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 the last meeting of my mechanics lecture class for twelfth week of the second term, we started on the topic of friction.

I gave the two types of friction: static and kinetic, when they are used and their differences in numeric value on the same surface.

I emphasized that even though an object is already moving along a surface, it doesn’t mean that there is no friction anymore, just that the friction is of course less than the force allowing the object to move.

From there I went to the equation of friction, which is the product of the coefficient of friction and the normal force of the surface on the object.

The coefficient, I said, is a ratio between zero and one. The closer it is to zero, the more frictionless the surface is. Here I gave two examples, the ice-skating rink and the air hockey table with the blowers working.

When the coefficient is one, the surface is adhesive, like tape or flypaper.

The coefficient, I underscored, has no unit.

From there I gave an example of horizontal motion, showing them that it was still important to be able to come up with equations for the summation of forces along the x and the y axis, because they are important, and not constant.

Thus the illustration is not optional anymore, but integral to the solution.

I also cleared up when there is no acceleration along the vertical (when the object has no upward or downward motion).

There is no acceleration along the horizontal when the object hasn’t started moving yet, or if it is moving with constant velocity.

I also gave an example where the pulling force (tension) is at an angle, and thus its x and y components had to be obtained before placing them in the two summation of forces equations.

Session 879 has not started moving yet, and thus has no horizontal acceleration. Class dismissed.