http://www.ck12.org Chapter 7. Momentum
FIGURE 7.11
coin will move. However, because of the effect of friction between the coins and the surface, you will not be able to
see the target coin move along with the same velocity as the incoming coin.
Check Your Understanding
Two identical marbles of massmare moving directly toward each other with equal speedsv. The momentum of the
system is:
a. zero
b.mv
c. 2mv
Answer:The correct answer is A. Since the marbles are moving directly toward each other, they must have opposite
velocities. Therefore, one marble has momentummvand the other marble has momentum−mv. The momentum of
the system ismv+(−mv) =0.
Illustrative Example 7.3.2
A car of mass 1000-kg at rest at a stop light is hit from behind when a second car of mass 900-kg moving at a speed
of 13.4 m/s, collides with it. If the two cars lock bumpers, what common speed do they move off with after the
collision?
We use Conservation of Momentum to solve the problem, writing the initial and final momenta of the system and
then setting them equal.
pi= ( 1000 kg)( 0 )+( 900 kg)(
13. 4
m
s)
pf= ( 1000 kg+ 900 kg)vf
pi=pf→( 1000 kg)( 0 )+( 900 kg)(
13. 4
m
s)
= ( 1900 kg)vfvf=( 900 kg)(
13. 4 ms)
1900 kg= 6. 347 → 6. 35
m
shttp://demonstrations.wolfram.com/PhysicsOfARearEndCollision/
Check Your Understanding
- In the last Illustrative Example, what wouldvfhave been if both cars had mass 900-kg?