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3rd Law of Motion

UC Irvine Physics Dept.

Sailboat.flv

Force - Sailboat.m4v

B) Free Body Diagrams - diagrams that show the forces acting on an object

Ex 1) Lamp on a table

F_n - normal force

(1) upward force supporting on an object

(2) Always drawn perpendicular  to surface where 2 objects meet

(3) When an object is placed on a horizontal surface

F_n = weight of object

Ex 2) Object resting on an incline plane

Don't copy this picture

weight force?

Normal Force?

Friction?

Weight force? B

Normal Force? C

Friction? D

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Calculating the Components of Weight Relative to a Ramp

by Dr. Michael R. Gallis  -  Penn State Schuylkill

Ex 2) Object at rest on an incline plane

(larger)
St. Mary's HS

On Ramp,
 
F_n (normal force) ≠ Weight Force

(It's less)

Larger
 St. Mary's HS

Ex 3) Force on a falling object
(before terminal velocity)

Downward Force - Weight Force

Upward Force - Friction

Ex 4) Block being pulled to the right
 at constant velocity

What force is on this block that is also on you?

Now draw the applied force

Draw the last force

Last force is the friction force

F_net = 0
 
(Always True at
 Constant Velocity)

How could this box be moving if
all the forces are balanced?

A force is not required to MAINTAIN motion

Ex 5) Forces on a projectile moving from left to right

What force is on this ball that is also on you?

If there is no forward force on this ball,
how could it move forward?

- Aristotle

Aristotle - (384 BC – 322 BC) was a Greek philosopher

Della Porta Collection then Borghese Collection; purchased in 1807

English: Portrait of Galileo Galilei by Justus Sustermans painted in 1636.

Galileo (and Newton) would say ...

An object in motion doesn't require a force to stay in motion

Gravity