Law of Conservation of
Momentum
The Law of Conservation for
Momentum
• Momentum is always conserved in a collision.
• It is never created or destroyed! (Just like
Energy)
INDESTRUCTABLE
• It can only go back and forth between objects.
Conceptual example
• Dodgeball Game: The ball is thrown at
someone. Dodgeball is moving (has
momentum), the person is not (no momentum).
• The dodgeball has all the momentum until it hits
the person. (collision)
• Then, it transfers some of its momentum to the
person, and the dodgeball has some left over.
-
• Before
• p = a lot
+
After
p=0
p = a little
p = a little
X
The TOTAL momentum is the
same on both sides!
X
Elastic Collisions
• Momentum can only be transferred through
collisions.
• Collisions where objects bounce off each other
are called elastic collisions.
Examples: dodgeball, pool, marbles, golf.
Elastic Collision
• A 2 kg dodgeball was going +10 m/s before it hit
someone (who was standing still). The guy’s
head has mass of 5 kg. After they collide, the
dodgeball has a velocity of -3m/s. What is the
speed of the guy’s head?
Practice
• Page 219; 1a, 2a, 3a ONLY
• Set up each problem using the chart from
your notes.
Inelastic Collisions
• When two objects collide and become
“stuck together” momentum is still
transferred, but the collision is called
inelastic.
• Examples: ballistics (bullets), football
tackles, car collisions
Bullet Example
• A bullet of mass 0.1 kg is fired at a speed
of 300 m/s into a block of wood with mass
10 kg. The bullet becomes stuck in the
block. What is the velocity of the block
with the bullet inside after the collision?
Practice
• Page 214; 1-3
• Set up each problem using the chart from
your notes.