Download Conservation of Momentum

Conservation of Momentum
Newton’s Third Law
• For every action, there is an equal and
opposite reaction.
https://www.youtube.com/watch?v=1xQozXJKbGI
https://www.youtube.com/watch?v=y97tlkGohoc
Conservation of Momentum
• Closed system
– does not gain or lose mass
– no external forces
• Law of Conservation of Momentum- The
momentum of any closed system does not
change.
• Velocity is a vector, so that makes Momentum a
vector!
• http://www.pbs.org/opb/circus/classroom/circusphysics/linear-momentum/
Collisions
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Elastic- KE does not change
Happens between hard objects (steel)
They hit and bounce
Inelastic- KE decreases (converted to heat)
Happens between soft, sticky objects (clay)
They stick together after colliding!
Equations
P = mv
Find the momentum of a 15 kg dog running at 5 m/s:
Elastic Collisions:
• M1V1(initial) + M2V2(initial) = M1V1(final) + M2V2 (final)
Inelastic Collisions:
• M1V1(initial) + M2V2(initial) = (M1 + M2) V(final)
Collision Problems
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Figure out what type of collision is taking place.
Use the correct equation! (elastic or inelastic?)
Make sure to identify direction!
Solve for the unknown variable.
• Two billiard balls are traveling in the same
direction. Each has a mass of 500 g. Ball #1 is
traveling at 4 m/s, while ball #2 is traveling at 6
m/s. They hit and stick together. How fast are
they moving together?
• M1V1(initial) + M2V2(initial) = (M1 + M2) V(final)
• (0.5 kg)(4 m/s) + (0.5 kg)(6 m/s) = (0.5 + 0.5) Vf
• Vf = 5 m/s
• Two bumper cars bounce against each other headon. One car has a mass of 75 kg and is traveling 6
m/s. The other car has a mass of 65 kg and is
traveling 4 m/s. They hit and bounce off each other.
If the 75 kg car is moving at 3 m/s after the
collision, how fast is the other car moving?
• M1V1(initial) + M2V2(initial) = M1V1(final) + M2V2 (final)
• (75)(6) + (65)(-4) = (75)(-3) + (65) V2 (final)
• V2 (final) = 6.4 m/s
Bellwork
• What is the mass of a duck with a
momentum of 135 kg m/s flying at a
velocity of 24 m/s?
Two ice skaters, with masses of 45 kg and 65 kg, are at
rest and facing each other over a surface of ice. They
push off from each other and the 45 kg skater moves
away with a velocity of 6 m/s. Find the final velocity
of the 65 kg skater.
Impulse
• What does it mean to “follow-through?” in
sports?
• Increase the amount of time that the force is
in contact with the object
• Leads to greater velocity of object
• The product of Force and the Time over
which it is applied is called Impulse (J)
J = FΔt
• F Δt = Δp
Impulse is change in momentum
• F Δt = m Δv
• F Δt = m (vf-vi)
A soccer player kicks a 0.1 kg ball that is
initially at rest so that it moves with a velocity
of 20 m/s. What is the impulse the player
imparts to the ball? If the player’s foot was in
contact with the ball for 0.01 s, what was the
force exerted by the player’s foot on the ball?
•Minimizing the impact force:
•What do you do when you land from a jump of
any height?
•A boxer moves away from a punch to increase
impact time
Δt
F
= Δp
•Maximizing the impact force:
•Hulk smash, karate, etc.
http://www.youtube.com/watch?v=JgFWDkhkOk&NR=1&feature=endscreen
Tiger Woods hits a 0.050 kg golf ball, giving it a
speed of 75 m/s. What impulse does he impart to
the ball?