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Name: Noam
Status: student
Grade: K-3 
Country: Israel
Date: April 4, 2011

I have a ball made of soft rubber with liquid inside. When I roll it quickly on the floor, it changes into a wheel shape. Why does it do that?

Hi Noam,

I am not quite sure of the toy you described, but it sounds very much like centripetal force is the reason for the squishy ball to flatten and turn into a wheel shape. Centripetal force is what you feel when you are revolving around a point in circular motion. You feel centripetal force when you are traveling in a car and you are going around a curve, and you might feel yourself pushed against the car door when this happens. This force is really your inertia at work. You tend to go in a straight line (like Newton's 1st law says), but the car door is pushing on you to make you travel the same curved path that the car does.

In the same way, the inertia of the liquid in the ball would tend to move it in a straight line, but it is constrained to go in a curved path by the round ball shape. So it pushes against the ball surface and this stretches the ball in the radial direction, but this means the ball contracts in the axial direction. So it turns into more of a wheel shape. The faster you roll it, the more it will flatten out and look like a wheel.

John C Strong

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