Momentum

1
Chapter 7

• Momentum

2
7.1 Momentum

• Mass is inertia and momentum is inertia in motion
• momentum mass velocity
• momentum mv
• Measured in N?s or kg?m/s
• Moving object can have large momentum if it has a
  large mass, a high speed, or both.

3
7.2 Impulse Changes Momentum

• Impact is force and impulse is force time
• A force sustained for a long time produces more
  change in momentum than does the same force
  applied briefly
• Impulse Ft
• Impulse change in momentum
• Therefore, Ft ?mv
• Measured in N?s

4
Math Practice

• What is the momentum of a 10-kg bowling ball
  rolling at 3 m/s?
• If the bowling ball rolls into a pillow and stops
  in 0.5 seconds, calculate the average force it
  exerts on the pillow
• What impulse occurs when an average force of 20 N
  is exerted on a cart for 3 s?
• What change in momentum does the cart undergo?

5
Your Turn

• What is the momentum of a 20-kg carton that
  slides at 5 m/s across an icy surface?
• The sliding carton skids onto a rough surface and
  stops in 2 s. Calculate the force of friction it
  encounters.
• What impulse occurs when an average force of 10 N
  is exerted on a cart for 2 s?
• What change in momentum does the cart undergo?

6
Case 1 Increasing momentum

• Apply a large force for the longest amount of
  time possible
• Example Golf club hitting the golf ball
• Example Bat hitting a baseball

7
Case 2 Decreasing Momentum

• Example Car and haystack
• Example Landing a jump

8

• Example Riding with the punch
• Example Egg catch
• Same change in momentum (impulse), but if the
  time is large, the force is small and if the time
  is small, the force is large

9
7.3 Bouncing

• Impulses are greater when an object bounces
• Greater impulse is required to stop an object and
  then throw it back than to simply stop it
• Example Karate chop
• Example Pelton Wheel

10
7.4 Conservation of Momentum

• Law of conservation of momentum in the absence
  of an external force, the momentum of a system
  remains unchanged
• Momentum is conserved
• Momentum is a vector quantity
• It has magnitude and direction
• Example Rifle and bullet

11
7.5 Collisions

• Momentum is conserved in collisions
• net momentum before collision net momentum
  after collision
• Elastic Collisions
• When objects collide without being permanently
  deformed and without generating heat

12

• Inelastic Collisions
• When colliding objects become tangled or couple
  together

13
Math Practice

• A 4-kg blob of putty moving at 6 m/s slams into a
  4-kg blob of putty at rest
• Calculate the speed of the two stuck-together
  blobs of putty immediately after colliding.
• Calculate the speed of the two blobs if the one
  at rest was 2 kg

14
7.6 Momentum Vectors

• Momentum is conserved in all directions (not just
  in a straight line)