Newton

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Newtons Third Law
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Newtons Third Law
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Which vehicle exerts a greater force ? the tow
truck or the car?
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Newtons laws of motion

1. The law of inertia. An object in motion remains
   in motion with constant velocity if the net force
   on the object is 0.
2. Force and acceleration. If the net force acting
   on an object of mass m is F, then the
   acceleration of the object is a F/m. Or, F
   ma.
3. Action and reaction. For every action there is an
   equal but opposite reaction.

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For every action there is an equal but opposite
reaction.
Newtons third law
Whenever one object exerts a force on another
object, the second object exerts an equal but
opposite force on the first object.
Forces always occur like this, in pairs.
We will see that this is very hard to accept! It
is just not common sense. That is why it took a
great genius like Newton to figure it out.
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Example A collision
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Playing catch with a medicine ball
A
B
A throws the ball and B catches it. ? four
forces
When A throws the ball he exerts a force on the
ball (toward the right) and the ball exerts a
force on him so he recoils (toward the left).
? Newtons third law for the throw
When B catches the ball he exerts a force on the
ball (toward the left to stop it) and the ball
exerts a force on him so he is knocked back
(toward the right).
? Newtons third law for the catch
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Which vehicle exerts a greater force ? the tow
truck or the car?
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Forces obey Newtons third law.

• Well consider two examples
• The force of universal gravitation
• The spring force

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Universal Gravitation --- an example of
Newtons third law
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The Earth pulls the apple down (action).
The apple pulls the Earth up (reaction).
The two forces are equal (but opposite).
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When does a scientific theory become accepted as
true?
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For a laboratory measurement, the gravitational
force is really very weak.
The force on the 1 kg mass is 3.3 x 10-10 N. The
force on the 5 kg mass is 3.3 x 10-10 N. (
means to the right, i.e., increasing x)
Henry Cavendish, 1798 first measurement of G
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What makes g?
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Weighing the Earth
Calculate the mass of the Earth.
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The spring force ---another example of 3rd law
The forces are equal but opposite.
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Example One end of a spring is attached to a
wall. When the other end is pulled with a force
of 50 N, the spring is stretched by 3 cm. What
force would be required to stretch the spring by
5.5 cm?
Answer 91.7 N
Hookes law The strength of a spring force is
proportional to the displacement (extension or
compression). F k x where k is called
Hookes constant for the spring.
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A puzzle
The truck pulls to the right. According to
Newtons third law, the car pulls to the left
with an equal force. So how can they start
moving, or accelerate?
Resolution Consider each part separately, and
dont forget that other forces are also acting.
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A puzzle Tug of War
Which team will end up in the puddle?
But arent the forces equal but opposite !?
Resolution Dont forget that there are other
forces acting.
Each team exerts a force on the Earth, so the
Earth exerts a force on the team (3rd law!). The
net force on either team is toward the left.
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Momentum, p mv
momentum mass x velocity
As vectors, p m v.
Newtons third law implies conservation of
momentum.
Total momentum is constant.
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Momentum, p mv
Total momentum is conserved
Proof
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A puzzle
The two forces are equal. They must be, by
Newtons third law!
A small car (Cooper Mini) collides with a big
truck (Mack). Which is greater the force
exerted by the truck or the force exerted by the
car?
equal
equal
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When does a theory become accepted as true?
The law of conservation of momentum states that
the combined momentum is constant when particles
interact. It is verified by many experiments.
Since momentum conservation is equivalent to
Newtons third law, the third law became an
accepted fact a law of nature not just a
hypothesis.
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Harold Edgertons high speed photograph of Wes
Kessler kicking a football.
The force exerted by the ball on the toe
(reaction) is equal to the force exerted by the
toe on the ball.
Really hard to accept!
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Quiz Question
The planet is pulled toward the moon (and vice
versa). Calculate the gravitational force on the
planet.
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• When you walk or run, what forces occur?
• At constant velocity the horizontal force is 0
  and you continue to move because of inertia.
• To accelerate, you push backward against the
  floor the reaction force, which is a friction
  force exerted by the floor on your foot, pushes
  you forward.

This reaction force may be hard to visualize, but
imagine what would happen if you were on a
frictionless surface cant accelerate!
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A puzzle Horse and Cart
So how can they start moving, or accelerate?
Resolution Consider each part separately, and
dont forget that there are other forces acting.
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The four fundamental forces

• Gravity
• Electromagnetic forces
• Strong nuclear force
• Weak nuclear force

All the fundamental interactions obey
conservation of momentum (verified by
experiments), which is equivalent to Newtons
third law.
Nature appears to be complex but beneath the
surface, nature is simple.