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 afternoon of the third day of the fifth week of classes, in my second mechanics lecture session for the week, I continued with the discussion of the three scenarios in free fall, or constant acceleration along one dimension of the vertical.

The second scenario, which I had already told them during the last meeting, is when the object is given an initial velocity downwards.

So the only difference from the first scenario is that initial velocity is a negative value instead of zero. But everything else is still the same: acceleration due to gravity is constant, time is positive and increasing, final velocity is still numerically less than initial velocity, and displacement is negative because initial position is higher than final position.

But I did give them the “warning” that this scenario is relatively rare and occurs only around once every ten problems.

Lastly we went to the most complicated third scenario, which is when the object is given an initial velocity upwards. Then for a time, the velocity continues upward but is decreasing. Displacement is also positive.

At the highest point, or maximum height, velocity becomes zero. From that point, velocity is now downwards or negative, and continues to decrease. Time still moves forward, but displacement is now decreasing positive until it reaches the original height, when it becomes zero.

In other words, it just becomes the same as the first scenario.

I also gave them some shortcuts. When the object returns to the same height, the time from there to the top is the same for both “trips”. The velocity for both also has the same magnitude, but opposite in direction.

This, I told them, was the reason why it is still dangerous for gun owners to shoot their firearms in the air during New Year’s Day celebration or any other occasion. When the bullet returns to the ground, it will, more or less, still have the same velocity as it just left the barrel.

We also computed for the impact velocity of someone or something falling from even as low as a four storey building. Then, to be able to relate more, we converted the answer to kilometers per hour. It came out as being in the sixties, so it wasn’t as “soft” as they initially thought.

Even though I gave several examples, in the end I told them that the third scenario would not be included in the upcoming exam, which to them was a big relief.

Session 811 is not included in the coverage of the upcoming exam as well. Class dismissed.