3rd Law of Motion

UC Irvine Physics and Astronomy Videos

School blocks YouTube? Download file below.

Sailboat.flv or Force - Sailboat.m4v



Click on the button below to start the game.

One of these things is not like the other!


The idea for this game and the graphic above come from a great physics teacher named Kelly O'Shea.



Review - Explain this using
Newton's Laws of Motion





Video Instruction




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


Ex 1) Lamp on a table


        [ Flash ]       

Fn - 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


Fn = weight of object





Object Resting on an
Incline Plane

weight force?

Normal Force?





Weight force? B

Normal Force? C

Friction? D





Ex 2) Object at rest on an incline plane


Drag and Drop the vector arrows to show the forces acting on a box resting on a ramp.


Click on the picture above




On A Ramp,
Fn (normal force) ≠ Weight Force

(Fn is less)




Calculating the Components of
Weight Relative to a Ramp

| Onsite |

by Dr. Michael R. Gallis - Penn State Schuylkill

Penn State   Schuylkill

Go Nittany Lions!!



 St. Mary's H.S.


Ranking Task

Rank the parallel component on the boxes below, from largest to smallest. Justify your answer.






Video Instruction





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






Fnet = 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






I Will Derive!!




Which plots below represent an object in a state of equilibrium?







A, C, D








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Use any material on this site (w/ attribution)