Student "edition" found at {csi dot journalspace dot com}.

Maybe I shouldn't have started this blog now, not with everything that's been going on.

In another of the meetings of my mechanics lecture class for the twelfth week of the second term, I discussed friction on non-horizontal surfaces.

For this one I told them that we would be temporarily abandoning the x and y horizontal and vertical axes for the more effective diagonal parallel and perpendicular axes to the inclined surface of motion.

For this one, the two sides of the axes pointed upwards would be designated as positive, and the ones pointed downwards designated as negative.

In the first example I gave them, where the object was on a tilting plank that started moving downwards at a certain angle, I showed them how the formula they used in the lab experiment was arrived at, and how it is universally applicable as long as there are no other external forces acting on the system.

That means that if the weight is the only force exerting on the object, the equation for the summation of forces parallel to the inclined surface are the friction minus the parallel component of the weight, since they are opposite in direction.

This is equal to the product of the mass and the acceleration, which is zero because the object has no acceleration since it has not started moving yet.

For the summation of forces perpendicular to the inclined surface, it is equal to the difference between the normal force and the perpendicular component of the weight, again opposite in direction from each other. This is the same as the product of the mass and the acceleration along the perpendicular, which is again zero because the object does not have any motion upwards from the surface or through the surface.

I also gave them the equations that the parallel component of the weight is equal to the mass of the object, multiplied by the acceleration due to gravity, multiplied by the sine of the tilt of the surface with respect to the horizontal. For the perpendicular component of the weight, it is the cosine instead of the sine, but basically still the same equation.

Session 895’s force cannot resist the opposing friction. Class dismissed.