Transcript Free Body Diagram Notes

Forces and
Free Body Diagram
Notes
What is a force and how is it illustrated?
• A force is a push or a pull.
• The metric unit: Newtons.
• A force is usually shown as a vector, which includes
both magnitude (size) and a direction.
head
tail
Vector vs Scalar
These measurements have a
size and direction:
Vector Quantities
• Postion (displacement)
– 10 meters South
• Velocity
– 10 m/sec South
Force!
10 Newtons South
Also: Acceleration
These have only a size:
Scalar Quantities
• Distance
– 10 meters
• Speed
– 10 m/sec
• Also: Temperature, Time,
etc.
Types of Forces
•
•
•
•
•
Friction
Tension
Normal Force
Air Resistance
Applied Force
• Gravity
• Electrical
• Magnetic
Force of Gravity
• AKA: weight
• Caused by the gravitational
pull of a planet (LARGE object).
• To calculate weight:
– multiply mass times acceleration
of gravity Fg = m x g
– Basically, that’s just F=ma!
• On Earth, g = 9.8 m/s2
Weight and Mass are NOT the Same
• Mass:
– Amount of matter an object has
– The same everywhere
– Measured in kilograms (kg)
• Weight:
– Force of gravity pulling on the object
– Different on different planets
– Measured in Newtons (N)
Mass and Weight: Example
m = 10 kg
Fg = 98 N
g = 9.8
G
=
weight
m/s2
10 kg
m = 10 kg
Fg = 16 N
weight
g = 1.6 m/s2
What is a free-body diagram?
• A free-body diagram illustrates the relative
magnitude and direction of all forces acting upon an
object.
FN
• Dog sitting on chair
dog
Fg
Normal Force
• Normal Force (FN): the force the chair exerts on the
dog. Any time the object is on a surface there will
be a normal force!
FN
Fg
Net Force
• Net Force: sum of all forces acting on an object.
Net force is zero if all forces
cancel.
We also say “No net force”
or EQUILIBRIUM!
So, if there is
“No net force”
the object is:
•Not moving or
•Moving with
constant velocity
If all forces don’t cancel,
We say there is a
“Net force”
But, if there is a
“Net force”
the object is:
•Accelerating
Example:
FN
Net Force?
FNet = 0
Fg
No Net Force!
The dog is not
moving
Example:
tennis ball falling into commons
Net Force?
FNet ≠ 0
Fg
•Accelerating
NOT
ZERO!
Tension
• Tension: force exerted by a string or rope
supporting an object.
FT
Fg
Air Resistance
• Air Resistance: Force opposing the motion of
an object falling in air.
Fair
Fg
Air Resistance
• If the frog is accelerating as it falls.
Fair
FNet ≠ 0
Fg
Air Resistance
• If the frog is falling at a constant velocity.
Fair
FNet = 0
Fg
Friction
• Friction: Force opposing the motion of an
object sliding against a surface.
Ff
Fa
Applied Force (FA): Someone
applying a force on the
object.
Friction
• The force of friction between the desk and the
floor is 100 N. To keep the desk moving at a
constant velocity you need to push with a
force of :
Ff
Fa