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 my first mechanics lecture meeting for the eleventh week of classes, I started on the specific types of forces, not just the general pushing and pulling that we took up the previous lecture meeting.

From our previous discussion where we only talked about forces in the two dimensions of the horizontal plane as seen from the top view (which would give it the direction of north, east, south and west), I went to forces in two dimensions as seen from the side view, which now have the direction of up, down, left and right.

I started with weight, then tension, then the normal force and finally friction.

Weight is the product of mass and the acceleration due to gravity, and is always pointed downwards.

Tension is along the length of a taut rope directed away from the object, but if two forces are pulling and there is overall motion in the system and the rope (or chain or cable) holds, then the tension from both ends of the rope, directed towards the middle, have equal magnitude.

Normal force is always directed perpendicular to the surface towards the object. If I’m pressing the eraser to the board so that it does not move, the normal force of the board to the eraser is directed horizontally away from the board.

Finally, friction is always parallel to the surface and away from the direction of intended, but not necessarily enacted, motion.

But I didn’t give examples of those yet. Unlike in the previous problems we had the motion of the object could be in any direction along the plane, here I had to show them that even though the object is being pushed or pulled at an angle, it doesn’t necessarily mean that the object is moving at that angle as well.

Instead, they only have to consider the component of the force along the direction where the force is free to move, which means horizontal, or in the case of an inclined plane, along the direction of the plane either upwards or downwards.

Its motion is certainly not downwards, and unless the upward force can counteract the weight, not upwards either.

Again in the examples that I used I tied it in to getting the initial velocity, the current and final velocity, the displacement and the time, because it was still part of motion with constant acceleration.

Session 867 does not have any upward or downward motion. Class dismissed.