Download Free Body Diagrams

Friction is a Force
Friction slows down all moving objects.
• Friction is a force that
resists motion.
• One reason Aristotle and lots of other people
didn’t understand inertia…didn’t believe that a
moving object would/will move forever unless
some force slowed or stopped it.
• Friction is that force! Everything we know slows
down and stops eventually because of friction.
• Friction is a net, external force that negatively
accelerates most moving objects we witness.
• …it is due to
microscopic roughness
on all surfaces.
• … it slows down all
moving objects.
Examples of Forces
► Ff
- - Friction (opposes sliding)
- - Applied Force (an external push or pull)
► FN - - Normal (perpendicular to a surface)
► FW - - Weight (force due to gravity)
► Fsp - - Spring (push or pull of a spring)
► FT - - Tension (spring, rope, cable)
► …many more…
more…
► FA
All Forces have an
Agent and an Object
Agent: the cause of a force
(what does the pushing or pulling)
Object: the ‘victim’
victim’ of a force
(what gets pushed or pulled)
Agents and Objects of
Forces
Example: A car is towed by a tow truck:
AGENT:
The tow truck
OBJECT:
The car
 Forces have magnitude and direction.
 We can represent forces by drawing arrows
 Example:
This diagram shows
all of the forces
acting on an object:
object:
Conventional Notation:
FTYPE (AGENT, OBJECT)
Forces are Vectors
FA (T,C)
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Drawing Force Vectors
► The
direction of the arrow represents the
direction of the force.
► The
length of the arrow represents the
magnitude of the force.
Free Body Diagrams
A picture of a ‘body’ with all the force
vectors acting upon it represented
graphically is called a
FREE BODY DIAGRAM
► We
always draw force vectors as ‘pulls’
pulls’ on
objects, not pushes (arrow starts at object)
Free Body Diagrams
Rules for drawing free body diagrams:
1.
2.
3.
4.
5.
Draw the object as a dot
Only draw forces acting on that object
Draw all forces as ‘pulls’
(arrow point away from the dot)
Draw and label every force acting on the object.
Length of each arrow must reflect magnitude
(stronger forces  longer arrows!)
(equal forces  same arrow length.)
Free Body Diagrams...
FN (normal force)
Book
FW (weight force)
Normal means perpendicular;
not ordinary or regular!
Try it...
Draw a free body
diagram of your
book sitting on your
desk.
Fnet and Free Body Diagrams
 Remember, the sum of all the forces is called
the net force. (F
(Fnet or ΣF)
 A free body diagram makes it easy to see whether
Fnet = 0 or not.
 Fnet = ma, soooooooooooo…
soooooooooooo….
If Fnet equals 0,
If Fnet is upwards,
If Fnet is sideways,
then a equals 0
then a is upwards
then a is sideways
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Free Body Diagrams...
The Answer...
FN
What forces are acting on a
skier as she races down a hill?
Ff
W or FW
A Free Body Diagram shows all of
the forces acting on an object:
The Answer...
FN
FN
Ff
Ff
surface
Hmmm…
What is Fnet ?
W or Fw
W or FW
Fnet and Free Body Diagrams
 A free body diagram makes it easy to see whether
Fnet = 0 or not.
then a equals 0
then a is upwards
then a is sideways
Fw or W = Weight Force
FN
= Normal Force
FA
= Applied Force
Ff
= Friction Force
A Free Body Diagram shows all of
the forces acting on an object:
 Remember, the sum of all the forces is called
the net force. (F
(Fnet or ΣF)
If Fnet equals 0,
If Fnet is upwards,
If Fnet is sideways,
FA
Physics
FN
Ff
FA
Physics
surface
W or Fw
Fw or W = Weight Force
FN
= Normal Force
FA
= Applied Force
Ff
= Friction Force
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Uniform Circular motion
Lab:
Centripetal Force
and Circular Motion
 Remember Newtons First Law?
So what is going on here?
F
Constant Speed
…not Constant Velocity
Velocity is Changing
…so acceleration exists
…therefore a force exists.
Uniform Circular motion
 A centripetal force causes circular motion
Circular motion
A
B
Where there is an
unbalanced force, there
will be an acceleration.
a
In the case of
uniform circular motion,
that unbalanced force is
centripetal (centerpointing)
Circular motion
A
If the ball is
moving at a
constant speed, can
it be accelerating
at all?
B
The instantaneous
velocity is always
tangential (sideways),
but the centripetal force
causes the ball to
change direction.
Circular motion
A
B
Acceleration
means change in
velocity, not speed.
Speed is constant,
velocity is not.
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