Workshop Lecture (From Physics 101) (Newton's Laws

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Workshop Lecture (From Physics 101) (Newton's
Laws Buoyancy)

• Assumptions
• You, as students, have done the preflights on
  the web.
• 16/24 people did (thanks!).
• You, as students, have read the textbook.
• I know you havent, however for this lesson it
  wont matter much.
• This material would usually be spread over
  several lectures.
• Lectures usually have quantitative examples (but
  not today)

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Newton's Laws
1. An object moving with constant velocity
will keep moving with that same velocity (both
speed and direction) unless a force acts on it.
(demo)
Both magnitude direction
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Act 1 (Pre-Flights 1-2)

• Driving your car on I-57 you encounter a bug
  which (sadly) splatters on your windshield.
  During the collision between the car and the bug
• 1. The force exerted by the car on the bug is
  BIGGER than the force exerted by the bug on
  the car.
• 2. The force exerted by the car on the bug is
  SMALLER than the force exerted by the bug on
  the car.
• 3. The force exerted by the car on the bug is THE
  SAME AS than the force exerted by the bug on
  the car.

VOTE
The car has greater mass. I think force is
something like a product of mass and change in
speed.
The car had higher speed.
For every action there is an equal and
opposite reaction.But a bug can't withstand the
same amount of force as a windshield, so it
squishes.
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Act 1 (Pre-Flights 1-2)
This is a beautiful response
I seem to remember a phrase stating that for
every force, there is an equal and opposite
force. For example, if I push against the wall,
there is an equal and opposite force pushing
back. But, if I push against a door and it
closes, I have to reason that the opposite force
cannot be equal because the door is moving. I
guess I feel that way about the bug.
Can anyone see the very subtle flaw in this
argument ??
This was part of the same answer - it reminds
me of a line from the Man of La Mancha, whether
the stone hits the pitcher or the pitcher hits
the stone, it's going to be bad for the pitcher.
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Act 2

• Follow-up During the collision between the car
  and the bug, which one experiences the greatest
  acceleration?
• 1. The car has a greater acceleration.
• 2. The bug has a greater acceleration.
• 3. The accelerations will be the same.

VOTE
F m a
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Act 3 (Pre-Flights 3-4)

• In Case 1 shown below, a weight is hung from a
  rope (over a pulley) and is attached to one side
  of a spring. The other side of the spring is
  attached to a wall using a second rope. In Case
  2, instead of being attached to a wall, the
  second rope is attached to a second identical
  weight.

In which case is the spring stretched the most?
1. Case 1 2. Case 2 3. Same in both cases
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• In which case is the spring stretched the most?
• 1. Case 1
• 2. Case 2
• 3. Same in both cases

VOTE
The net force on the spring is zero in each
case. In both cases the weight on the right
exerts the same force W. So there must be an
equal force on the left side of each spring
Tension exerted on the string attached to the
wall is equal to the downward force of the
weight. When the wall is replaced by a weight of
equal mass, then like the wall it exerts tension
on the string that is equal to the downward force
of the opposite weight.
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• In which case is the spring stretched the most?
• 1. Case 1
• 2. Case 2
• 3. Same in both cases

Case 1-fixed on one end therefore all of
weight acts to distract coils of spring.Case
2-opposite distractive forces but ends not fixed
so can move to pint where opposite forces
"cancel" each other out and eliminate distractive
forces on spring
The stretched spring must counter 1 times
weight in case 1, and 2 times weight in case 2.
There seems to be more total weight pulling on
the spring.
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Buoyancy
DEMOS
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Act 4 (Pre-Flights 5-6)

• An ice cube floats in a full glass of water as
  shown below.
• When the ice melts, the level of the water will
  
• 1. Go up, causing the water to spill out of the
  glass.
• 2. Go down.
• 3. Stay the same.

VOTE
The amount of ice that projects above the
water line is exactly equal to the extra volume
that is created when the liquid water is expanded
by freezing. The volume of the liquid water from
the melted ice cube will equal the volume of the
ice cube that is below the water.
The amount of liquid displaced is equal to
the weight of the object it is supporting. Even
though the density of ice is less than water, the
mass, and therefore the volume in the liquid
form,is the same.
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Act 4 (Pre-Flights 5-6)

• An ice cube floats in a full glass of water as
  shown below.
• When the ice melts, the level of the water will
  
• 1. Go up, causing the water to spill out of the
  glass.
• 2. Go down.
• 3. Stay the same.

VOTE
The idea is that a volume of a liquid is
increasing when some more liquid is added, and
ice melts into water eventually and takes more
space than the glass can hold
Ice cubes take up a larger volume than water.
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• My Favorite Answer

Archimedes's principle says that the weight
of water displaced equals the weight of the body.
When the ice melts, it provides exactly this
amount of water to the glass.
Now consider a boat floating on
a lake, and what happens to the level of the lake
when the boat's anchor is thrown overboard. ACT
5 Does the level of the lake 1. Go up2.
Go down3. Stay the same
VOTE