Transcript L09_Reaction - Virtual Homeschool Group

Upcoming Deadlines
Sixth Homework (Outline of First Term Paper)
Due Wednesday, September 30th
Seventh Homework – Cancelled due to furloughs
(Automatic 20 points of credit)
Campus-wide Furlough Day
Monday, October 19th
(Art/Phys 123 will meet on Wed., Oct. 21st)
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ArtPhysics123.pbworks.com
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First Term Paper
The Laws of Physics in an Animation Universe
Modern science is based on the principles of
experimental observation and theoretical analysis;
in this assignment you will apply these principles in
a critical analysis of animation and special effects.
Specifically, you will select an animation film (or a
live-action film featuring CGI animation special
effects) and formulate three distinct scientific
hypothesis for the universe portrayed in that film
(which may or may not obey the same physical laws
as the real world).
First Term Paper
For example, in your animation’s universe the laws
gravity may be different (e.g., heavy objects may
fall faster than light objects). Your hypotheses
should be such that there is relevant observational
evidence in the film; you need to describe what
that evidence is and how it supports your theories.
You will also formulate a competing theories and
present evidence that the universe portrayed in
the film does not follow those alternate
hypotheses.
First Term Paper
The required length of the paper is 1500 words, which
is about 6 double-spaced typewritten pages (not
counting images).
Upload your term paper to your blog in a post entitled
"The Laws of Physics in an Animation Universe."
If you find that you significantly deviate from the
outline, please add a brief explaination at the end
of your term paper (which does not count towards
the 1500 words).
This assignment is due by 8am on Wednesday,
October 14th.
125 points (if late, 75 points)
Extra Credit Opportunity
Visit the Walt Disney Family Museum in
San Francisco (opens Oct. 1st).
Give me your ticket receipt for
ten points extra credit.
Hours: Wednesday-Monday: 10a.m.-6p.m
Admission: $15.00 for students
Location: 104 Montgomery Street
Inside The Presidio of San Francisco
Extra Credit Opportunity
Visit the Exploratorium in San Francisco.
Give me your ticket receipt for
ten points extra credit.
Hours: Tuesday-Sunday: 10a.m.-5p.m
Admission: $11.00 for students
Location: 3601 Lyon Street, SF 94123
Next to Palace of Fine Arts complex
Collisions,
Crashes, and
Combat
Action-Reaction Principle
For every action force there is an equal
reaction force in the opposite direction.
This is also known as Newton’s Third Law of Motion
Action and Reaction
Common expression for this principle is,
To every action there’s an equal and opposite
reaction.
What’s an “action”?
A force exerted by one object on second object.
What’s a “reaction”?
A force exerted by second object back on the
first object that is causing the action.
How can reaction be “equal” and “opposite”?
Equal in magnitude but opposite in direction.
Action-Reaction Pairs
Here are examples
of actionreaction pairs.
Think of other
examples of an
object exerting a
force on a second
object.
Practice identifying
action-reaction
pairs.
????
????
????
????
Demo: Equal Magnitudes
Action
Reaction
5
With spring scales, we verify that action and
reaction forces have equal magnitudes.
15
5
15
0
Hold
0
10
10
Pull
Class Demo: Mutual Attraction
What happens when:
• Mr. A pulls, Mr. B holds.
• Mr. A holds, Mr. B pulls.
• Mr. A & Mr. B
both pull.
Mr. A
Mr. A has
less mass
(weighs less)
than Mr. B
Mr. B
Class Demo: Mutual Attraction (cont.)
If only Mr. A pulls on Mr. B then Mr. B accelerates.
Reaction force of equal magnitude so Mr. A also moves.
Who moves faster? Mr. A, Mr. B, or the same?
Reaction
Action
Mr. A
Mr. A has
less mass
than Mr. B
Accelerations
Mr. A goes faster
(greater acceleration)
since his mass is less.
Mr. B
Class Demo: Mutual Attraction (cont.)
When both persons pull then there are two action
forces and two reaction forces.
If both pull with same force, how much greater is the
acceleration than when only one pulls?
Reaction A
Action A
Mr. A
Twice the force,
twice the
acceleration
Action B
Reaction B
Accelerations
Mr. B
Class Demo: Mutual Attraction (cont.)
