Download 4-3 - mrhsluniewskiscience

Objectives: The student will
be able to:
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State Newton's 3rd law of motion and give
examples that illustrate that law.
Apply Newton’s 3rd law of motion to
various situations.
Explain the tension in ropes and strings in
terms of Newton’s third law.
Determine the value of the normal force by
applying Newton’s 2nd law.
Warm Up
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What is Newton’s third law of motion?
Demo
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Push on wall
Push on wall with cart
Newton’s Third Law of
Motion
Whenever one object exerts a force on a second object,
the second exerts an equal force in the opposite
direction on the first.
Newton’s third law of motion
Newton’s third law of motion
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Newton noticed that forces were always in pairs and
that the two forces were always equal in size but
opposite in direction. He called the two forces action
and reaction.
Section
4.3
Interaction Forces
Identifying Interaction Forces
When you exert a force on
your friend to push him
forward, he exerts an equal
and opposite force on you,
which causes you to move
backwards.
The forces FA on B and FB on A
are an interaction pair.
An interaction pair is two
forces that are in opposite
directions and have equal
magnitude.
Section
4.3
Interaction Forces
Drag Force and Terminal Velocity
Newton’s 3rd Law
• For every action there is an equal and
opposite reaction.
Book to
earth
Table to
book
Think about it . . .
What happens if you are standing on a
skateboard or a slippery floor and push against
a wall? You slide in the opposite direction
(away from the wall), because you pushed on
the wall but the wall pushed back on you with
equal and opposite force.
Why does it hurt so much when you stub
your toe? When your toe exerts a force on a
rock, the rock exerts an equal force back on
your toe. The harder you hit your toe against
it, the more force the rock exerts back on your
toe (and the more your toe hurts).
Newton’s Third Law
• A bug with a mass of
5 grams flies into the
windshield of a
moving 1000kg bus.
• Which will have the
most force?
• The bug on the bus
• The bus on the bug
Newton’s Third Law
• The force would be
the same.
• Force (bug)= m x A
• Force (bus)= M x a
Think I look bad?
You should see
the other guy!
Action and Reaction on Different Masses
Consider you and the earth
Action: earth pulls on you
Reaction: you pull on earth
Newton’s Third Law of Motion
Helpful notation: the first subscript is the object
that the force is being exerted on; the second is
the source.
This need not be
done indefinitely, but
is a good idea until
you get used to
dealing with these
forces.
(4-2)
Concept Question
What makes a car
go forward?
Concept Question answer
By Newton’s third law,
the ground pushes on
the tires in the
opposite direction,
accelerating the car
forward.
Reaction: road pushes on tire
Action: tire pushes on road
Concept Question
Which is stronger, the
Earth’s pull on an
orbiting space shuttle
or the space shuttle’s
pull on the earth?
Concept Question Answer
According to Newton’s Third
Law, the two forces are equal
and opposite. Because of the
huge difference in masses,
however the space shuttle
accelerates much more
towards the Earth than the
Earth accelerates toward the
space shuttle.
a = F/m
Action - Reaction
• You constantly use actionreaction force pairs as you
move about.
• When you jump, you push
down on the ground.
• The ground then pushes up
on you. It is this upward
force that pushes you into
the air.
Action - Reaction
• When the rocket fuel is ignited, a hot
gas is produced.
• As the gas
molecules collide
with the inside
engine walls, the
walls exert a force
that pushes them
out of the bottom
of the engine.
Action - Reaction
• This downward push is the action force.
• The reaction force is the upward push
on the rocket engine by the gas
molecules.
• This is the thrust that propels the rocket
upward.
Consider hitting a baseball with a bat. If
we call the force applied to the ball by the
bat the action force, identify the reaction
force.
(a) the force applied to the bat by the hands
(b) the force applied to the bat by the ball
(c) the force the ball carries with it in flight
(d) the centrifugal force in the swing
Other examples of Newton’s Third
Law
• The baseball forces
the bat to the left (an
action); the bat forces
the ball to the right
(the reaction).
• Why does the ball
accelerate if the
forces are equal?
Action - Reaction
“For every action there’s an
equal but opposite
reaction.”
• If you hit a tennis ball with a racquet,
the force on the ball due to the racquet
is the same as the force on the racquet
due to the ball, except in the opposite
direction.
• If you drop an apple, the Earth pulls on
the apple just as hard as the apple pulls
on the Earth.
• If you fire a rifle, the bullet pushes the
rifle backwards just as hard as the rifle
pushes the bullet forwards.
Earth / Apple
How could the forces on the tennis ball, apple, and
bullet, be the same as on the racquet, Earth, and
rifle? The 3rd Law says they must be, the effects are
different because of the 2nd Law!
0.40 kg A 0.40 kg apple weighs 3.92 N
(W = mg). The apple’s weight
3.92 N
is Earth’s force on it. The
apple pulls back just as hard.
So, the same force acts on
both bodies. Since their
Earth
masses are different, so are
3.92 N their accelerations (2nd Law).
The Earth’s mass is so big,
5.98  1024 kg it’s acceleration is negligible.
apple
Earth / Apple
(cont.)
The products are the same, since the forces are the same.
m
Apple’s
little mass
a
=
Apple’s big
acceleration
m
Earth’s
big mass
a
Earth’s little
acceleration
Newton’s 3rd Law
• Suppose you are taking a space
walk near the space shuttle, and
your safety line breaks. How
would you get back to the shuttle?
