ub, phy101: chapter 7, pg 1 physics 101: chapter 7 impulse and momentum l today’s lecture will...
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UB, Phy101: Chapter 7, Pg 1
Physics 101: Physics 101: Chapter Chapter 7 7Impulse and MomentumImpulse and Momentum
Today’s lecture will cover Textbook Sections 7.1 - 7.5
UB, Phy101: Chapter 7, Pg 2
UB, Phy101: Chapter 7, Pg 3
UB, Phy101: Chapter 7, Pg 4Demo: change in momentum
UB, Phy101: Chapter 7, Pg 5
Chapter 7, PreflightChapter 7, Preflight
Two identical balls are dropped from the same height onto 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 impulse given to the ball by the floor the biggest? [See conceptual example 3]
1. Case 1
2. Case 2
3. The same The impulse-momentum theory says that the impulse that acts on an object is given by the change in the momentum of the object, and this change is proportional change in velocity. The ball that sticks has a velocity of downward to zero, but the velocity of the ball that bounces goes downward then upward. This change in momentum is greater and therefore has a greater impulse on it.
correct
UB, Phy101: Chapter 7, Pg 6
Chapter 7, PreflightChapter 7, Preflight
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?
1. Upward
2. Downward
time
correct
UB, Phy101: Chapter 7, Pg 7
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) case 1
B) case 2
C) the same
In which case is the average force greater
A) case 1
B) case 2
C) the same
correct
correct
Define momentum conservation
UB, Phy101: Chapter 7, Pg 8
Impulse and Momentum SummaryImpulse and Momentum Summary
Favet I = pf - pi = p
For single object….
F = 0 momentum conserved (p = 0)
For collection of objects …
Fext = 0 total momentum conserved (Ptot = 0)
Fext = mtotal a
UB, Phy101: Chapter 7, Pg 9
Momentum Conservation (Fext=0)Momentum Conservation (Fext=0)
UB, Phy101: Chapter 7, Pg 10
Chapter 7, PreflightChapter 7, Preflight
Is it possible for a system of two objects to have zero total momentum while having a non-zero total kinetic energy?
1. YES
2. NO the example of the ice skaters starting from rest and pushing off of each other...the total momentum is zero because they travel in opposite directions but the kinetic energy is not zero (they start with none and gain a bunch)
if the objects have the same mass and velocities that cancel each other out, the total momentum is zero. but they still have kinetic energy, because they are moving, and the velocities are squared there so direction doesn't matter.
correct
UB, Phy101: Chapter 7, Pg 11
Chapter 7, PreflightChapter 7, Preflight
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?
1. greater than
2. same as
3. less than
If there is no bullet then pbullet = 0 so pgun = 0
correct
pgun = -pbullet
As if ice skater had no one to push…
UB, Phy101: Chapter 7, Pg 12
CollisionsCollisions
“before”
“after”
m1 m2
m1m2
ExplosionsExplosions
“before”
“after”
M
m1m2
• Draw “before”, “after”
• Define system so that Fext = 0
• Set up axes
• Compute Ptotal “before”
• Compute Ptotal “after”
• Set them equal to each other
Procedure
UB, Phy101: Chapter 7, Pg 13
Chapter 7, PreflightChapter 7, Preflight
A railroad car is coasting along a horizontal track with speed V when it runs into and connects with a second identical railroad car, initially at rest. Assuming there is no friction between the cars and the rails, what is the speed of the two coupled cars after the collision?
1. V
2. V/2
3. V/4
4. 0
CORRECT
UB, Phy101: Chapter 7, Pg 14
Chapter 7, PreflightChapter 7, Preflight
What physical quantities are conserved in the above collision?
1. Only momentum is conserved 2. Only total mechanical energy is conserved 3. Both are conserved 4. Neither are conserved
Momentum is conserved because the sum of external forces equal to zero. Total mechanical energy is not conserved because some of the energy is lost when the car runs into the other car.