We replace Mr. B with a solid wall and Mr. A pulls on the wall
(that’s the action force) .
Due to the enormous mass of the building, the wall does not move.
Wall exerts a reaction force, which pulls Mr. A towards the wall.
Mr. A
Reaction Action
Acceleration
Class Demo: Mutual Repulsion
Similar demonstration is to have Mr. A and Mr. B push
away instead of pull together.
Same results; if Mr. A pushes and Mr. B holds then both
move apart.
Action
Mr. A
pushes
Mr. B holds
Reaction
Large
Acceleration
Small
Acceleration
Class Demo: Mutual Repulsion (cont.)
If we replace Mr. B with a solid wall then Mr. A pushes on the wall
(that’s the action force) but due to the enormous mass of the
building, the wall does not move.
Wall exerts a reaction force, pushing Mr. A away from the wall.
Mr. A
Action
Reaction
Acceleration
Wile E. Coyote & Action/Reaction
Play
Wile E. Coyote demonstrates action/reaction in “Beep Beep” (1952)
Collisions & Crashes
A 2 ton car, going 60 m.p.h. hits a 5 ton truck, going 20 m.p.h..
The force of impact is greatest on which vehicle, the car or the
truck?
Forces of impact are equal.
(Force of car on truck) = (Force of truck on car)
The change in velocity (the acceleration) is greatest for which
vehicle?
For the car, which has less mass.
Law of acceleration says that
for a given force, the smaller
the mass, the greater the
acceleration.
Action/Reaction in KFP
Play
End credits of KFP play with the 3rd law, but consistently
IMPORTANT!!!
Action force & reaction
force NEVER cancel
because they act on
different objects!
Repeat this to yourself over and over again
Balance of Forces?
Miss A pushes the car (action); car
pushes back on her (reaction). Do
these forces cancel?
No, the two forces act on different
objects.
Force on Miss A is to the left; how can
she move forward (to the right)?
Miss A pushes on the floor with her
feet (action) towards the left;
reaction of floor on her is to the
right.
What if floor had zero friction?
Then Miss A can’t move forward.
Miss A
Action
Reaction
Reaction
Action
ActionReaction
Pairs
Balance of Forces? (cont.)
Miss B also pushes the car; can she
move the car by herself?
Obviously not.
In terms of Newton’s laws, why is this
not possible?
Because the net force exerted by Miss
B on the car is zero.
What other force does Miss B exert on
the car besides her hands?
The friction of the car seat on Miss B
(action) keeps her from moving.
Reaction (Miss B’s butt pushing on car)
balances force of her hands.
ActionReaction
Pairs
Miss B
Action
Reaction
Action
Reaction
Wile E. Coyote, Propelled
Play
Using an outboard motor in a tub for propulsion, as done
by Wile E. Coyote, would actually work, True or False?
Wall-E Fire, Propelled
Using a fire-extinguisher for propulsion, as used by Wall-E,
would actually work, True or False?
Class Demo: Extinguisher Rocket
Play
Using a fire-extinguisher for rocket-like propulsion.
Forces when Jumping
The three main forces on a person jumping are:
• Gravity (Downward)
• Support of the floor (Upward)
• Frictional force of the floor (Horizontal)
Only these forces can accelerate the person.
Gravity is constant but the
force exerted by the floor
can increase in reaction to
the action of the person
exerting a force on the floor.
Jumping Action/Reaction
Jumping is done by pushing
downward on the ground
(action) so the ground
pushes upward on you
(reaction).
How high you jump depends
on the force and on the
distance over which you
apply that force.
Can only push while in contact
with the ground so squatting
helps by increasing distance.
Reaction
Action
Action/Reaction Jumping Forward
To jump upward and
also forward, the
action force (pushing
downward with your
legs) needs to also be
pushing towards your
back so that reaction
force of the floor is
upward and forward.
