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Re: A problem (displacement of contact force)



Hello,

John Denker wrote an insightful explanation (again!) on the cube problem. Still I have a quite elementary question to ask.

When the cube is at rest, the upward force due to contact >is colinear with this weight vector, so there is no
torque.

This is very clear.

But during the deceleration, the weight and the contact >force are not colinear, resulting
in a torque.

I would like to get more explanation on this one. For instance, in case of a decelerating car I see why the contact force and the weight cannot be colinear: otherwise the torque exerted by the friction would not be balanced. But why does this "displacement" of the contact force take place? I am not totally satisfied with the answer "to keep the net torque zero". The contact force may not have intentions :-).

A guess. If the car decelerates, there must be a net force on every "slice" of the car. It means that the car is under varying tension. Of course this applies to the decelerating cube as well. Would this explanation have something to do with the "displacement" of the contact force?

Regards,

Antti Savinainen
Physics teacher
Kuopio Lyseo High School/IB
Finland