Unit 6, An Introduction to Forces and Newton’s Laws
Forces: a quick definition
One method of classifying forces:
Contact vs. field forces
Contact forces:
Field forces:
Forces are VECTORS
Force Units: metric vs non-metric
Mass
Units of mass
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Instruments to measure forces:
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Isaac Newton (1642-1727) and his
three laws of motion
(Guidelines that help us understand how
forces interact with objects)
STATES OF MOTION:
(1)
(2)
Newton’s first law of motion:
Lex I: Corpus omne perseverare in statu suo quiescendi vel movendi uniformiter in directum, nisi
quatenus a viribus impressis cogitur statum illum mutare.
Law I: Every body persists in its state of being at rest or of moving uniformly straight forward, except
insofar as it is compelled to change its state by force impressed.
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An object that is at rest will stay at rest unless an unbalanced force acts upon it.
An object that is in motion will not change its velocity unless an unbalanced force acts upon it.
Galileo uses the term, “INERTIA”
Famous example: seat belts
http://www.physicsclassroom.com/mmedia/newtlaws/cci.cfm
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What about Newton’s second and third Laws of Motion? (we will come back
to these later)
Some Common Forces and symbols
Weight (W or Fg)
Can we quantify weight and come up with an equation to
calculate it?
What is mass?
Mass (m) kg W (Fg)
Newtons
Weight or Fg (N)
30
25
20
15
10
5
0
0
0.5
1
1.5
2
2.5
3
mass (kg)
Drawing the weight vector:
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Let’s think about the weight vector and what it does…
Example…think about the Earth without an
atmosphere (no air resistance or friction of any sort).
Parachute would not work…
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Let’s think about the weight vector and what it does…
Example…think about a larger planet with more ‘g’
without an atmosphere (no air resistance or friction of
any sort).
Parachute would not work…
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Let’s think about the weight vector and what it does…
Example…think about the Earth WITH an
atmosphere (no air resistance or friction of any sort).
Parachute would not work…
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Support Forces
Tensions (Ft or T) and Normal (Fn or N) Forces
Tensions:
Examples…
Drawing these forces
More complicated cases…
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Normal Forces (Fn or N)
More Complicated cases:
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Other forces and their symbols
Friction (Ff):
Spring or Elastic Forces (Fs or Fel):
Air resistance or motion through air
Fair or D (Drag):
Thrust:
Buoyancy Forces:
Forces we apply: Fapplied = Fa
Drawing Forces on Objects: Force Diagrams or Free-Body Diagrams
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Newton’s Second Law. A relationship between a net or unbalanced force
and the acceleration.
Fnet = m∙a, (not your pa but your ...)
the net force (in Newtons) = mass of an object (kilograms) ∙ the acceleration
of the object (m/s/s)
kg∙m/s/s → Newton
Let’s do an activity (Investigating Newton's Second Law by Pulling a
Wagon)
and we’ll come back to this
What did we learn in the activity???
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Find the NET force
In this case, Fnet > 0 and we get:
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Fnet = ma
Non-equilibrium (unbalanced)
Equilibrium (balanced)
------------------------------------------------------------------------We see all three scenerios on K-10
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Newton’s Third Law of Motion (action-reaction)
Show with a demonstration
Forces are interactions between two objects
Doesn’t mean that the “effects” of a force interaction will be the same
Famous question on physics tests:
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What about a larger bug??????
What are the effects now?
Overview of unit:



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Forces are pushes and pulls
Forces have names
Quantifying some forces (Weight = mg)
Newton comes up with three laws that describe how things
move:
o First Law
o Second Law
o Third Law
 Net forces and what this means in terms of motion
o Equilibrium (balanced, Fnet = 0, a = 0)
 Velocity = 0 (stationary object)
 Velocity ≠ 0 (object moving with constant velocity)
o Non-equilibrium (unbalanced, Fnet ≠ 0, a ≠ 0)
 Object’s velocity is changing
 An acceleration is present
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