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.

For the second session of my mechanics lecture class for the eighth week of the second term, I wrapped up the topic of projectile motion.

First I gave the three scenarios of the initial angle, since the only ones we discussed last time had positive angles less than ninety degrees.

Here I had to give a short review as well of when vertical displacement is positive and when it is negative. By our convention, it is the first when final position is higher than initial position, and vice versa for the second.

Then I explained to them what happens when a car runs off a cliff or a pen or a ball rolls off the table, how the angle is zero and thus the initial velocity is equal to the horizontal velocity. Initial vertical velocity is therefore zero also. Thus, the trajectory looks like the second half of the path from the first scenario. I even had them compute this using the sine and cosine relations of zero degrees, and they ended up with the same answers.

I will have to remember to give next time that in an object thrown upwards has an angle of ninety degrees, so here it is initial horizontal velocity that is zero, and initial velocity is all along the vertical.

Next was for a ramp sloped down, which is sometimes given as angled “below the horizontal” and how the initial velocity along the vertical is now negative because it is pointed downward.

I also gave an example for determining if a projectile is going to hit the wall or the floor, and how and when they are supposed to used distances as displacement, and when not to – just like in that fish ball stand in the path of a braking car problem.

Here they had to remember again that time is the only quantity that can be used in both constant horizontal velocity (which has one equation) and constant vertical acceleration (which has five equations).

Lastly, I answered the most difficult problem of their seatwork, which is now their problem set.

This was the one where the angle of the initial velocity is asked for, given the initial velocity and one point in its path with vertical and horizontal displacement given.

I showed them that substituting all the values we know, we ended up with four equations in four unknowns. So it was solvable. Thus we had to replace each unknown we had in two of the equations until we ended up with one variable.

I will have to cut this tale short here and continue tomorrow. Session 843 is at the end of this trajectory here. Class dismissed.