Mechanics Lecture 11, Slide 1 Physics 211 Lecture 11 Todays Concept: Inelastic Collisions

Mechanics Lecture 11, Slide 1

Physics 211Lecture 11

Today’s Concept:Inelastic Collisions


Hour Exam 1Hour Exam 1High Score of 92%High Score of 92%

Charles HuhCharles HuhYixin YuYixin Yu

Mean score of 75.1%Mean score of 75.1%


Hour Exam 1Hour Exam 1High Score of 92%High Score of 92%

Charles HuhCharles HuhYixin YuYixin Yu

Mean score of 75.1%Mean score of 75.1%

Repeat of an important tip:Repeat of an important tip:NeverNever write an equation write an equation before making a diagram.before making a diagram.Don’t do anything before Don’t do anything before making a diagram.making a diagram.Diagrams are Diagrams are importantimportant..


What is the difference between conservation of Momentum and conservation of energy? How do I know if the Momentum is conserved or energy is conserved?

Momentum (Prelecture 11)

Energy (Prelecture 8)

Demos: NewtonDemos: Newton’’s cradle,s cradle,Happy/sad ballsHappy/sad balls


tot i ii

P m v

Collisions

Demo: Inelastic collision on air trackDemo: Inelastic collision on air track


ACT

A) Horizontal momentumB) Kinetic energyC) Both of the above.D) Neither.

Before After

As long as there are no external forces acting on the system

A wood block rests at rest on a table. A bullet shot into the block stops inside, and the bullet plus block start sliding on the frictionless surface. Which of the following (for the bullet+block system) remain constant during the collision?


p1

p2

p2

p1

pinit

pfinal

Homework Problem


CheckPoint

A) Yes, and it moves to the right.B) Yes, and it moves to the left.C) No, it remains in place.

Left Right

Conservation of momentum means the cart must move to the right since the ball moves to the left.

Suppose you are on a cart initially at rest that rides on a frictionless track. If you throw a ball off the cart towards the left, will the cart be put into motion?


? ? ? ?? ? ? ?Tell me moreTell me more

Tell me moreTell me more

Why? Why not? Tell me moreWhy? Why not? Tell me more



Left Right

CheckPointSuppose you are on a cart which is initially at rest that rides on a frictionless track. You throw a ball at a vertical surface that is firmly attached to the cart. If the ball bounces straight back as shown in the picture, will the cart be put into motion after the ball bounces back from the surface?


Left Right


CheckPoint


““spot on”spot on”


Great observation!Great observation!


““other”?other”?

? ? ?? ? ?! ! ! ! !! ! ! ! !

! ! ! ! !! ! ! ! !

What’s the What’s the finalfinal result? result?Which direction is “back”?Which direction is “back”?

Left? Good.Left? Good.

Great observation!Great observation!


ACT

A) IncreaseB) DecreaseC) Will not change

v

As long as there are no external forces acting on the system, Ptotal is conserved

Suppose you are on a cart that is moving at a constant speed v toward the left on a frictionless track. If you throw a massive ball straight up (relative to the cart), how will the speed of the cart change?

Left Right


Two balls of equal mass are thrown horizontally with the same initial velocity. They hit identical stationary boxes resting on a frictionless horizontal surface. The ball hitting box 1 bounces back, while the ball hitting box 2 gets stuck.

Which box ends up moving faster?

A) Box 1 B) Box 2 C) same

CheckPoint

1 2


Two balls of equal mass are thrown horizontally with the same initial velocity. They hit identical stationary boxes resting on a frictionless horizontal surface. The ball hitting box 1 bounces back, while the ball hitting box 2 gets stuck.

Which box ends up moving faster?

A) Box 1 B) Box 2 C) same

CheckPoint

1 2


The beginning of understanding.The beginning of understanding.

Momentum is also conserved in the first case.Momentum is also conserved in the first case.

I don’t know I don’t know howhow to dissipate momentum. to dissipate momentum.

Is “more conserved” like “more equal”?Is “more conserved” like “more equal”?

But the collisions are But the collisions are notnot identical. identical.

? ? ? ? ? ? ? ? ? ? True. How does that affect this? True. How does that affect this?

? ? ? ? ? ? ? ? ? ? No.No.

No.No.

A) AB) BC) They both have the same chance.


ACT

A B

Two equal-mass balls swing down and hit identical bricks while traveling at identical speeds. Ball A bounces back, but ball B just stops when it hits the brick. Which ball is more likely to knock the brick over?


A B

PAPB

The change in the momentum of the ball is bigger in A

PA > PB

Demo: super/clay ball pendulumDemo: super/clay ball pendulum


Hm

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?

v

Ballistic Pendulum

Show us a difficult problem that would involve momentum.

Which quantities are conserved before the collision?

A) momentum (horizontal)B) mechanical energyC) both momentum and mechanical energy

mM

vv

H

before during after

splat

Breaking it down into steps


Which quantities are conserved during the collision?

A) Momentum (horizontal) B) mechanical energyC) both momentum and mechanical energy

mM

vv

H

before during after

splat


( )mv m M V


Which quantities are conserved after the collision

A) momentum (horizontal)B) mechanical energyC) both momentum and mechanical energy

mM

vv

H

before during after

splat


2 2V gH


( )mv m M V ( )mV v

m M

2 2V gH

2 m Mv gHm

2 1 Mv gH

m

Hm

Mv

Ballistic Pendulum


Ballistic Pendulum Demo

Physics 211 Lecture 11, Slide 24

In the demo we measure forward displacement d, not H:

HmMv

L LL L

M m

d

L

d H

LH 222 )( HLdL

22 dLLH

for d << L

Lgd

mMv

1

v(mph) 3d(cm)

Final thoughts


Have we covered these?Have we covered these?

Documents

Mechanics Lecture 11, Slide 1 Physics 211 Lecture 11 Todays Concept: Inelastic Collisions