Peoples Physics Book Version-2

8.5. Examples http://www.ck12.org

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

=

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

.17kg

=.80m/s

Now we want to find the direction of the cue ball. To do this we will use the diagram below.

We know that the momentum in the y direction of the two balls is equal. Therefore we can say that the velocity in
the y direction is also equal because the masses of the two balls are equal.

mcvcy=mevey→vcy=vey

Given this and the diagram, we can find the direction of the cue ball. After 1 second, the 8 ball will have traveled
1 .2m. Therefore we can find the distance it has traveled in the y direction.

sin 25 o=

opposite

hypotenuse

=

x

1 .2m

→x=sin 25 × 1 .2m=.51m

Therefore, in one second the cue ball will have traveled.51m in the y direction as well. We also know how far in
total the cue ball travels in one second(.80m). Thus we can find the direction of the cue ball.

sin−^1

opposite

hypotenuse

=sin−^1

.51m

.80m

= 40 o