CORRECT
UB, Phy101: Chapter 7, Pg 15
Chapter 7, PreflightChapter 7, Preflight
Is it possible for a system of two objects to have zero total momentum and zero total kinetic energy after colliding, if both objects were moving before the collision?
1. YES
2. NO
if both objects are moving in oposite directions with the same mass and velocity they would have a resulting velocity of zero.
CORRECT
i really just dont think it is possible. but if i am wrong, i am sure you will have a great demo to make me feel dumb for answering the wrong question.
UB, Phy101: Chapter 7, Pg 16
Some TerminologySome Terminology
• Elastic Collisions: collisions that conserve energy
• Inelastic Collisions: collisions that do not conserve energy
* Completely Inelastic Collisons: objects stick together
Demo: perfectly inelastic collision
UB, Phy101: Chapter 7, Pg 17
UB, Phy101: Chapter 7, Pg 18
H
L LL L
m
M
A projectile of mass m moving horizontally with speed v strikes a stationary mass M suspended by strings of length L. Subsequently, m + M rise to a height of H.
Given H, what is the initial speed v of the projectile?
M + mv
V
V=0
See example 9 in textbook See example 9 in textbook
Chapter 7, Ballistic PendulumChapter 7, Ballistic Pendulum
UB, Phy101: Chapter 7, Pg 19
ExplosionsExplosions
“before”
“after”
M
m1m2
• Example: m1 = M/3 m2 = 2M/3
• Which block has larger momentum?
* Each has same momentum
• Which block has larger velocity?
* mv same for each smaller mass has larger velocity
• Which block has larger kinetic energy?
* KE = mv2/2 = m2v2/2m = p2/2m
* smaller mass has larger KE
• Is energy conserved?
* NO!!
v1v2
UB, Phy101: Chapter 7, Pg 20
Collisions or Explosions in Two Collisions or Explosions in Two DimensionsDimensions
y
x
before after
• Ptotal,x and Ptotal,y independently conserved
Ptotal,x,before = Ptotal,x,after
Ptotal,y,before = Ptotal,y,after
UB, Phy101: Chapter 7, Pg 21
ExplosionsExplosions
“before”M
A B
Which of these is possible?
A
B
both
UB, Phy101: Chapter 7, Pg 22
Assuming Collision is elastic (KE is conserved) Balls have the same mass One ball starts out at rest
Then the angle between the balls after the collision is 90o
ppf
ppi
FF PPf
before after
vvcm
Shooting Pool...Shooting Pool...
UB, Phy101: Chapter 7, Pg 23
Tip: If you shoot a ball spotted on the “dot”, you have a good chance of scratching !
Shooting Pool...Shooting Pool...
UB, Phy101: Chapter 7, Pg 24
Center of MassCenter of Mass
Center of Mass = Balance point
i
cm m
rmrmr 2211
Example 1:
L
m m
Example 2:
L
m 5m
xCM = (0 + mL)/2m = L/2
xCM = (0 + 5mL)/6m = 5L/6
X=0 X=L
UB, Phy101: Chapter 7, Pg 25
Center of Mass (Preflight 6)Center of Mass (Preflight 6)
Shown is a yummy doughnut. Where would you expect the center of mass of this breakfast of champions to be located?
Normally it's the geometric center, but since the geometric center isn't there....i'd have to guess that it'd be located evenly around the donut. The mass is, in other words, evenly distributed around the donut.
Homer: "mmmm.....donut...(slobbering)...center of mass in tummy...." Flanders: "why no diddly-o there Homer. The center of mass would be in the center of the hole." Homer: "Doe!"
UB, Phy101: Chapter 7, Pg 26
Center of MassCenter of Mass
Center of Mass of a system behaves in a SIMPLE way- moves like a point particle!- velocity of CM is unaffected by collision if Fext = 0
Ptot = MtotVcm Fextt = Ptot = MtotVcm
So is Fext= 0 then Vcm is constant
Also: Fext = Mtotacm
UB, Phy101: Chapter 7, Pg 27
UB, Phy101: Chapter 7, Pg 28