Transcript File - Physical Science

Impulse and Momentum
Science Starter
• Show mathematically why an 80,000 pound (36,000
kg) big rig traveling 2 mph (0.89 m/s) has the SAME
MOMENTUM as a 4,000 pound (1,800 kg) sport utility
vehicle traveling 40 mph (18 m/s).
Answer
• Momentum is the product of an object's mass and velocity.
The formula is
• p = mv. The product of each is equivalent.
• The SI unit for momentum is the kilogram x meter/second
(kg x m/s).
• Truck momentum = (36,000 kg)(0.89 m/s) = 32,000 kg x m/s
• SUV momentum= (1,800 kg)(18 m/s) = 32,000 kg x m/s
Remember
• F=ma
• Impulse = Force * Change in Time
• Momentum = Mass * Velocity
Throwing an egg at a Pillow vs. Wall
• If their momenta are equal before the
• Which egg experienced the
collisions (same mass and velocity),
greater impulse, the egg that both eggs experience identical
hit the wall or the bed sheet? impulses because both are stopped
by the collision.
(Be careful here!)
• Which egg experienced the • The egg that hit the wall experienced
the greater impact force due to the
greater force of impact?
shorter impact time.
• Which egg experienced the
• The egg that collided with the bed
greater time of impact?
sheet experienced the greater time of
impact, thereby experiencing a
smaller stopping force over a longer
time interval.
Car Crash
Would you rather be in a head
on collision with an identical
car, traveling at the same
speed as you, or a brick wall?
Assume in both situations you
come to a complete stop.
http://techdigestuk.typepad.com/photos/uncategorized/car_crash.JPG
Car Crash (cont.)
Everyone should vote now
Raise one finger if you think it is
better to hit another car, two if
it’s better to hit a wall and
three if it doesn’t matter.
And the answer is…..
Car Crash (cont.)
The answer is…
It Does Not Matter!
Look at FΔt= mΔv
In both situations, Δt, m, and Δv are
the same! The time it takes you to
stop depends on your car, m is the
mass of your car, and Δv depends
on how fast you were initially
traveling.
Major Theme
• The shorter the impact time, the greater the force!!!
• An impulse is needed to change momentum!
Crash Test Lab
• Procedure:
• Students work with your table partner(s)
• Groups watch their assigned crash test video
• Students research the necessary information to complete the
worksheet
• Students complete equations in order to determine the forces
• Groups look up the crash test rating for their vehicle
• Find your car’s rating:
http://www.consumerreports.org/cro/cars/safetyrecalls/index.htm