Physics 218: Mechanics Instructor: Dr. Tatiana Erukhimova Lecture 2.
Physics 218: Mechanics Instructor: Dr. Tatiana Erukhimova Lecture 18.
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Transcript of Physics 218: Mechanics Instructor: Dr. Tatiana Erukhimova Lecture 18.
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