photo credit: paul (dex)

A person fires a pellet gun off the top of a building straight downward with an initial velocity of 20 m/s.

After 0.25 seconds how far has the pellet travelled?

Important Points Regarding this Physics Problem:

In the solution posted above there are two starred points that I want to discuss in a bit more detail…

1. Acceleration of Gravity Not Given

I didn’t make the “rule” so don’t blame the messenger but it’s a rule you need to get used to.  In physics, it is very rare for a gravity related problem to outright tell you that there is a gravity component.  The expectation is that YOU will read the problem, see that the forces of gravity are going to be playing a role, and then for YOU to incorporate the necessary information into the problem.  If you have gotten into the habit of looking for an equation to fit the data provided in the problem this may become and issue for you because what they really want you to do is UNDERSTAND the problem and then choose an appropriate equation.  In the case of the problem above, to solve the problem you needed to understand that gravity plays a role, that it is in the positive direction and so that means that a = 9.81 m/s/sup>.

2. Changing the X to a Y

In the problem above I changed all the distance symbols from X to Y.  Right now it may seem completely USELESS to do that.  A symbol is a symbol and for all we care, we could call distance :) . BUT … a few weeks down the road, you will see that there is a really good reason to get in the habit of using X to identify things that are horizontal and Y to identify things that vertical.  That’s because in a few weeks we are going to be working on problems that deal with more than one dimension at a time.  That is, instead of going just up or down, or side to side they will be doing BOTH at the same time!