Peoples Physics Concepts

7.3 Elastic Collisions

7.3 Elastic Collisions

• Solve problems involving elastic collisions.

Key Equations

∑pinitial=∑p

final The total momentum does not change in closed systems

∑KEinitial=∑KE

final The total kinetic energy does not change in elastic collisions

Example 1

MEDIA

Click image to the left for use the URL below.

URL: http://www.ck12.org/flx/render/embeddedobject/1894

Example 2

Question: Chris and Ashley are playing pool. Ashley hits the cue ball into the 8 ball with a velocity of 1.2m/s. The
cue ball(c)and the 8 ball (e) react as shown in the diagram. The 8 ball and the cue ball both have a mass of.17kg.
What is the velocity of the cue ball? What is the direction (the angle) of the cue ball?

Answer: We know the equation for conservation of momentum, along with the masses of the objects in question as
well two of the three velocities. Therefore all we need to do is manipulate the conservation of momentum equation
so that it is solved for the velocity of the cue ball after the collision and then plug in the known values to get the
velocity of the cue ball.

mcvic+mevie=mcvf c+mevf e

vf c=

mcvic+mevie−mevf e

mc

vf c=

.17kg× 2 .0m/s+.17kg×0m/s−.17kg× 1 .2m/s

.17kg

vf c=.80m/s