Transcript 04.Notes-2010-FBD weight and normal PP


Types of Forces
◦ Contact Forces:





Frictional force: Ff
Tensional force: FT
Normal force: FN
Air resistance force: Fa
Applied force
(push or pull): Fapp
◦ Action-at-aDistance-Forces:
 Gravitational force: Fg= W
 Electrical force: Fe
 Magnetic force: Fb


Draw a force diagram on
the book at rest on the
table:
FN

Fg



This normal force will
always be perpendicular to
the surface and may differ
from the object’s weight.

The table pushes back up on
the book with as much force as
the downward force of gravity
on the book.
You can imagine it like a
spring!
If you push down on a spring
and compress it, the spring
pushes back up on you!
As a book lying on the table
compresses the atoms in the
table, the microscopic atoms
in the table act as little springs
that squeeze upward on the
book.
They produce the normal, or
support force.
BALANCED:
•Forces are equal and opposite
•NO net force
•NO acceleration (either at rest or
constant velocity
UNBALANCED:
Larger force in one direction
than the other
•Net force is the sum of all forces,
taking direction into account
Object accelerates (speeds up or
slows down) in direction of net force

Scenario:

FBD:
FN
◦ A book is at
rest on a table
top
◦ A car is
coasting to
the right and
slowing down

Fg
FN
Balanced/
Unbalanced:
◦ x: no forces
◦ y: balanced
◦ x: unbalanced
◦ y: balanced
Ff
Fg
◦ An egg is
free-falling
from a nest in
a tree (ignore
air resistance)
◦ x: no forces
◦ y: unbalanced
Fg
 Try
the next four
on your own!

FN
Fg
A spring
mechanism gets
squeezed between
the force of gravity
pulling down on
you and the atoms
in the floor pushing
back up with an
equal and opposite
force.
Rory (9.30 kg) sits in his 3.70 kg high
chair
A. Draw a free body diagram for Rory, and
find the normal force exerted by the chair
on Rory.
B. Draw a free body diagram for the chair,
and find the normal force exerted by the
floor on the chair.

• Mass = measure of inertia
• Same at any location
•
•
Weight = measure of the force of gravity
Depends on your mass and location
• Mass = m
• Units of m: Kilograms (kg)
•
•
•
The pull of earth’s gravity (Fg)
Weight = W = mg = Fg
Units of Fg: Newtons (N)
Note: We are interested (for now) only in the
magnitude of the force of gravity, therefore we
will use 9.8 m/s2 for “g” and assign directions
(positive or negative) later.

Kerry, the ballet
dancer, has a mass of
45.0 kg.
◦ What is Kerry’s weight
on Earth?
 W = mg
 W = (45)(9.8)
 W = 441 N

Now Kerry takes a trip
to Jupiter where the
acceleration due to
gravity is 25 m/s2.
◦ What is Kerry’s mass on
Jupiter?
 45 kg
◦ What is Kerry’s weight on
Jupiter?
 W = mg
 W = (45)(25)
 W = 1125 N

http://www.youtube.com/watch?v=_Z0X0yE8Ioc