Day 138: Conceptualizing Impulse and Gravitatonal Energy

Advanced Physics:

Yesterday I gave the students a set of conceptual questions about impulse and momentum. I don’t know about you, but I find many of my students can easily hide behind the quantitative problems and think they really ‘get’ the topic.  In other words, they are really petty good at putting the numbers into  the correct equation.  Granted, sometimes a sign messes them up, but for the most part they are pretty solid at that.  They still struggle at times to use the models/equations (sans numbers) to explain things.  I will not bore you with all the questions on the assignment, but we did add a few demo’s that I hope helped solidify the concepts.

One question dealt with a clay ball hitting a wall and then a same mass super ball hitting the wall. Which had the greater change in momentum. Not too bad on this one, but then asked which imparts a greater impulse on the wall… that made a few think.  Here is the demo I did to help:(will add later, sorry)

Another question was the traditional question about why airbags work.  I did the classic egg thrown in to the bed sheet, but then also showed them the cup and ball toy. We talked about how it is easier to catch the ball, if you draw your hand down and away as you catch it.  We then made our own version:


We also had a series of questions about comparing aspects of a collision between a Hummer and a VW bug.  Compare the forces, the impulse, the change in momentum, and finally the acceleration.  I tried for about 45 minutes to figure out how demonstrate this with two dynamics carts and WDSS force sensors.  Each time the forces were about 2-3 newtons off.  It was just not good enough for me… especially with the maximum force only being about 20N.  Tomorrow we will WB some quantitative problems.


General Physics:

All data gathering today!  The purpose was to develop the model for gravitational energy.  To get at it, we simply allowed a dynamics cart to roll down a shallow incline from various starting heights and measured the velocity with a photogate near the bottom of the ramp. We calculated the (kinetic) energy from the velocity  and plotted this as a function of the change in height.  Tomorrow we will discuss it… here’s hoping the slope turns out to be the weight of the dynamics cart!


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