Today’s Topics:
 Review
 Perfectly Elastic Collisions
Perfectly Elastic Collisions (no external forces)
Conservation of momentum:
Homework Review
Conservation of energy:
Throw takes 0.2 seconds,
A. Velocity of man?
B. Normal force on ice?
C. KE after the throw
Throw takes 2.4 seconds
D. Velocity of the man?
E. Normal force on ice?
Solving: In a perfectly elastic collision, the relative velocity changes _______,
but not __________.
A 75 kg Dr. Schwartz wearing ice skates
stands motionless on the ice when he
throws a 9 kg block with a velocity as
shown in the figure. Assume he keeps his
legs rigid during the throw and neglect
friction and the motion of his arms. Use
right and up as the positive directions.
Fun with algebra:
Rewrite COM:
Rewrite COE:
Divide COE equation by COM equation:
or
EF 151 Spring, 2017 Lecture 3-6
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Example: Perfectly Elastic Collision
A 14 lb bowling ball rolls down the alley at 27 ft/s. It directly strikes
a single 3.5 lb pin. Assume the collision to be perfectly elastic.
What are the velocities of the bowling ball and pin after the
collision?
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EF 151 Spring, 2017 Lecture 3-6
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Perfectly Elastic Collisions (no external forces)
v1
m2
v2
Billiard Ball
v
Billiard Ball
0
Billiard Ball
v
Bowling
Ball
0
Bowling
Ball
v
Billiard Ball
0
Bowling
Ball
v
Billiard Ball
-v
EF 151 Spring, 2017 Lecture 3-6
An air puck with a mass of 0.15 kg and velocity (-1.7î - 2.0ĵ)m/s collides
with a second air puck of mass 0.22 kg and a velocity of (3.6î)m/s.
Assume a direct, perfectly elastic collision. What are the velocities of
the air pucks after the collision? (magnitude-angle format)
v1  v2  v1  v2 
m1v1  m2 v2  m1v1  m2 v2
m1
Example: 2D Perfectly Elastic Collision
v1’
v2’
A.
B.
C.
D.
E.
-2v
-v
0
v
2v
5
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