Name: ___________________________
Date: ______________
Physics I H
Mr. Tiesler
Solutions to Momentum Homework Problems 6-10
6.) A 95-kg halfback moving at 4.1 m/s on an apparent breakaway for a touchdown is tackled
from behind. When he was tackled by an 85-kg cornerback running at 5.5 m/s in the same
direction, what was their mutual speed immediately after the tackle?
The tackle will be analyzed as a one-dimensional momentum conserving situation. Let “A” represent the
halfback, and “B” represent the tackling cornerback.
pinitial  pfinal  mAv A  mB vB   mA  mB  v 
v 
mAvA  mB vB
mA  mB

 95 kg  4.1m s   85 kg  5.5 m s 

 95 kg   85 kg 
4.8 m s
7.) A 9300-kg boxcar traveling at 15.0 m/s strikes a second boxcar at rest. The two stick together
and move off with a speed of 6.0 m/s. What is the mass of the second car?
Consider the motion in one dimension, with the positive direction being the direction of motion of the
first car. Let “A” represent the first car, and “B” represent the second car. Momentum will be
conserved in the collision. Note that vB  0 .
pinitial  pfinal  mAv A  mB vB   mA  mB  v 
mB 
mA  v A  v   9300 kg 15.0 m s  6.0 m s 

 1.4  104 kg
v
6.0 m s
8.) A 23-g bullet traveling 230 m/s penetrates a 2.0-kg block of wood and emerges cleanly at 170
m/s. If the block is stationary on a frictionless surface when hit, how fast does it move after the
bullet emerges?
Consider the motion in one dimension with the positive direction being the direction of motion of the
bullet. Let “A” represent the bullet, and “B” represent the block. Since there is no net force outside
of the block-bullet system (like frictions with the table), the momentum of the block and bullet
combination is conserved. Note that vB  0 .
pinitial  pfinal  mAv A  mB vB  mAvA  mB vB 
vB 
mA  vA  vA 
mB

 0.023 kg  230 m s  170 m s 
2.0 kg
 0.69 m s
9.) A 3800-kg open railroad car coasts along with a constant speed of 8.60 m/s on a level track.
Snow begins to fall vertically and fills the car at a rate of 3.50 kg/min. Ignoring friction with the
tracks, what is the speed of the car after 90.0 min?
Momentum will be conserved in the horizontal direction. Let “A” represent the car, and “B” represent the
snow. For the horizontal motion, vB  0 and vB  vA . Momentum conservation gives the following.
pinitial  pfinal  mAv A   mA  mB  vA
vA 
mA v A
mA  mB

 3800 kg 8.60 m s 
 3.50 kg  90.0 min
3800 kg  


 min 
 7.94 m s  7.9 m s
10.) An atomic nucleus initially moving at 420 m/s emits an alpha particle in the direction of its
velocity, and the remaining nucleus slows to 350 m/s. If the alpha particle has a mass of 4.0 amu
and the original nucleus has a mass of 222 amu, what speed does the alpha particle have when it
is emitted?
Consider the motion in one dimension, with the positive direction being the direction of motion of the
original nucleus. Let “A” represent the alpha particle, with a mass of 4 u, and “B” represent the new
nucleus, with a mass of 218 u. Momentum conservation gives the following.
 mA  mB  v  mAvA  mB vB 
 mA  mB  v  mB vB  222 u  420 m s    218 u  350 m s 
pinitial  pfinal 
vA 
mA

4.0 u
 4.2  103 m s
Note that the masses do not have to be converted to kg, since all masses are in the same units, and a
ratio of masses is what is significant.