Transcript momentumAndImpulserodenx

Unit 7
&
Essential Questions
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Essential Questions:
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In a head-on collision between a Mack
Truck and a Volkswagon Bug, why does
the Mack Truck always win?
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How does an air bag help to reduce the
injury to a person in a car crash?
Momentum
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Any mass in motion
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Inertia depends on mass (Newton’s 1st law)
Momentum = mass x velocity
p  mv
Unit is kg x m/s
Momentum is directly proportional
to both the mass and the velocity.
Momentum is a vector quantity.
p
m v
Example 1
Impulse p
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A change in momentum
Requires a force F  ma
v
F  m( )
t
F  t  m  (v)
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Impulse = force x time
Impulse is how long it takes a force to
change the momentum of an object
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∆p = F x t = m x ∆v
Impulse is directly proportional to both force and
time.
Example 2
Example 3
Example 4
Example 5
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A 1400 kg car moving westward with a velocity of 15 m/s collides
with a utility pole and is brought to rest in 0.30 s. Find the magnitude
of the force exerted on the car during the collision.
Example 6
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A 2250 kg car traveling to the west slows down uniformly from 20.0 m/s to
5.00 m/s. How long does it take the car to decelerate if the force on the car is
8450 N to the east? How far does the car travel during the deceleration?
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When the egg hits the
plate, the time decreases
& force increases
 F  t  mv
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When the egg hits the
pillow, time increases
& force decreases
 F  t  mv
Essential Questions
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In a head-on collision between a Mack Truck
and a Volkswagon Bug, why does the Mack
Truck always win?
The Mack Truck has more Momentum
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How does an air bag help to reduce the injury
to a person in a car crash?
The air bag decreases the force by increasing the
time over which it is applied.
Conservation of Momentum
Newton’s Third Law of Motion
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For every action (force) there is an equal and
opposite reaction (force)
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If the net force is zero then the total change in
momentum is zero
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... in every interaction, there is a pair of forces acting
on the two interacting objects.
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The size of the force on the first object equals the size of the
force on the second object.
The direction of the force on the first object is opposite to
the direction of the force on the second object.
Forces always come in pairs - equal and opposite actionreaction force pairs.
Conservation of Momentum
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Momentum is never lost , but is simply
transferred from one object to another.
Elastic vs. Inelastic Collisions
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Elastic Collisions
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two objects collide and then bounce off each other.
Both objects return to their original shape.
Objects move separately after the collision.
Total kinetic energy is conserved
 ½mv2 1i + ½mv2 2i = ½mv2 1f + ½mv2 2f
Total momentum is conserved
m1v1i  m 2v 2i  m1v1 f  m 2v 2 f
Elastic collision where a larger mass
hits a ball that is initially at rest
Elastic collision where a smaller mass
hits a larger mass
Collisions That are not Head On
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Inelastic Collisions
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Two objects collide and stick together and move
as one mass.
The velocity of the two objects after the collision is
the same (obviously since they are stuck together).
KE is NOT conserved
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Turned into other forms of energy:
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Sound energy
Heat/Thermal energy
Friction
Inelastic Collision
m1v1i  m2 v2i  (m1  m2 )v f
Example 7
Example 8
Example 9