Newton’s 3rd Law
• The thing to do would be to take one of the tools
from your tool belt and throw it is hard as you
can directly away from the shuttle. Then, with
the help of Newton's second and third laws, you
will accelerate back towards the shuttle. As you
throw the tool, you push against it, causing it to
accelerate. At the same time, by Newton's third
law, the tool is pushing back against you in the
opposite direction, which causes you to
accelerate back towards the shuttle, as desired.
Demolition Derby
When two cars of
different size collide,
the forces on each
are the SAME (but in
opposite directions).
However, the same
force on a smaller
car means a bigger
acceleration!
Newton’s Third Law
• Newton’s Third Law of Motion
– When one object exerts a force on a
second object, the second object exerts
an equal but opposite force on the first.
Newton’s Third Law
• Problem:
– How can a horse
pull a cart if the cart
is pulling back on
the horse with an equal but opposite
force?
– Aren’t these “balanced forces” resulting
in no acceleration?
NO!!!
Newton’s Third Law
• Explanation:
– forces are equal and opposite but act on
different objects
– they are not “balanced forces”
– the movement of the horse depends on the
forces acting on the horse
Example of 3rd Law
A horse harnessed to a
cart exerts an equal and
opposite force to the
cart as it exerts a force
against the ground.
Putting the three laws together
• http://science360.g
ov/obj/video/642db
496-d506-432e85b44e38f75d9142/new
tons-three-lawsmotion
Section
4.3
Interaction Forces
Earth’s Acceleration
Draw free-body diagrams for the two systems: the ball and Earth and
connect the interaction pair by a dashed line.
Section
4.3
Interaction Forces
Earth’s Acceleration
Identify known and unknown variables.
Known:
Unknown:
mball = 0.18 kg
FEarth on ball = ?
mEarth = 6.0×1024
kg
aEarth = ?
g = 9.80 m/s2
Section
4.3
Interaction Forces
Earth’s Acceleration
Step 2: Solve for the Unknown
Section
4.3
Interaction Forces
Earth’s Acceleration
Use Newton’s second and third laws to find aEarth.
Section
Interaction Forces
4.3
Earth’s Acceleration
Substitute a = –g
Section
4.3
Interaction Forces
Earth’s Acceleration
Substitute mball = 0.18 kg, g = 9.80 m/s2
Section
4.3
Interaction Forces
Earth’s Acceleration
Use Newton’s second and third laws to solve for FEarth on ball and
aEarth.
Section
4.3
Interaction Forces
Earth’s Acceleration
Substitute FEarth on ball = –1.8 N
Section
4.3
Interaction Forces
Earth’s Acceleration
Use Newton’s second and third laws to find aEarth.
Section
4.3
Interaction Forces
Earth’s Acceleration
Substitute Fnet = 1.8 N, mEarth= 6.0×1024 kg
Section
4.3
Interaction Forces
The Normal Force
The normal force is the perpendicular contact force
exerted by a surface on another object.
The normal force is important when calculating
Section
4.3
Interaction Forces
Forces of Ropes and Strings
Tension forces are at work in a tug-of-war.
If team A, on the left, is exerting a force of 500 N and the rope
does not move, then team B, must also be pulling with 500 N.
Click the Back button to return to original slide
Section Review
Question 1
Explain Newton’s third law of motion.
Answer
The third law says that forces always act in
equal but opposite pairs. For every action,
there is an equal and opposite reaction.
Section Review
Question 2
If they are “equal but opposite,” why don’t
action-reaction pairs cancel?
Answer
Action-reaction pairs don’t cancel because
they act on different objects, not on the
same object. Equal and opposite forces
acting on the same object would cancel.
Homework
• Tentative test Wednesday Period 3 and
Thursday Period 4B
• Hand out on Newton’s Third Law
Closure
• Kahoot
If P is the magnitude of the contact force between the
blocks, draw the free-body diagrams for each block.
An Atwood’s machine is two masses connected by a
strong light string that are hung over an ideal pulley
(light and frictionless). The masses have identical
velocity and acceleration magnitudes at every instant.
If we define up on the left and down on the right as
positive directions, then the masses have identical
velocities and accelerations period. This simplifies the
analysis a lot.
Atwood Machine
Would this move?
100 kg 100 kg
Atwood Machine
Would this move?
Which way?
FT
FT
Forces?
100 kg
200 kg
Fg = w
Fg = w
Atwood Device
Assume m1 < m2 and that
the clockwise direction is
+.
T
T
m1
m1g
m2
m2g
If the rope & pulley have
negligible mass, and if the
pulley is frictionless, then
T is the same throughout
the rope.
If the rope doesn’t stretch,
a is the same for both
masses.
Atwood Analysis
Remember, clockwise has been defined as
+.
2nd Law on m1: T - m1g = m1a
2nd Law on m2: m2g - T = m2 a
T
Add equations:
T
m1
m1g
m2
m2g – m1g = m1a + m2
a
(The T’s cancel out.)
Solve for a:
a=
m2g
m2 – m1
m +m
g
Atwood Machines
Example without numbers
An Atwood’s machine has m1 = 2 kg, m2 = 3 kg, hung
from an ideal pulley. What is the acceleration of the
masses? Calculate the tension in the string attached to
each mass.