Workshop Lecture (From Physics 101)
(Newton's Laws & Buoyancy)
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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 haven’t, however for this lesson it won’t
matter much.
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This material would usually be spread over several lectures.
Lectures usually have quantitative examples (but not today)
4/10/01 Workshop, Pg 1
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)
a
2. Ftot = ma
(demo)
Ftot
m
Both magnitude
& direction
FF-C
3. If object 1 exerts a force F on object 2,
then object 2 exerts an equal but
opposite force (-F) on object 1. (demo)
FC-F
4/10/01 Workshop, Pg 2
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.
VOTE
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.
Preflight responses
correct
The car has greater mass. I think force is something like
a product of mass and change in speed.
The car had higher speed.
38%
0%
62%
0%
20%
40%
60%
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.
80%
4/10/01 Workshop, Pg 3
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.
4/10/01 Workshop, Pg 4
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.
correct
VOTE
3. The accelerations will be the same.
F = m a
bug
car
=
bug
car
x
bug
car
4/10/01 Workshop, Pg 5
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.
down
wall
Case 1
Case 2
In which case is the spring stretched the most?
1. Case 1
2. Case 2
3. Same in both cases
4/10/01 Workshop, Pg 6
down
In which case is the spring stretched the most?
1. Case 1
2. Case 2
3. Same in both cases
wall
Case 1
correct
VOTE
Case 2
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
6%
38%
56%
0%
20%
40%
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.
60%
4/10/01 Workshop, Pg 7
down
In which case is the spring stretched the most?
1. Case 1
2. Case 2
3. Same in both cases
wall
Case 1
correct
Case 2
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
6%
38%
The stretched spring must counter 1 times weight in case 1,
56% and 2 times weight in case 2.
There seems to be more total weight pulling on the spring.
0%
20%
40%
60%
4/10/01 Workshop, Pg 8
Buoyancy
This is why an object floats if
it is less dense than the liquid.
Upward (buoyant) force FB
= weight of displaced liquid.
FB
FB
mg
DEMOS
mg
4/10/01 Workshop, Pg 9
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.
CORRECT
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.
19%
6%
75%
0%
20%
40%
60%
80%
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.
4/10/01 Workshop, Pg 10
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.
CORRECT
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
19%
6%
Ice cubes take up a larger volume than water.
75%
0%
20%
40%
60%
80%
4/10/01 Workshop, Pg 11
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 up
2. Go down
3. Stay the same
CORRECT
VOTE
4/10/01 Workshop, Pg 12