8.4. Key Applications http://www.ck12.org
8.4 Key Applications
- Two cars collide head-on—two subatomic particles collide in an accelerator—a bird slams horizontally into a
glass office building: all of these are examples ofone-dimensional (straight line) collisions. For these, pay
extra attention to direction: define one direction as positive and the other as negative, and be consistent with
signs. Remember, in one dimension vectors are just numbers with signs. - A firecracker in mid-air explodes—two children push off each other on roller skates—an atomic nucleus
breaks apart during a radioactive decay: all of these are examples ofdisintegrationproblems. The initial
momentum beforehand is zero, so the final momentum afterwards must also be zero. Momenta along any set
of perpendicular vectors (like(~x,~y,~z)) must also be 0. - A spacecraft burns off momentum by colliding with air molecules as it descends—hail stones pummel the top
of your car—a wet rag is thrown at and sticks to the wall: all of these are examples ofimpulseproblems,
where the change in momentum of one object and the reaction to the applied force are considered. What is
important here is the rate: you need to come up with an average time∆tthat the collision(s) last so that you
can figure out the force~F=
∆~p
∆t, according to [4]. - Remember as well that if a particle has momentum~p, and it experiences an impulse that turns it around
completely, with new momentum−~p, then the total change in momentum has magnitude 2p. It is harder to
reflect something than to stop it. - Momentum vectors add just like any other vectors. Refer to the addition of vectors material in Chapter 1.