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

In my mechanics lecture class last Thursday I had another recitation at the start. For this one some of the students were already raising their hands to answer my question when they saw that the person I initially asked was blanking out. That was a marked improvement from last time.

For the lecture proper, we started on what may very be the last topic (that is, if I don’t get to talk about spring constants and their contribution to energy). We talked about conservation on linear momentum.

First I introduced the concept of momentum, and how it could be expressed in several ways: as the product of mass and velocity, the product of force and time, and the quotient of energy and velocity. This is because the unit of momentum is kilogram meter over second.

Then I gave them the definition of conservation of linear momentum, which, just like with conservation of mechanical energy, meant no external forces. In fact I gave them two equations for conservation, because besides linear momentum for before and after collisions being equal, the same is also still true for energy. The two sides of each equation refer to the quantities before and after collisions.

I told them that for linear momentum, we would have two pairs of categories to discuss: collisions where the target is stationary and where the target is moving, and one-dimensional and two-dimensional, making four sub-categories in all.

For the stationary target, they did not have to worry anymore about the second velocity, because it would be zero, which means that the same object’s momentum before the collision would also be zero.

I initially told them that we would have the quiz on the next meeting, which they readily agreed to seeing how they only had one new equation to deal with. I backed out on the day of the quiz itself though, not because I didn’t want it too easy for them, but because I couldn’t find enough questions of varying degrees of challenge to fill a fifty-point quiz without grossly over-inflating their values.

Yesterday (we followed the Monday schedule of classes because there had been simply too many holidays falling – with a loud thud – on Mondays) in the same class I began again with a recitation. This time, since last meeting we still took longer than I had expected, I told them they would all be standing until they could answer the questions. Not so productive though, as it only made the frustration on their faces more marked every time they got a question wrong.

Finally afterwards, we tackled two dimensional collisions. For this one I had to emphasize to them that unlike our previous discussions on two-dimensional motion, which were usually seen from the side view to show both horizontal and vertical directions, for linear momentum, because we hardly have to consider the gravitational potential energy of the object (in other words, it does not change height) then we would be showing the two dimensions from the top view of scenarios.

In preparation I had to practice them on how to determine angles given in such terms as “north of east” or “west of south”, something that next time, I will probably have to drill them on when we are discussing composition of forces in two dimensions instead of this late.

And that’s all for today. I’ll continue with the discussion tomorrow. Class dismissed.