LECTURE DEMONSTRATION MANUAL | Instructional Research Lab : ucla physics
A Set of Momentum and Collision Demos

A Set of Momentum and Collision Demonstrations

Conservation of momentum alone is best demonstrated with the Reaction Carts M.6.3. Here one merely observes that the larger mass cart moves away more slowly. If you want to make a measurement, the dynamics track can be used with two carts. The carts are placed touching each other and are pushed apart by unsnapping the plunger on one of the carts. The velocities of the retreating carts can be measured either using Data Studio or with timers and will be found inversely proportional to the masses of the carts.

Equal mass elastic collisions can be demonstrated with either the Collision Balls M.6.1, the Dynamics Track Collisions M.6.2 using the magnetic end of two carts or the Two Balls Hanging M.6.6. In each illustration the incoming mass stops dead, and the other mass moves away with all the momentum and energy. Both conservation of momentum and conservation of energy are needed to derive this result. The collision of two highly inelastic squash balls contrasts with the elastic case. A few other elastic collisions to show are a large mass object hitting a smaller mass object and vice-versa. In the first case (bowling ball hitting a BB) the large mass "plows through" and both masses go forward; in the second case (BB hitting a bowling ball) the small mass bounces back.

Inelastic collisions are easy to show with the Dynamics Track Collisions M.6.2 by using the velcro end of the carts. Here conservation of momentum alone determines the result. Either timers r Data Studio may be used to show, for example, that if a moving 1-unit mass collides with a stationary 2-unit mass, the coupled 3-unit mass will move away with one-third the initial velocity. (To get an accurate velocity measurement with this demonstration send the initial cart in slowly; otherwise, the sticking carts can derail.) Two dimensional collisions can be shown with the Hover Disk Collisions M.6.5.

A final dramatic demonstration is the Ballistic Pendulum, M.6.7 (See M.10.1). Using both conservation of momentum and conservation of energy you compute the speed of a ball shot by a spring gun. Using this result and the ballistics equations, you then predict how far the gun will shoot across the floor. A target is placed at the predicted position and then struck squarely to the resounding applause of the class.

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