Download 4.2 Force and Inertia

Warm up
• What is force measured in?
• What does inertia mean in
Italian?
• What is the difference between
mass and weight?
Force
• Push or pull
• Causes acceleration
• Vectors have direction (up,
down, left or right)
• And magnitude: Newton (N)
(1N ~ ¼ pound)
Inertia
• Means “lazy” in Italian
• Tendency of an object to keep
doing what it is doing
• To say something has a lot of
inertia is to say something has a
lot of mass.
Mass
• Measure of inertia
• Measure of the amount of
“stuff” (matter) in an object
• Scalar (no direction)
• Unit: kilograms (kg) (1 kg =
2.2 pounds) (1 kg weighs 9.8N)
Aristotle vs Newton
• Aristotle believed that
the natural position of
all objects is the state
of rest.
SCORE
Aristotle
Zip
Newton
3
Newton’s
st
1
Law*
• An object at rest stays at rest, an
object in motion stays in motion
in a straight line at a constant
speed, unless acted upon by a
NET force.
* Quiz Question
•F
Force
• FNet Net force (resultant of all forces)
• Types of Forces:
•
•
•
•
•
W
FN
Ff
FA
T
Weight (or Fg )
Normal (at 90 degrees to a supporting surface)
Friction
Applied (a push or pull on an object)
Tension (or FT)
Free Body Diagrams
• Pictures that show the forces
acting on an object.
• Box or circle represents the
objects
• Arrows for the forces
Practice Problem #1
• Draw a free body diagram of a
10 N book sitting on a desk.
• Identify the type and amount of
all forces acting in each
direction.
FN
10 N
W
10 N
Net Force = 0 N
• Practice Problem #2
• Draw a free body diagram of a
10 N book that is being pushed
to the right across a desk with a
force of 2 N at a constant
velocity.
• Identify the type and amount of
all forces acting in each
direction.
FN
10 N
Ff
FA
2N
2N
W
Net Force = 0 N
10 N
• Practice Problem #3
• Draw a free body diagram of a
10 N book that is being pushed
to the right across a desk with a
force of 2 N and is accelerating.
• Identify the type and amount of
all forces acting in each
direction.
FN
10 N
Ff
FA
1N
2N
W
Net Force = 1 N
To the right
10 N
• Practice Problem #4
• Draw a free body diagram of a
500 N person who is hanging
from a swing supported by two
ropes.
• Identify the type and amount of
all forces acting in each
direction.
Static Tension
FT
FT
250 N
250 N
W
500 N
• Practice Problem #5
• Draw a free body diagram of a
500 N person who is hanging
from a swing supported by two
ropes hanging at a 45° angle.
• Identify the type and amount of
all forces acting in each
direction.
FT
FT
400 N
400 N
W
500 N
• Practice Problem #6
• Draw a free body diagram of a
500 N person who is hanging
from a swing supported by two
ropes hanging at a 80° angle.
• Identify the type and amount of
all forces acting in each
direction.
FT
FT
1200 N
1200 N
W
500 N
Homework
•
•
•
•
•
Problems 6-26
Draw a free body diagram for each
Label the type of force
Label the magnitude of the force
Consider air resistance and friction when
told.