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Chapter 4
• 1642, several months
after Galileo died,
Isacc Newton was
born. Newton is the
father to: Classical
Mechanics
• Born prematurely and
barely survived on
Christmas Day, 1642.
• Wrote one of the
greatest scientific
books ever:
Principia
Mathematica
Philosophiae
Naturalis
Definition of Force
• A force is a push or pull on an object in a
particular direction.
• Forces affect how objects move. They can
make objects:
– start or stop moving
– move faster or slower
– change direction
– change shape.
Definition of Forces
• If forces cause objects to speed up, slow
down, and change direction then that means
that they affect _____________.
• If they affect velocity then they affect
_______________.
• Therefore forces cause ACCELERATION.
Definition of Forces
• There are a lot of misconceptions about what
a force is and what it isn’t.
Definition of Force
• It is important to remember that FORCES do
not exert forces, objects exert forces.
• You cannot say that the force on an object is
because of these:
•Centripetal force
•Friction
•Acceleration
•Gravity
•Inertia
•Velocity
•Tension
•Drag
• You can use these terms:
•String
•Earth
•Hand
•Table
•Floor
•Air
•Water
•Magnet
Definition of Forces
• Example:
– When a ball is kicked only the earth and air act
upon the object not gravity and drag.
– The road, through friction makes a car move
forward not friction, velocity, or momentum.
• When in doubt just remember…
Free – Body Diagrams
• When representing the forces acting upon an
object, it is important to use a free-body
diagram.
– A well made diagram makes the problem simpler
and minimizes the negatives in an equation.
Free – Body Diagrams
1. Use arrows to show the direction and
magnitude of the force applied.
2. Label the forces.
3. Choose a positive and negative direction.
– Often the positive direction will be the larger of
the two forces.
Free – Body Diagram Example
1. Use arrows to show
the direction and
magnitude of the force
applied.
2. Label the forces.
3. Choose a positive and
negative direction.
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FEarth on people
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Fpeople on Earth
Free – Body Diagram Practice
For each of the following draw a free-body diagram.
•
A flowerpot falls freely from a windowsill.
- Fair on flower
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•
FEarth on flowers
A car traveling at a constant velocity crashes into a brick wall.
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Fwall on car
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Fcar on wall