Hey guys. I got this lab called Ballistic Pendulum. Its where we take a small projectile and launch it through a stationary block, with a hole in it, so it becomes embedded in the block. Then, the momentum of the system brings the ball up to a maximum height.

We measure the change in height from the bottom of the block, as opposed to the center of mass of the block. I need to explain why?

I think its got something to do with the mass not changing. Also, the gravitational potential energy is 0 for the second part of the mechanism. And thirdly, It is too difficult to measure an exact height during a half of second reading, so using a flat surface is beneficial.

But I really don't understand the answer. I want the physical reason as to why we can treat an object as a point particle outside of the center of mass.

how many strings do you have?
look at this

then this

this second one can only occur when the string stretches

for small angles, i'm pretty sure they'll follow the first picture if it's designed as a 4 bar linkage
if you make the bottom your reference and measure from it to the new bottom height, it shouldn't matter much

This post was edited by saber_x3 on Apr 27 2014 02:24pm

It looks like the one at the top, where the final block holds the same orientation as it did initially.

well, then it's makes perfect sense to measure to the bottom
if you want to measure to the center, then your reference point would shift up by half the height

you're measuring delta energy
everything rises by h regardless where in the block

This post was edited by saber_x3 on Apr 27 2014 02:54pm

Right, I understand this. However, is there a physical reason as to why we can measure a point outside of the center of mass, and treat it as the center of mass? Is it because the mass in the center is equal to the center of mass in the corner?

because every point in the block rises by the same ammount , rigid body
measuring the bottom is physically easiest in many cases