Why Can't the Motorcycle's Contact Point with the Wall Serve as a Pivot?

[bobbytkc]

look here: http://www.mut.ac.th/~physics/PhysicsMagic/wall.htm"

scrolling to the motorcycle section, my question is, why is it that you cannot consider the point of contact of the motorcycle with the wall as a pivoting point? I have done some calculations, and this leads to a physically nonsensical answer.

I recognize that any line of force not acting through an object's centre of mass would produce a torque about an axis through the centre of mass,and that would indeed lead to a coherent answer, but my problem is, why can the point of contact NOT be considered a pivot? The motorcycle would rotate about that point after all.

Another point to clarify, the principle of moment states:

For rotational equilibrium, the sum of the clockwise moments about any point must be equal to the sum of the anti-clockwise moments about the same point.

Does 'any point' refers literally to any point within the frame of reference, of simply any point that can act as a pivot only?

Thanx.

 

[Doc Al]

To analyze the rotational motion of an object, calculate torques about its center of mass. If you use any other point as your pivot, be sure you are using one fixed in an inertial reference frame. The point of contact of the tire with the ground is accelerating, so it is not an inertial reference frame. (This same issue comes up in analyzing the lean angle of a bicycle or motorcycle turning a corner. If you use the center of mass, it's easy; if you use torques about the contact point with the ground you cannot simply apply Newton's laws without modification.)

As far as the "principle of moments" that you quoted, realize that that is usually applied to systems in static equilibrium. Any point can be used to calculate torques, since the system is viewed from an inertial frame.