EQ: HOW IS THE MOTION OR STATE OF REST OF AN OBJECT
REPRESENTED BY FREE BODY DIAGRAMS?
WHAT IS A FORCE?
A force is a push or pull upon an object resulting from
the object's interaction with another object. It is a
vector quantity. 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.
Forces only exist as a result of an interaction.
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F
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C
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Contact Forces
Action-at-aDistance Forces
Frictional Force
Gravitational Force
Tension Force
Electrical Force
Normal Force
Magnetic Force
Air Resistance Force
Applied Force
Spring Force
Gravitational Force (Weight): Fg This is the only force that will always be present
and will always point straight down.
Tension Force : Ft This type of force is caused by a rope, chain, string, etc. and goes
F Normal Force:
O Applied Force:
R Frictional Force:
C Net Force:
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in the direction of the rope, chain, string, etc.
Fn This type of force is present when the object is in contact with a
surface. It always points perpendicular to the surface.
Fa This type of force is used when an outside agent is acting on the
object such as a person.
Ff This type of force is the result of two surfaces being drug across
each other, creating friction. It always points in the direction opposite of the movement.
Fnet Sum of all forces
*Vector: The size of the arrow should be proportional to the size of the force.
Larger forces should have larger arrows!!! Arrow should represent direction of
force.
FREE BODY DIAGRAM
FBD
A physical representation that shows the forces
acting on a system. Accelerations are the result of
unbalanced forces. If a motion diagram show that
an object is accelerating, a free body diagram of
that object should have an unbalanced force in the
same direction as the acceleration.
FREE BODY DIAGRAM – ANALYZING FORCES
A force is applied to the right to drag a sled across loosely
packed snow with a rightward acceleration. Neglect air
resistance. A free-body diagram for this situation looks like
this:
FREE BODY DIAGRAM
a. A physics book rests upon a level table.. A free-body
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diagram for this situation looks like this:
b. A flying squirrel is gliding (no wing flaps) from a tree to
the ground at constant velocity. Consider air resistance. A
free-body diagram for this situation looks like this:
FREE BODY DIAGRAM
c. A girl is suspended motionless from the ceiling by two
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ropes. A free-body diagram for this situation looks like this:
d. An egg is free-falling from a nest in a tree. Neglect air
resistance. A free-body diagram for this situation looks like
this:
CALCULATING NET FORCE
In each of the above situations, there is an unbalanced
force. Unbalanced force indicates object is accelerating (velocity is
changing – slowing down, speeding up, changing direction)
FIND THE NET FORCE
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Free-body diagrams for four situations are shown below. For each situation,
determine the net force acting upon the object and the direction of
acceleration.
FIND THE NET FORCE
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A 2.0-kg bucket is tied to a rope and accelerated upward out of a
well at a rate of 1.5 m/s/s. Neglect air resistance. Diagram the forces
acting on the bucket.
FBD:
ΣFx = ma = (2.0)(0)
ax = 0
ΣFy = ma = (2.0)(1.5) = 3.0 N
2
FT
ay = 1.5 m/s
Fg
Find the individual forces
acting on the bucket:
FT = ____
Fg = ____