It's not a dumb question at all.

However, I'm not sure of its practical implications for you. Afterall, if the car falls off of the jacks with you under it, then I doubt you would care what direction it rolls since you'll be good and squished.

I believe you chalk the back of the rear wheels because as you lift the front, the car would have a tendency to roll backwards. It's a relatively simple statics problem. With the car at an angle, the force between the car and the ground has a component in the backwards direction. There is no component in the forwards direction, so there is no real danger of the car rolling forwards unless the stand is positioned on the wrong side of the center of mass. If the stand is positioned wrong, though, you'll know before you can remove the jacks!

Now, it would be possible for another car to bump the back providing a force in the forward direction. However, whether there is a chalk in the front or not, I'd imagine the bump would have to be great enough to knock the thing off the stands anyway. So you're squished either way.

Chris wrote:

More simply: you're right and the head mechanic is wrong.
The head mechanic is the boss, though.

Well hes just the head mechanic, we're actually equals hes just more experienced than me, but here we go.

This is the rebbutal he gave me.

This is indeed a simple statics problem. Due to the fact that it is a static problem, there is no inertia involved. We’ll refer to Newton’s First and Third Laws of Motion.

1. A physical body will remain at rest, or continue to move at a constant velocity, unless an outside net force acts upon it.
3. To every action there is an equal and opposite reaction.
Newton’s first and third law states a body at equilibrium will remain at equilibrium unless acted on by an external force and that every action has an opposite and equal reaction.

The car raised up on one end, and resting on the floor on the other. It is in equilibrium. This means that the force of its weight is being held in place by an equivalent reactant force. Now think of which way gravity points, straight down. No matter how an object is positioned gravity acts straight down on it. The tendency to roll either forward or backward comes from the resultant force which the surface exerts on it.

Think of a car sitting on some stairs, where none of the wheels are not resting on the edges of a stair. Neglecting friction, the surface can only react in a direction perpendicular to itself. The car on the stairs has no resulting forces in any other direction than vertical, since the stairs surfaces are all at an angle of zero. The car on a slope, however will roll. Due to the slope reacting in a direction other than vertical, it adds a small component to the reactant force in the horizontal direction. The horizontal force is just a result of the angle.

Even though some weight may be transferred to the rear while lifting the car, this is due to moving the vehicle's center of mass. Even though the forces change, they still act in the same direction. Assuming the floor and the jack are parallel, the car on the jack is like the car on stairs. Therefore, it has no tendency to roll in either direction.

If your looking for credible information to back this up, take a look in any statics book. Forces and motion are not really all that complicated, always start with a sketch, and be sure to ignore things that are not important, while taking into account what is.

Last edited by curiousbynature (2008-05-10 00:54:39)