Physics 105

March 31, 2003

Today we will continue our study of collisions.

During a collision a strong collision force acts between the objects involved in the collision. The collision force is related to the change in the linear momentum of the objects on which it is acting. Newton concluded that when two objects exert a force on each other, than the force experienced by each object is equal in magnitude but directed in an opposite direction. This implies that when two objects collide, the change in the linear momentum of each object is equal in magnitude, but opposite in sign. Thus, the total linear momentum of the system is unchanged:

total linear momentum before the collision = total linear momentum after the collision

We have tested this relation last week by looking at elastic collisions. In elastic collisions, the total kinetic energy (0.5*m*v^2) is conserved, and this allows one to make predictions about the outcome of the experiment given the initial conditions. In inelastic collisions, kinetic energy is not conserved, and some of the kinetic energy is converted into internal energy (e.g. energy of deformation, heat, etc.). Using the same setup we used in experiment P17 we will study today inelastic collisions (in which the carts stick together after the collision). In order to carry out inelastic collisions, we use the velcro bumpers on the carts so that the carts stick together atfer the collision. Following the same procedure we used in our studies of elastic collisions, carry out a series of measurements with different initial velocities. For each collision carry out the following calculations:

• Calculate the linear momentum of each cart before and after the collision.
• Calculate the total linear momentum of the system before and after the collision. Is linear momentum conserved?
• Calculate the kinetic energy of each cart before and after the collision.
• Calculate the total kinetic energy of the system before and after the collision. How much energy is lost in the collision?

Repeat the series of inelastic collisions after changing the mass of the carts (study at least two additional combinations of cart masses).

© Frank L. H. Wolfs, University of Rochester, Rochester, NY 14627, USA

Last updated on Monday, March 31, 2003 9:43