I realized tonight, as I start to plan my lessons for the week, that I don’t have much here on momentum. This is a pretty straight-forward section. It’s easy to teach and should not be confusing to students that do the barest amount of studying.
As a quick summary, I teach momentum and impulse, skip angular momentum, then teach conservation of momentum. Not all the books discuss elastic and inelastic collisions, but I think that is rather critical to the subject. I also stop short of including the energy section.
Once I start conservation of momentum, I talk about what happens when people don’t wear seatbelts. I teach high school seniors in Philadelphia, many of them are just getting their license because they use public transportation to get around. I drum into their head the need for seatbelts. One way to do this is to figure that a car traveling at 25 m/s (about 55mph) hitting a tree would compress perhaps 1.5 meters while coming to a stop. I calculate the time to stop is about .12 seconds. Using the impulse-momentum theorem, I get a figure of about 300,000 Newtons of force. I’m sure car manufacturers have better numbers, these are just an estimate.
For demos, I have something call “slippery alley” that was purchased through Frey. There is a sled that wants to separate because of a rubber band, but it is held together with a string. The alley is a metal trough filled with plastic microspheres creating a nearly frictionless surface. Since one part of the sled is twice the mass of the other, when they seperate (in opposite directions), the lighter mass is traveling twice as fast as the heavy one.
The worksheet I’ve attached was very carefully researched. All the masses are accurate, as are the velocities of the projectiles. If you find any errors, please let me know.
In the third paragraph [since corrected and updated thanks to Jerry - Scott] you include too much momentum. The car imparts very little momentum on the occupant (the compression of the springs in the seat) and the impact on the windshield is usually at less than the initial velocity (25 m/s in your example). It might be a great encouragement to wearing seatbelts, but not good science.
Respectfully,
Jerry Fountain
Comment by Jerry Fountain — January 7, 2009 @ 12:09 pm |