Chapter 6 Newtons 3rd Law

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Chapter 6Newtons 3rd Law

• For every action, there is an equal and opposite
  reaction

If you push something, something must push back
Forces are not a thing within itself, but rather
all forces happen as a result of interactions
between objects
There is never a single isolated force in nature.
If you look closely, there is always an
oppositely-directed counterpart
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Newtons 3rd Law

• Does a stick of Dynamite contain a force?
• A) Yes B) No

This stick of dynamite does not contain a force,
but it does contain energy, And that energy can
be used to create a force remember the
definition of Force is a push or pull on an object
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Newtons 3rd Law

• A car accelerated along a road, what is the force
  that moves the car forward?
• The engine
• The road
• The tires
• The gasoline

What direction is the tire pushing the road?
If the tire pushes back on the road, the
road must push on the tire.
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Newtons 3rd Law

• If a swimmer pushes the water backward, the water
  pushes the swimmer forward.

Swimming slow Swimming fast
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Newtons 3rd Law

• Friction was the main factor in the past couple
  of examples. What happens when friction isnt
  there.
• Think about walking on ice. There is much less
  friction, so when you try to push back on the ice
  to move forward, the ice pushes you forward
  equally.

To move forward on skates, you increase By
having your skates not straight
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Newtons 3rd Law

• Identifying the action and reaction is not always
  obvious.
• A 500 kg boulder falls off the cliff and heads
  towards the ground. Is Earth pulling the boulder
  with a force?
• A) Yes B) No

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Newtons 3rd Law

• How much force is the Earth pulling on the 500 kg
  rock?
• A) 500 N B) 5,000 N C) 50,000

Force Mass x acceleration
According to Newton, if Earth pulls on the rock
with 5,000 N of force, there must be an equal and
opposite force.
Earth is pulling the rock at 5,000 N
The Rock pulls the Earth at 5,000 N
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Newtons 3rd Law

• Confused yet?

To understand how the rock pulls the Earth, think
about what causes the Earth to pull the rock.
GRAVITY
If Earth pulls the rock with a gravitational
force, the rock pulls the earth with its own
gravitational force (granted it is small in
comparison to the earth).
So if Earth pulls the rock at 5,000 N, how can
rock pull the Earth at 5,000 N???
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Newtons 3rd Law

• Stay with me here!

Both the rock and the Earth pull of each other
with 5,000 N. Referring to Newtons 2nd
Law Fma The rock 5,000 N 500 kg x 10 m/s²
The Earth 5,000 N 5,970,000,000,000,000,000,000
,000 kg x acceleration F 5,000
N Rearranged a m 5.97 x 1024 kg
1.19 x 10-15 m/s2 This is the gravitational
acceleration the rock is pulling on the earth.
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Newtons 3rd Law

• The difference in masses between objects
  determines how much the force accelerates the
  objects.
• A force creates an equal and opposite force
• Forces makes the things move (Accelerate)
  Things move (accelerate) according to
  their mass

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Newtons 3rd Law

• We know that the Earth pulls the moon, does the
  moon pull on the Earth?
• Yes B) No

• Which pull is stronger?
• Earth
• B) Moon
• C) Same

They pull with the same force on each other, but
because the masses are different, the
accelerations towards each other are different.
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Newtons 3rd Law

• The rocket pushes the gas
• The ______ pushes the rocket
• Atmosphere
• Gas
• Force

How can gas push a rocket?
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Newtons 3rd Law

• The forces are the same
• The mass of the rocket and gas are different
• The acceleration of both are different

This is Molecular Man!
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Newtons 3rd Law

• If forces are equal and opposite, why dont they
  cancel?
• You have to look at each system