Goal: To understand momentum

1
Goal To understand momentum

• Objectives
• To Learn about What momentum is
• To learn about how to calculate Momentum in 2
  dimensions
• To understand How is momentum changed?
• To understand the Conservation of momentum
• To learn about Why momentum is useful to
  understand.
• In the 2nd hour To learn about applications to
  the conservation of momentum

2
What is momentum?

• You have probably used the word momentum tossed
  out in everyday life but not necessarily 100
  correctly.
• With a neighbor discuss where you have heard
  momentum talked about, and try to figure out from
  that what the average person probably thinks
  momentum means.

3
Will the real momentum please stand up?

• In reality momentum is quite simply a measure of
  your mass times your velocity.
• Momentum mass velocity
• Anytime you have a collision or separation it
  will be a momentum problem
• Lets do some a sample
• 1) A car with mass of 500 kg moves at a velocity
  of 20 m/s. What is the cars momentum?

4
Another example

• Two cars are headed towards one another.
• The first car has 700 kg of mass and moves at a
  velocity of 20 m/s North
• The 2nd car has 1400 kg of mass and moves at a
  velocity of 10 m/s South.
• A) How much momentum does each car have in the
  North direction (yes momentum has direction)?
• B) What is the combined momentum of the cars?

5
Momentum in 2 dimensions

• Each dimension has momentum.
• So, you have to find the total momentum for each
  dimension separately.
• Then at the end you can get a magnitude if you
  want, but usually it is more useful to keep them
  separate much like you keep a checking account
  separate from a savings account.

6
Changing momentum

• How do you change momentum?
• You use what is called an impulse.
• Impulse force time
• Note that force mass acceleration
• So, Impulse mass (acceleration time)
• What does acceleration time equal?

7
Impulse

• Acceleration time change in velocity
• So, Impulse mass change in velocity in
  essence.
• However, you will almost always be given a force
  and time to find it.

8
Example

• A car runs into a mailbox.
• The mass of the mailbox is 10 kg and the mass of
  the car is 800 kg.
• If the car imparts a 2000 N force to the mailbox
  for 0.4 seconds find
• A) The impulse on the mailbox
• B) The new velocity of the mailbox (set impulse
  to mass change in velocity)?
• C) What is the impulse the mailbox imparts on the
  car? (What, you have forgotten about Newtons
  3rd law already?)
• D) How much does the cars momentum change?
• E) What is the net change in momentum (i.e. if
  you add the changes in momentum of the car and
  mailbox what do you get)?

9
Conservation of momentum!

• Momentum is almost always conserved in a
  collision.
• In fact it is conserved for each dimension.
• Quick question will kinetic energy be
  conserved?
• KE 0.5 mass velocity velocity

10
Energy?

• Sometimes kinetic energy is also conserved.
• Collisions that conserve kinetic energy are
  called elastic collisions.
• Collisions where energy is not conserved are
  called inelastic collisions.
• However, what happens to the lost energy for an
  inelastic collision?

11
Oooh, oooh, fender benderThe pips from that
car commercial

• In many collisions energy is transferred.
• Energy is transferred to sound energy, heat
  energy, and used to crumple a car.
• These collisions are always inelastic collisions.
• So, if you get hit by a car, you want it to be an
  elastic collision!
• You will fly faster and further, but the initial
  impact wont use energy to bend and break things.

12
Uses

• Well, using momentum we can better predict what
  will happen in many collisions.
• When might this be useful?

13
Useful when

• Playing pool
• Bowling
• Making safety features for cars or other things
• Making a racecar safe (parts fly off at high
  speed so that the rest of the car can more safely
  loose momentum protecting the driver).
• Military bombs ect especially if you want to
  prevent hurting innocent bystanders
• Sports

14
Conclusion for this hour

• We learned that momentum mass velocity
• Momentum has direction and breaks into dimensions
• Changing momentum requires impulses
• Momentum is conserved even when kinetic energy is
  not
• Knowing about momentum helps

15
In this hour

• We will apply the conservation of momentum to
  some real life problems.
• Example 1 importance of the follow through
• Two hitters hit a 0.6 kg ball coming at them at
  40 m/s South (90 mph).
• The first applies a force of 320 N North for 0.2
  seconds.
• The second applies a force of 80 N North for 1.2
  seconds (he follows through)
• What will the velocities of the hit balls be for
  each hitter and which did a better job of hitting
  the ball?

16
Rear end crash

• A speeding car of mass 800 kg attempting to elude
  the police crashes into a 600 kg car sitting
  parked at the intersection.
• Ignoring breaks and friction, if the initial
  velocity of the speeding car is 50 m/s and the
  final velocity of the speeding car is 10 m/s then
  what will the final velocity of the other car be?

17
Head on collision

• Car 1 25 m/s East and a mass of 800 kg.
• Car 2 30 m/s West and a mass of 900 kg.
• A) What is the momentum of each car.
• B) What is the net momentum of the two cars
  combined.
• C) After the crash Car 1 moves West at a velocity
  of 5 m/s. What will the final velocity of car 2
  be?

18
Ball off a wall

• You bounce a 0.1 kg ball off of the wall.
• The ball hits the wall at 20 m/s and when it
  bounces it returns at 80 of the speed of when it
  hit the wall.
• A) What is the change in velocity for the ball
  (remember direction)?
• B) What is the change in momentum?
• C) If the ball is in contact with the wall for
  0.6 seconds then what is the average force that
  the wall imparts to the ball?
• D) What is the acceleration the wall gives the
  ball?
• E) If you ran into the wall and were given that
  acceleration what would happen?

19
Conclusion

• Momentum mass velocity
• Momentum is conserved!
• Momentum is conserved in every direction!
• If you run into something or it runs into you
  at high velocity dont bounce!