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This presentation looks at the classic motion of a vehicle moving along a level road, where it could be speeding up or slowing down. In the example presented, the vehicle is slowing down due to its brakes, and in fact, we are told the wheels lock up. What is required in the example is the friction forces on the wheels.
This is an application of the equations of motion, three equations in 2D, where two involve summing forces and one involves taking moments. In Statics, moments can be taken about any point since all points are fixed. Not so with Dynamics. For this problem, there is actually only one point that is valid, the center of gravity. However, the solution in the example uses one of the points where the wheels touch the ground. As explained in a follow on presentation, not picking the center of gravity is due to making Newton’s 2nd Law, F = ma, into a Statics equation by moving “ma” to the left side of the equation, meaning F – ma = 0. The “ma” is treated as an “inertia” force, however no such force exists. It has units of force, but it’s not a force.
Therefore, my advice is take moments only about the center of gravity unless there is a clear fixed point. Trying to make a Dynamics problem into a Statice problem is asking for trouble.
-Tom
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