Forces and Newton

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Forces and Newtons Laws of Motion
To explain all nature is too difficult a task for
any one man or even for any one age. Tis much
better to do a little with certainty, and leave
the rest for others that come after you, than to
explain all things. -Isaac
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Objectives What You Should Know about Forces

• Describe how forces affect the motion of objects.
• Interpret and construct free-body diagrams.
• Explain the relationship between the motion of an
  object and the net force acting on it.
• Determine the net external force on an object.
• Predict the direction and magnitude of the
  acceleration caused by a known net external
  force.
• Recognize the conditions required for
  equilibrium.
• Identify action-reaction pairs.

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A Very Brief History of Statics and Dynamics

• The idea that a force causes motion goes back to
  the 4th century B.C., when the Greeks were
  developing ideas about science.
• Aristotle (384-332 BC) studied motion, and
  divided it into two types, natural and violent.

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Natural Motion

• Natural motion was thought to be straight up or
  straight down on earth. Objects would seek their
  natural resting places heavy things would fall
  to the ground, light things (like smoke) would
  rise. The heavens natural motion was circular.
  Since these motions were considered natural,
  they were not thought to be caused by forces.

Newtonian
Aristotelian
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Violent Motion

• Violent motion was imposed motion. It was the
  result of forces that pushed or pulled. A cart
  moved because it was pulled. Violent motion had
  to have a cause.
• For nearly 2000 years, it was commonly believed
  that an object on earth moved against its
  nature.

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Galileo Galilei (1564-1642)

• Until Galileo Galilei, the premier scientist of
  the 17th century, common sense" told people that
  heavier objects fall faster.
• Galileo showed the power ofexperiment over
  logic.

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Enter Newton

• In 1668, Newton built the first reflecting
  telescope.
• During his lifetime he was involved in the
  development of the calculus AND the laws of
  motion. Neither are small accomplishments.
• Newton struck upon the laws of motion and the law
  of gravitation.
• Newton was one of the greatest scientists the
  world has known for his ability to explain
  seemingly dissimilar phenomena by the formulation
  of a coherent system of laws.

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Forces
A force is a push or pull upon an object
resulting from the object's interaction with
another object.

• Whenever there is an interaction between two
  objects, there is a force upon each of the
  objects.
• When the interaction ceases, the two objects no
  longer experience the force.

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Forces

• Force is a vector quantity. Forces cause changes
  in velocities (accelerations) more specifically,
  unbalanced forces cause changes in velocities.

Unbalanced Forces
Acceleration
Weight
FN
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SI Unit for Forces

• The newton (N) is the SI unit for force the
  pound is the English unit for force.

One newton is the force required to accelerate
a mass of 1 kg at a rate of 1 m/s2
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Free-Body Diagrams

• Free-body diagrams (FBDs) are graphical
  representations of objects where all forces
  acting ON the object are represented. Forces
  exerted by the object on other objects are not
  included in the diagram.
• FBDs are used to analyze situations. When you
  isolate an object, and identify all of the forces
  acting on it, you can predict how the object will
  move.

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Newtons First Law of Motion

• Newtons First Law of Motion, also known as the
  Law of Inertia, states that a body in motion will
  remain in motion at a constant velocity, and a
  body at rest will remain at rest unless acted
  upon by an unbalanced force.

Inertia is the resistance of an object to
changes in its motion.
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More on Inertia

• Galileo developed the concept of inertia. Galileo
  reasoned that moving objects eventually stop
  because of a force called friction.
• Mass is a measure of inertia.

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Implications of Newtons First

• A body in motion at constant velocity does not
  necessarily require a force to keep it in motion
  at constant velocity.

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More Implications
Galileos Thought Experiment

• If there were no friction, objects moving along
  a horizontal surface would not slow down.

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AND If an Object is at RestOR Moving with
Constant Velocity

• All forces acting on that object must add (as
  vectors) to be zero.
• The object is said to be in equilibrium.

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Equilibrium
?Fx 0 ?Fy 0
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What If There Were An Unbalanced Force?
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Newtons Second Law
Forces
mass
acceleration
summation
Is the product of a vector (acceleration) and a
scalar (mass) the direction of the acceleration
is ALWAYS in the direction of the net force.
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Newtons Second Law

• ?F is the summation of forces acting on the
  object under consideration. ?F may also be called
  the net force, or the unbalanced force. Lets
  consider the scenario below

?F
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Newtons Second Law

• If there is an unbalanced force acting on an
  object, the acceleration of the object is
  directly proportional to the unbalanced force
• and inversely proportional to the mass of the
  object

?F ? a
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Implications of Newtons Second

• Bigger forces yield bigger accelerations.

?F ma 10 N 5 kg (a) a 2 m/s2
m 5 kg
?F ma 50 N 5 kg (a) a 10 m/s2
m 5 kg
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Implications of Newtons Second

• Forces accelerate smaller masses more than larger
  masses.

?F ma 10 N 2 kg (a) a 5 m/s2
m 2 kg
?F ma 10 N 4 kg (a) a 2.5 m/s2
m 4 kg
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More Implications

• If you know the mass of an object, AND its
  acceleration, you know the net force acting on
  it even if you dont know the nature of these
  forces

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Newtons Third Law of Motion

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

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Implications of Newtons Third

• If you push/pull on something, it pushes/pulls
  back on you with an equal but opposite force.
• Forces always occur in pairs.

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Equal and Opposite/Action-Reaction Pairs

• The force the earth exerts on the dinosaur is
  equal but opposite to the force the dinosaur
  exerts on the earth. Lets say the dinosaur
  weighs 33,000 N. The weight of the dinosaur is
  the gravitational force the earth exerts on the
  dinosaur, and that 33,000 N is the gravitational
  force the dinosaur exerts on the earth.

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More Action Reaction
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And Even More
Why does anything move? These forces seem to
cancel.
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The Forces Acting ON an Object Determine its
Motion
The action/reaction pairs are forces that are
exerted on two objects by each other.