Reaction
Action
Jump Magnification
Define jump magnification as
Jump Magnification =
Jump Magnification = 2
Jump Height
Push Height
Jump Magnification = 8
Get a large jump magnification when:
• Push Height is large
• Push Time is short
Push Factor
Can calculate jump magnification with this:
Jump Magnification = (Push Factor) x (Push Height in Feet)
Push Time
(in frames)
Push Factor
1
36
2
9
3
4
4
2 1/4
6
1
8
9/
16
Push Factor = 36 / (Push Time in Frames)2
Average Push Force
You can determine the average
force exerted when jumping as:
(Jump Force) = (Jumper’s Weight)
x (Jump Magnification)
Remember that
Jump Magnification =
Jump Height
Push Height
Jump Force
(Action)
The Incredible Hulk
The Incredible Hulk is big, let’s say 10 feet tall.
Say his push height when he jumps is 3 feet.
If you animate 2 frames from
crouch to take-off, how high does
he jump?
For a push time of 2 frames the push
factor = 9 so the jump multiplier is
(Jump multiplier) = (9) x (3) = 27
(Push Factor) x (Push Height)
He jumps 81 feet into the air since his
push height of 3 feet gets magnified by a
factor of 27 (the jump multiplier).
Play
Force Plate Analysis of Jumps
Understanding how
this force varies
during the jump helps
you animate secondary
actions like the drag
of hair and clothes.
Force
Can measure the force during a jump using a force plate.
Normal
Weight
Play
Time
Force Plate Analysis of Jumps
Force
a) Standing still before starting the jump.
Force on the ground is just our normal weight.
Normal
Weight
Time
Force Plate Analysis of Jumps
Force
b) Start coming down.
Force goes below normal weight as we accelerate
downward.
Normal
Weight
Time
Force Plate Analysis of Jumps
Force
c) Midpoint of downward motion.
Pushing upward to decelerate the motion.
Normal
Weight
Time
Force Plate Analysis of Jumps
Force
d) Lowest point in the crouch.
Pushing upward with near maximum force.
Normal
Weight
Time
Force Plate Analysis of Jumps
Force
e) Rising out of the crouch, just before lifting off.
Force decreases as feet are nearly losing contact.
Normal
Weight
Time
Force Plate Analysis of Jumps
Force
f) Lift-off! Moment you lose contact with the ground.
Normal
Weight
Time
Force Plate Analysis of Jumps
Force
g) Apex. Motion goes from upward to downward.
Normal
Weight
Time
Force Plate Analysis of Jumps
Force
h) Landing. Moment you regain contact with the ground.
Normal
Weight
Time
Force Plate Analysis of Jumps
Jump magnification for this jump is roughly equal to one
(jump height = push height)
Force
Average jump force is
roughly equal to my
weight.
Push
Time
Jump
Force
Normal
Weight
Time
More Incredible Hulk
If The Hulk has a push height of
2 feet and he makes a huge jump,
rising a height of 200 feet, how
much force does he push with?
Since The Hulk is twice as tall as
a normal person, his weight is at
least 8 times larger (probably
closer to 10-12 times larger). So
the push force of this jump would
be about 1000 times the weight
of a normal person if he jumps
straight up and about 1400 times
if he jumps at a 45 degree angle.
Similar force on
jumping and landing
(if similar push and
crouch height)
Swinging Arms in a Jump
The natural motion when
jumping is to swing the arms
upward as fast as possible
while the feet are in
contact with the ground.
Swinging the arms
raises the center
of gravity and
also increases the
downward action
force pushing off
the ground.
Swinging Arms in a Jump
The natural motion when jumping is to swing
the arms upward as fast as possible while the
feet are in contact with the ground.
Swinging the arms raises the CG and also increases
the downward action force pushing off the ground.
Swinging Arms in a Jump
The height of a jump is significantly lower
(almost 30% lower) if you don’t swing your
arms during the take-off portion of a jump.
However, if you
swing your arms
after leaving the
floor, then the
height of the jump
is much lower.
Home Demo: Jumping & Arm Swing
First, jump normally, that is, swing your arms
upward while feet are still on the ground.
Now try swinging your arms upward after you
leave the ground; you’ll notice a big difference.
Home Demo: Somersault
Now let’s try it with a backwards somersault
Tuck increases
rotation speed
Motion of the arms is also useful here for control of the rotation.
Arm Motion while in the Air
While in the air, moving your arms can shift the
center of gravity and also cause rotation but it
cannot change time in the air or distance jumped.
Next Lecture
Balance
First Term Paper due on
Wednesday, October 14th
(Two weeks from today)
Please return the clickers!