http://www.ck12.org Chapter 7. Momentum
FIGURE 7.5
A 0.074-kg mouse.In order to find the velocity of the mouse we equate their momenta (plural form of momentum).
pc=pm→ 3. 05 = ( 0 .074 kg)vm→vm=3. 05 kgs·m
0 .074 kg=41m/sThis is about 92 mph. Massive objects moving at low speeds can easily have more momentum than light objects
traveling at their fastest. (An economy car, for example, would need a speed of about 67 m/s to have the same
momentum as an 18-wheeler with a speed of 4.5 m/s. What is the ratio of their masses?)
http://www.youtube.com/watch?v=y2Gb4NIv0Xg
A Change in Momentum
It is important to remember that momentum is a vector quantity. This can best be illustrated by considering a change
in momentum.
Illustrative Example 7.1.2
Earlier, we stated that Aroldis Chapman of the Cincinnati Reds was credited with throwing the fastest baseball during
a game. The ball had a speed of 46.95 m/s and a mass of 145.0 g. We will calculate the change in momentum for
two cases. In Case 1, Chapman makes his throw, it’s a “swing and a miss” by the batter, and the ball is caught by the
catcher. In Case 2, the ball is “bunted” by the batter. A bunt means that the bat is brought to rest at about waist level
and the ball is permitted to bounce off it. We will assume that the bunted ball leaves the bat opposite to the direction
it was thrown and at the same speed it was thrown.
Case 1:We calculate initial momentum of the ball as it leaves Chapman’s hand and the final momentum of the ball
in the catcher’s mitt. The work is shown below:
pi=mvi= ( 0 .145 kg)( 46 .9m/s) = 6. 81kg·m
s
pf=mvf= ( 0 .145 kg)( 0 .00m/s) = 0. 00
kg·m
s