You are standing on a sheet of ice that covers the football stadium parking lot in Buffalo; there is
negligible friction between your feet and the ice. A friend throws you a 0.400-kg ball that is
traveling horizontally at 10.0 m/s. Your mass is 70.0 kg. (a) If you catch the ball, with what
speed do you and the ball move afterward? (b) If the ball hits you and bounces off your chest, so
afterward it is moving horizontally at 8.0 m/s in the opposite direction, what is your speed after
the collision?
Identify: Apply conservation of momentum to the system of you and the ball. In part (a) both
objects have the same final velocity.
Set Up: Let +x be in the direction the ball is traveling initially.
0.400 kg mA

(ball).
70.0 kg mB

(you).
Execute:
(a)
P P 1 2 x x

gives
2
(0.400 kg)(10.0 m/s) (0.400 kg 70.0 kg)   v
and
2
v  0.0568 m/s
.
(b)
P P 1 2 x x

gives
2
(0.400 kg)(10.0 m/s) (0.400 kg)( 8.00 m/s) (70.0 kg) B    v
and
2 0.103 m/s B
v 
.
Evaluate: When the ball bounces off it has a greater change in momentum and you acquire a
greater final speed.