Need some help with concept of torque

[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.

 

[charng]

I would like to clarify the concept of torque and its relation to the point of contact between the motorcycle and the wall. Torque is a measure of the force that causes an object to rotate around a pivot point or axis. In this case, the pivot point or axis is the point of contact between the motorcycle and the wall.

It is important to note that the point of contact is not a fixed pivot point, as it can change depending on the force applied and the resulting movement of the motorcycle. Therefore, it cannot be considered a pivot point in the traditional sense.

Additionally, the principle of moment states that 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. This means that any point within the frame of reference can be used as a pivot point, as long as the clockwise and anti-clockwise moments are balanced.

In the case of the motorcycle against the wall, the point of contact can be considered as a pivot point only if the forces acting on the motorcycle are balanced and there is no net torque. However, if there is a net torque, the motorcycle will not rotate around the point of contact, but around a different pivot point determined by the forces and their distances from the point of contact.

I hope this helps clarify the concept of torque and its relation to the point of contact in this scenario. It is important to consider all the forces and their effects on the object in order to accurately determine the pivot point and the resulting movement.