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Physics Chapter 6. Impulse and Momentum. Momentum. Momentum depends on… Mass Velocity Momentum = Mass x Velocity The property of momentum… Variable: p Unit: Scalar or Vector ? Equation: p = mv. Example #1. - PowerPoint PPT Presentation
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Physics Chapter 6
Impulse and Momentum
Momentum• Momentum depends on…
– Mass– Velocity– Momentum = Mass x Velocity
• The property of momentum…– Variable: p– Unit:
– Scalar or Vector?
– Equation: p = mv
kg m
s
Example #1
Which has more momentum, a 1-ton car moving at 100 km/h or a 2-ton truck moving at 50 km/h?
Example #2
How fast would a 18 kg kid on a tricycle have to go in order to have the same momentum as a 55 kg car traveling at 25 m/s?
Impulse• Momentum can change. Most often, the
mass of an object remains the same, while the velocity changes.
• p = mv• v acceleration a = v/t v=at• p = mv becomes p = m x a x t• p = Force x time = Ft• This is called Impulse
– I = Ft
F
Example #3If a baseball bat applies a 22-N force to a baseball for 0.13 s, what is the impulse experienced by the ball?
Impulse & Momentum
• Impulse equals a change in momentum
• I = p
• VariationsFt = mv
Ft = mvf – mvi
I = mv
I = mvf – mvi
Ft = p
Example #4
What is the impulse needed to stop a 10-kg bowling ball moving at 6 m/s?
Example #5A car crashes into a wall at 25 m/s and is brought to a rest in 0.1 s. Calculate the average force exerted on a 75-kg test dummy by the seat belt.
Impulse & MomentumIncreasing Momentum
mv = FtIncrease the time to increase the velocity
• Examples:– Follow through in sports
• Basketball shoot• Baseball hit• Soccer kick• Golf swing
– Long-range cannons have long barrels
Impulse & MomentumDecreasing Force
mv = FtIncrease the time to decrease the Force
• Examples:– Catching
• Egg toss• Water balloon• Football
– Cars: Crumple Zones & Air Bags– Roll with the punches– Mighty Ducks– Jumping and landing (sprung floors)– Running Shoes– Any padding– Bungee Jumping
Problems
• Review Questions1-11, 13
• Exercises1-4, 6, 9, 10
• Problems1, 2, 5
Conservation of Momentum• Newton’s Second Law…
– If we want an object to accelerate, we must apply a force.
• Impulse and Momentum…– If we want a change in momentum, we must
apply an impulse.
• In both cases, the force or impulse must be exerted on the object or any system of objects by something external.
Conservation of Momentum
• If we define the system to include all the objects we want to study…
all forces become internal; there can be no impulse; no impulse means no change in momentum!
• When a physical quantity remains unchanged during a process, that quantity is said to be…
CONSERVED.
Collisions• Momentum is conserved
Momentum before = Momentum after
• Three types of collisions…– Elastic objects rebound off each
other without lasting deformation.– Inelastic objects are deformed by
the collision; think car crashes.– Perfectly inelastic both objects
stick together
Example #1aThe blue car is moving at 10 m/s, with a mass m. The orange car, also mass m, is at rest. If the freight cars are coupled together by the collision, what is their combined velocity?
v = 0
v = 10 m/s v = ?
Example #1bIf the freight cars in the previous example do not couple and experience an elastic collision, what is the velocity of the orange car?
v = 0
v = 10 m/s v = ?
v = 0
Example #2Two trucks with equal masses, m, experience a head on collision. After collision, the coupled wreck remains at the point of impact with zero momentum. If the green truck was moving at 10 m/s, how fast was the red truck moving?
v = ?v = 10 m/s v = 0
Example #3A 3 kg object traveling at 5 m/sec collides head-on with a 2 kg object that is traveling at 2 m/sec in the opposite direction. What is the velocity of the second object after the collision if the first object is moving at 1 m/sec?
Problems
• Review Questions18-23
• Exercises13, 21
• Problems6-10