Ch 6 - Momentum

1
Ch 6 - Momentum
2
What is momentum?

• Momentum a vector quantity defined as the
  product of an objects mass and velocity
• p mv (momentum mass x velocity)
• SI Unit kgm/s (kilogram meter per second)

3
A 2250 gram toy truck has a velocity of 4 m/s to
the east. What is the momentum of the toy?

• M 2250 g 2.25 kg
• V 4 m/s
• p mv 2.25 x 4 9 kgm/s east

4
Momentum Continued

• A change in momentum takes force and time
• When a soccer ball is moving very fast, the
  player must exert a large force over a short time
  to change the balls momentum and quickly bring
  the ball to a stop

5
Impulse Momentum Theorem

• Impulse for a constant external force, the
  product of the force and the time over which it
  acts on an object OR, the change in momentum of
  an object
• F?t ?p mvf mvi
• Impulse change in momentum
• final momentum initial momentum

6
A 1400kg car moving westward with a velocity of
15 m/s collides with a utility pole and is
brought to rest in 0.30s. Find the magnitude of
the force exerted on the car during the
collision.

• M 1400kg
• ?t 0.30s
• Vi 15 m/s west -15 m/s
• Vf 0 m/s
• F ?

7
6.2 Conservation of Momentum
8
Law of Conservation of Momentum

• The total momentum is conserved
• That is, the total momentum at the beginning of
  the situation has to equal the total momentum at
  the end
• This formula can be used in lots of different
  examples, like collisions, explosions, or when
  objects push away from each other.

9
A 76kg boater, initially at rest in a stationary
45kg boat, steps out of the boat and onto the
dock. If the boater moves out of the boat with a
velocity of 2.5 m/s to the right, what is the
final velocity of the boat?
10
Momentum Continued

• The conservation of momentum fits with Newtons
  Third Law
• Every action has an equal but opposite reaction

11
Real World vs. Physics World

• In real life, forces during collisions are not
  constant
• In physics world, we will work as if we are using
  the average force in our calculations

12
6.3 Elastic and Inelastic Collisions
13
Types of Collisions

• Perfectly Inelastic Collisions
• Two objects collide and stick together, moving
  together as one mass
• Momentum is Conserved

NOTE You will get the same results using the
equation we already learned for conservation of
momentum. This just reminds you that the masses
stuck together!
14
Speed of combined fish 4 km/hr
15
(No Transcript)
16
(No Transcript)
17
Perfectly Inelastic Collisions, Cont.

• Kinetic Energy is NOT constant (conserved) in
  inelastic collisions
• When the two objects stick together, some energy
  is lost
• Deformation of objects (crunching of cars)
• Sound
• Heat

Then compare the initial KE to the final KE to
see how much energy was lost
18
Type of Collisions

• Elastic Collisions
• Two objects collide and then move separately
• Both Momentum and Kinetic Energy are Conserved

19
(No Transcript)
20
Real World vs. Physics World

• In the real world, most collisions are neither
  elastic nor perfectly inelastic
• In physics world, we act as if they fall into one
  of the two categories

21
Review

• Perfectly Inelastic Collision
• Stick together
• Momentum Conserved
• Kinetic Energy NOT Conserved

• Elastic Collision
• Bounce off
• Momentum Conserved
• Kinetic Energy Conserved