Physics 101: Lecture 11 Momentum and Impulse

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Physics 101 Lecture 11Momentum and Impulse

• Todays lecture will cover Textbook Sections
  7.1-7.5

• EXAM TONIGHT
• Bring calculator
• Bring ID
• No cell phones. No exceptions.
• Homework due on Wednesday (not tomorrow)
• No quiz in discussion this week.

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Power (Rate of Work)

• P W / Dt
• Units Joules/Second Watt
• How much power does it take for a (70 kg) student
  to run up the stairs in 141 (5 meters) in 7
  seconds?

P W / t m g h / t (70 kg) (9.8
m/s2) (5 m) / 7 s 490 J/s or 490 Watts
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Last Time

• Last Time Work-Energy
• S F m a multiply both sides by d
• S F d m a d (note a d ½ Dv2)
• S F d ½ m Dv2
• S W DKE Define Work and Kinetic Energy
• This Time Impulse-Momentum

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Momentum and Impulse

• Momentum
• p m v a vector
• units kg m / s
• S F m a multiply both sides by Dt
• S F Dt m a Dt note a Dt Dv
• S F Dt m Dv
• S F Dt Dp
• Impulse
• I S F Dt also a vector
• I Dp

m
v
Fave?t ? I pf - pi ?p
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Preflight 1

• A magician yanks a tablecloth out from under some
  dishes. Briefly explain why the dishes were not
  given much impulse by the tablecloth.

Because time is almost equal to 0, the impulse
is very small.
Since impulse is the change in momentum, then
the change in momentum was very small when the
table cloth was yanked. There was a very small
change in time which would give a small impulse.
tablecloth demo
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ACT
Drop demo

• Two identical balls are dropped from the same
  height onto the floor. In each case they have
  velocity v downward just before hitting the
  floor. In case 1 the ball bounces back up, and in
  case 2 the ball sticks to the floor without
  bouncing. In which case is the magnitude of the
  impulse given to the ball by the floor the
  biggest?
• A. Case 1
• B. Case 2
• C. The same

Bouncing Ball I Dp mvfinal m
vinitial m( vfinal-vinitial) 2 m v
Sticky Ball I Dp mvfinal m
vinitial m( 0-vinitial) m v
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ACT

• In both cases of the above question, the
  direction of the impulse given to the ball by the
  floor is the same. What is this direction?
• A. Upward
• B. Downward

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ACTs
You drop an egg onto 1) the floor 2) a thick
piece of foam rubber. In both cases, the egg does
not bounce. In which case is the impulse
greater? A) Floor B) Foam C) the same
I DP Same change in momentum
In which case is the average force greater A)
Floor B) Foam C) the same
egg demo (uh oh)
Dp F Dt F Dp/Dt Smaller Dt large F
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Pushing Off

• Fred (75 kg) and Jane (50 kg) are on skates
  facing each other. Jane then pushes Fred w/ a
  constant force F 45 N for a time Dt3 seconds.
  Who will be moving faster at the end of the push?
• A) Fred B) Same C) Jane

rubber band glider demo
Fred F 45 N (positive direct.) I 45 (3)
N-s 135 N-s I Dp mvf mvi I/m vf -
vi vf 135 N-s / 75 kg 1.8 m/s
Jane F -45 N Newtons 3rd law I -45 (3)
N-s -135 N-s I Dp mvf mvi I/m vf -
vi vf -135 N-s / 50 kg -2.7 m/s
Note Pfred Pjane (1.8) 75 (-2.7) 50 0!
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Momentum is Conserved

• Momentum is Conserved meaning it can not be
  created nor destroyed
• Can be transferred
• Total Momentum does not change with time.
• This is a BIG deal!

Hammer demo (uh oh)
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Momentum ACT

• A car w/ mass 1200 kg is driving north at 40
  m/s, and turns east driving 30 m/s. What is the
  magnitude of the cars change in momentum?
• A) 0 B) 12,000 C) 36,000 D) 48,000 E) 60,000

Pinitial m vinitial (1200 Kg) x 40 m/s
48,000 kg m/s North Pfinal m vfinal (1200
Kg) x 30 m/s 36,000 kg m/s East
North-South Pfinal Pinitial (0 48000)
-48,000 kg m/s
30 m/s
East-West Pfinal Pinitial (36000 - 0)
36,000 kg m/s
40 m/s
Magnitude Sqrt(P2North P2East ) 60,000 kg
m/s
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Preflight 2 3

• Is it possible for a system of two objects to
  have zero total momentum while having a non-zero
  total kinetic energy?
• 57 1. YES
• 43 2. NO

Jane and Fred
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ACT

• Movies often show someone firing a gun loaded
  with blanks. In a blank cartridge the lead bullet
  is removed and the end of the shell casing is
  crimped shut to prevent the gunpowder from
  spilling out. When a gun fires a blank, is the
  recoil greater than, the same as, or less than
  when the gun fires a standard bullet?
• A. greater than
• B. same as
• C. less than

If there is no bullet then pbullet 0 so pgun 0
As if ice skater had no one to push
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Summary

• Momentum p mv
• Momentum is VECTOR
• Momentum is conserved (when SF 0)
• S mvinitial S mvfinal
• Impulse I FDt
• Gives change in momentum I Dp
• Chapter 7, problems 3, 5, 13, 21

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