CHAPTER 7 MOMENTUM AND COLLISIONS

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CHAPTER 7MOMENTUM AND COLLISIONS
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MOMENTUM

• Momentum is the product of the mass and velocity
  of a body. Momentum is a vector quantity that has
  the same direction as the velocity of the object.

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MOMENTUM
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PROBLEM 1
A 2250 kg pickup truck has a velocity of 25 m/s
to the east. What is the momentum of the truck?
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IMPULSE-MOMENTUM THEOREM

• Impulse is the product of the average force and
  the time interval during which the force is
  exerted.

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Applications of Impulse Momentum Theorem

• Used to determine stopping distances and safe
  following distances of cars and trucks.
• Used in designing safety equipment that reduces
  force exerted on the object during collisions.
  Example-nets and giant air mattresses for
  fire-fighters.
• Also used in sports equipment and games.

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PROBLEM 3
A baseball of mass 0.14 kg is moving at 35 m/s.
A) Find the momentum of the baseball. B) Find
the velocity of a bowling ball, mass 7.26 kg, if
its momentum is the same as the baseball.
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PROBLEM 6

• The brakes exert a 640 N force on a car weighing
  15689 N and moving at 20 m/s. The car finally
  stops.
• What is the cars mass?
• What is the cars initial momentum?
• What is the change in the cars momentum?
• D) How long does the braking force act on the car
  to bring it to a halt?

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CONSERVATION OF MOMENTUM
States that the momentum of any closed, isolated
system does not change.
Isolated System has no net force acting on
it. Closed System is a system in which objects
neither enter nor leave .
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PROBLEM 11
Glider A of mass 0.355 kg moves along a
frictionless air track with a velocity of 0.095
m/s. It collides with a glider B of mass 0.710 kg
moving in the same direction at a speed of 0.045
m/s. After collision glider A continues in the
same direction with a velocity of 0.035 m/s. What
is the velocity of glider B after collision?
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INTERNAL FORCES
Internal forces are forces between objects
within a system. Example If we consider a
single car as our system, forces are exerted on
objects within the car during a collision (i.e. a
crash dummy)
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INTERNAL FORCES
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EXTERNAL FORCES
External force is a force exerted by an object
outside the system. Example Our single car
from the previous example is still considered our
system. If the car collides with a tree, then
the force the tree exerts on the car is an
external force.
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EXTERNAL FORCES
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PROBLEM 17
Two campers dock a canoe. One camper steps on the
dock. This camper has a mass of 80 kg and moves
forward at 4 m/s. With what speed and direction
do the canoe and the other camper move if their
combined mass is 110 kg?
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Types of Collisions

• ELASTIC objects are apart after the collision
• INELASTIC objects stick together after the
  collision

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COLLISIONS

• INELASTIC
• Momentum is conserved
• Some KE is changed into other forms

• TOTALLY ELASTIC
• Momentum is conserved.
• KE is conserved.

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ELASTIC COLLISIONS

• http//www.youtube.com/watch?NR1vSBesU12g58I

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INELASTIC COLLISIONS

• http//www.youtube.com/watch?vwFoPawE0LxA

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SKATER/MEDICINE BALL EXAMPLE

• A 15 kg medicine ball is thrown at a velocity of
  20 km/hr to a 60 kg skater who is at rest on ice.
  The skater catches the ball and slides with the
  ball across the ice. Determine the velocity of
  the skater and the ball after the collision.

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The collision occurs between a skater and a
medicine ball. Before the collision, the ball has
momentum and the person does not. The collision
causes the ball to lose momentum and the skater
to gain momentum. After the collision, the ball
and the skater travel with the same velocity
("v") across the ice.
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SKATER/MEDICINE BALL EX
Before Collision After Collision
Skater 0 60 v
Medicine ball 300 15 v
Total 300 300
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GRANNY/RALPH EXAMPLE
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GRANNY/RALPH EXAMPLE
Before Collision After Collision
Granny 80 6 480 80 v
Ralph 0 40 v
Total 480 480
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CAR/TRUCK EXAMPLE
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CAR/TRUCK EXAMPLE
Before Collision After Collision
Truck 3000 10 30 000 3000 v
Car 0 1000 15 15 000
Total 30 000 30 000
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Elastic
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Elastic
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Elastic
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Above is a representation of  3 bullets with
equal mass running into 3 blocks of wood with
equal mass. The first bullet passes through the
block and maintains much of its original momentum
  As a result, very little momentum gets
transferred to the block. The second bullet,
expands as it enters the block of wood which
prevents it from passing all the way through it. 
As a result, most of its momentum gets
transferred to the block.  (This is an inelastic
collision.) The third bullet (a rubber bullet)
bounces off the block transferring all of it's
own momentum and then borrowing some more from
the block.  This has the most momentum
transferred to the block.  (This is an elastic
collision.)
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KE and Collisions
Totally (Perfectly) Elastic Collision KE is
conserved no sound or heat produced Example
sub atomic particles (electrons), attracting
fields http//www.youtube.com/watch?vx6n-QgjM4Ss
(Partially) Elastic Collision some KE is lost
as heat or sound Is a broad range containing
most collisions Example billiard balls
http//www.youtube.com/watch?vCgDDiDa3Kzk
http//www.youtube.com/watch?vwFoPawE0LxA
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KE and Collisions
Completely Inelastic Collision KE is conserved
no sound or heat produced Example sub atomic
particles (electrons), repelling fields
http//www.youtube.com/watch?vNN_wwbx6Bew Inela
stic Collision more KE is lost as heat or sound
the colliding objects stick together Example
Coupling railroad cars http//www.youtube.com/watc
h?vqzV8ovAobfE
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PROBLEM 20

• A 2575 kg van runs into the back of a 825 kg
  compact car at rest. They move off together at
  8.5 m/s. Assuming no friction with the ground,
  find the initial speed of the van.

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PROBLEM 21

• A 5g bullet is fired with a velocity of 100 m/s
  toward a 10 kg stationary solid block resting on
  a frictionless surface. What is the change in
  momentum of the bullet if it becomes embedded in
  the block?

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