Physics 211

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relative kinetic energies in different frames of
references What is an elastic collision,
inelastic collision, and completely elastic
collision? And are there any other types of
collisions I should know about? I think we should
have more days off of lecture in the
future." Very good prelecture, I felt like I
understood all of it." i hereby declare that this
will be the last attempt to get on the board for
the rest of the year. I get depressed when I go
without physics for a week ( I'm glad the soy
bean aphids are gone. Thanks.When I die, I want
to go peacefully like my Grandfather did, in his
sleep -- not screaming, like the passengers in
his car. The pudding is a lie Why can't Juice NOT
turn the ball over? Seriously...oh by the way,
the last question was a little confusing, but not
as confusing as our offense. I think, because I
usually end up doing these later during the night
before the lecture, there's usually not a chance
to end up having my comments read with the
slightest intention of putting them on the board.
So maybe even writing this now is something of an
irony, but perhaps the comments are compiled the
morning of. Well... who knows. It's off to bed!
And maybe just some practice with the kinetic
energy of a system, thank you for asking. These
are all really easy, I mean I am the second
coming of Einstein.
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Physics 211 Lecture 13
Today's Concepts More on elastic collisions,
Average force during collisions
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Motivator

• Get 80 on all of the clicker questions and Ill
  play the worlds best song ?
• (and you can see a video of Mats doing a demo
  from this class on TV)

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The prelecture said that the Kinetic energy of a
system of particle would be different for
different relative observer frame. how does that
make sense?
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More on Elastic Collisions
In CM frame, the speed of an object before an
elastic collision is the same as the speed of the
object after.
m2
v1,i
v2,i
m1
m2
v2,f
m1
v1,f
So v1,i - v2,i v1,f - v2,f
But the difference of two vectors is the same if
we add the same constant to both (which is what
you do if you view them in another reference
frame).
Rate of approach before an elastic collision is
the same as the rate of separation afterward, in
any reference frame!
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Act

• Consider the two elastic collisions shown below.
  In 1, a golf ball moving with speed V hits a
  stationary bowling ball head on. In 2, a bowling
  ball moving with the same speed V hits a
  stationary golf ball.
• In which case does the golf ball have the greater
  speed after the collision?

A) 1 B) 2 C) same

V
V
1
2
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Act

• A small ball is placed above a much bigger ball,
  and both are dropped together from a height H
  above the floor. Assume all collisions are
  elastic.
• What height do the balls bounce back to?

H
A
B
C
Before
After
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Explanation

• For an elastic collision, the rate of approach
  before is the same as the rate of separation
  afterward

m
v
v
M
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Preflight
A block slides to the right with speed V on a
frictionless floor and collides with a bigger
block which is initially at rest. After the
collision the speed of both blocks is V/3 in
opposite directions. Is the collision elastic? A)
Yes B) No C) Need more information
V
Before Collision
V/3
V/3
After Collision
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Forces during Collisions
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Two identical blocks, A and B, initially are
moving on frictionless air tracks. The initial
speed of block A is twice as that of block B. You
now apply the same constant force to both blocks
for exactly one second.
Act
v
2v

• The change in momentum of block A is
• A) Twice the change in momentum of block B
• B) The same as the change in momentum of block B
• C) Zero.
• D) Half the change in momentum of block B

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Act

• Two boxes, one having twice the mass of the
  other, are initially at rest on a horizontal
  frictionless surface. A force F acts on the
  lighter box and a force 2F acts on the heavier
  box. Both forces act for exactly one
  second.Which box ends up with the biggest
  momentum?

A) Bigger box B) Smaller box
C) same
F
2F
2M
M
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A constant force acts for a time Dt on a block
that is initially at rest on a frictionless
surface, resulting in a final velocity V. Suppose
the experiment is repeated on a block with twice
the mass using a force thats half as big. For
how long would the force have to act to result in
the same final velocity?
Preflight

• A) Four times as long.
• B) Twice as long.
• C) The same length.
• D) Half as long.
• E) A quarter as long.

Only 50 got this right Lets try it again !
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The experiment is repeated on a block with twice
the mass using a force thats half as big. For
how long would the force have to act to result in
the same final velocity?

• A) Four times as long.
• B) Twice as long.
• C) The same length.

A) the change in momentum when the experiment is
repeated is twice the amount of the first time,
and since the force is half as big, when you
solve for dt it will be 4 times as big as the
initial experiment.
B) Mass does not matter, only the force.
C) The forces and the mass canceled out
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Preflight
Identical balls are dropped from the same initial
height and bounce back to half the initial
height. In Case 1 the ball bounces off a cement
floor and in case 2 the ball bounces off a piece
of stretchy rubber. In which case is the average
force acting on the ball during the collision
the biggest? A) Case 1 B) Case 2 C)
Same in both cases
Only 59 got this right Lets try it again !
Case 2
Case 1
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In which case is the average force acting on
the ball during the collision the biggest? A)
Case 1 B) Case 2 C) Same in both cases
A) the force on the ball depends on the time it
is in contact with the floor. since it is much
less in case 1, then the force is much larger
B) its in contact with the ball for longer
C) If both balls reach the same height the
impulse will be the same because they both leave
with the same speed.
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Egg Throwing Video
The best song ever (well maybe not)