ub, phy101: chapter 7, pg 1 physics 101: chapter 7 impulse and momentum l today’s lecture will...

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 4Demo: change in momentum


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



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


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


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


Momentum Conservation (Fext=0)Momentum Conservation (Fext=0)



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



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…


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



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



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



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.


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


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



“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


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



“before”M

A B

Which of these is possible?

A

B

both


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...


Tip: If you shoot a ball spotted on the “dot”, you have a good chance of scratching !

Shooting Pool...Shooting Pool...


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


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!"


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 1 physics 101: chapter 7 impulse and momentum l today’s lecture will...

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