common explanation that inertia of the ball keeps it from moving till the card is out from under it. Be honest. Consider the card. The net vertical force on it is zero. While the spring is pushing the card with relatively large horizontal force, the opposing horizontal forces are the friction with the cup and the friction with the ball. These two forces are fixed by the weight of the ball and the surface conditions of ball and cup, and are quite small compared to the push of the spring. The mass of the ecard is small; the acceleration high; this means that once motion has started the card is out from under the ball in a very short time. During this very short time, the net vertical force on the ball is zero. The only horizontal force on the ball is the friction with the card; this is (by design) a rather small force fixed by the weight of the ball and the surfaces of ball and card. The mass of the ball is quite large; hence, the acceleration of the ball is very small; and in the very small time interval when the card is moving and in contact with the ball, the ball acquires a very small horizontal velocity in the direction of the card (and usually rotates, but forget it). As soon as the card has lost contact with the ball, the ball continues to move in the same direction as the card with this small acquired velocity but in addition starts to fall vertically with acceleration g. The design of the equipment is such that by the time the ball has struck the side or lip of the cup because of its horizontal motion, it has fallen to such a low position that it will not hop out of the cup, but settles back into the cup."
Video Demonstration
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