Levitation Homopolar motor





Once the magnet is set

in motion, why does

it stay in motion?











Once an object is set in

motion, a force is not

required to keep it in






III. Definition - study of relationship between force & motion


Isaac Newton

A) 3 Laws of Motion




1st Law - Law of Inertia




An object at rest...



...tends to stay at rest.



An object in motion
tends to stay in
motion, .....







..... unless acted on by an unbalanced force.





Balanced Forces


Unbalanced Forces




Identify all the
UNBALANCED Forces below



In other words ...
If a moving object has
no unbalanced force

on it,...

it would
maintain its speed
and direction  forever






Unbalanced Forces
Change Motion








Explain this using Newton's 1st Law

(Flash) (HTML5) Inspired by Dr. Richard Feynman



Why doesn't the ball roll backward
after the cart stops and starts up again?

Notice that at the exact moment the ball reaches the front of the cart, the cart instantly moves ahead at the same velocity it had previously. Therefore the ball never actually comes to rest.  It maintains its 'moving inertia'. If there had been even the smallest time interval between when the ball reached the front of the wagon and when the wagon began moving again we would have had different results. The ball would have bounced off the front of the wagon and when the wagon continued forward, the ball would then have rolled to the back of the wagon at an even faster rate.

Observe the animation again, but this time follow the movement of the ball with a ruler and you will see that once the ball starts moving, it never really stops moving.




An object in motion
tends to maintain
its' speed and direction...




unless acted on by an unbalanced force






John Paul Stapp



Sonic Wind I - 1954

Stapp decelerating in the G-Whiz sled. This five frame 'movie' shows pretty well the stress he experienced.  Courtesy EAFB History Office

John Stapp stops short after travelling 421 mph

Stapp suffered repeated and various injuries including broken limbs, ribs, detached retina, and miscellaneous traumas which eventually resulted in lifelong lingering vision problems caused by permanently burst blood vessels in his eyes. In one of his final rocket-propelled rides, Stapp was subjected to 46.2 times the force of gravity.[5]










"Why is it initially so

difficult to pedal a bike

when you start from




 "And why is it so easy

to pedal the bike once

you obtain your desired





Inertia - Object at rest tends to stay at rest
(force required to change motion)

Object in motion tends to stay in motion
(force only needed to offset friction)





[ Flash ][ HTML5 ]





Moon has Unbalanced Force On It!

[ HTML5 ]

St. Mary's HS Physics

Unbalanced Forces


St. Mary's HS Physics




Mass and the Inertia









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