Physics 218: Mechanics Instructor: Dr. Tatiana Erukhimova

Lecture 18

>80

Exam 2 results

0

5

10

15

20

1 2 3 4 5 6 7

score

nu

mb

er

of

stu

de

nts

Series1

>80 >70 >60 >50 >40 >30

0

5

10

15

20

25

1 2 3 4 5

grade

nu

mb

er

of

stu

de

nts

Series1

>80=A >70=B >55=C >38=D

Average=62.4 Median = 66High=95

Sec. 807 – 59.2Sec. 808 – 67.8Sec. 809 – 59.8

Momentum is a vector!

vmp

cmi

N

ii

N

iitotal vMvmpP

11

N

iiicm vm

Mv

1

1

cmtotal vMP

Vector equation!

dt

Pd

dt

vdMFaM totalcm

externalcm

externaltotal Fdt

Pd

If ,0externalF

0dt

Pd total

ConstPtotal

Conservation of Momentum

If there is no external force on a system, then the total momentum of the system is a constant

ConstPtotal

)()( afterPbeforeP

True in X and Y directions separately!

totalx

totalx Fdt

dP total

y

totaly Fdt

dP

Problem Solving

For Conservation of Momentum problems:

1. BEFORE and AFTER

2. Do X and Y Separately

Before

X

Y

After

YX

Inelastic collision

A collision in which the total kinetic energy after the collision is not equal to the kinetic energy before the collision is called an inelastic collision.

A

Av

B

0BvBEFORE

AFTERA B

Vafter?

Perfectly elastic collision

A collision in which the total kinetic energy after the collision is the same than that before the collision is called an elastic collision.

A B

Av

A block of mass m is moving along x axis with a velocity of V0. It collides with a block of mass M, initially at rest.1) What is the change in kinetic energy of the system of two balls:

a) if the collision is perfectly elastic;b) if the collision is perfectly inelastic

(balls stick together after collision).2) For m = M = m0, find the velocity of each ball after a perfectly elastic collision.

Quiz 1

Problem 2 from handout

Have a great day!

Reading: Chapter 10Hw: All Chapter 10 problems and exercises