THE NEWTON’S THREE LAWS OF MOTION

Brian Hwang

Newton’s Three Laws of Motion

The laws explain the relationship

between the net force on a body

and its motion.

The three laws were presented through Issac Newton's book Philosophiæ Naturalis Prin-cipia Mathematica (Mathematical Principle of Natural Philosophy)

Philosophiæ Naturalis Principia Mathematica The book was released on July 5th, 1687. It is considered to be one of the most impor-

tant scientific books. It states Newton's laws of motion, Newton's

law of universal gravitation,

and a derivation of Kepler's law of

planetary motion.

F=ma

This simple equation can be used to explain the first two Newton’s laws of motion.

The First Law of Motion

“Lex I: Corpus omne perseverare in statu suo quiescendi vel movendi uniformiter in directum, nisi quatenus a viribus impressis cogitur statum illum mutare.”

It states that every object continues in a state of rest or in a state of motion at a constant speed along a straight line, un-less it is compelled to change that state by a net external force.

A state of rest can be merely considered as a state

of motion at a constant speed of zero

along a straight line.

The First Law of Motion (Cont.)

The first law is also known as the Law of Inertia.*Inertia – resistance to acceleration.

(Inertia is proportional to mass)

F= ma -> a = F/m

This demonstrates that the acceleration is di-rectly proportional to the net force.

The Second Law of Motion“Lex II: Mutationem motus proportionalem esse vi motrici

impressae, et fieri secundum lineam rectam qua vis illa imprimitur.”

The second law is the most important law of the three laws.

It states that When a net force (ΣF) acts on an object of mass (m), the acceleration (a) that results is directly proportional to the net force and has a magnitude that is inversely proportional to the mass. The direction of the acceleration is the same as the direction of the

force.

This law states the formula F=ma

Example of the Second Law For example, if one flicks a small pebble, the

pebble will bounce away. That means the pebble accelerated at a high rate.

However, if one flicks a huge, heavy rock, it will hardly move away. That means the rock accelerated at an insignificantly slight

rate.

SI Unit Newton

From the second law, the unit of force N (Newton) can be derived.

F = m*a

= kg * m/s2

= N

The unit is the amount of net force required to accelerate a mass of 1 kilogram at a rate of 1 meter per second squared.

The First and Second Law Now, if we look back at the first law, we see

that the first law is just a special case of the second law when ΣF is zero.

ΣF = m*a (2nd law)

0 = m*a

m is always bigger than 0

so a = 0 when ΣF= 0 (1st Law)

The Third Law

“Lex III: Actioni contrariam semper et æqualem esse reactionem: sive corporum duorum ac-tiones in se mutuo semper esse æquales et in partes contrarias dirigi.”

It states that whenever a body exerts a force on a second body, the second body exerts an op-positely directed force of equal magnitude on the first body.

Simply “Equal and opposite”

Example of the Third Law When one runs into someone who weighs just as

much, both will bounce off to opposite direction but the same distance.

 However, if one runs into someone who weighs

much more, the one will bounce off further (F=ma) in one direction than the heavier one who will bounce off less

far (F=ma) to the other

direction.

Conclusion The Newton’s three law of motion is one of the

most important and interesting topics in Physics.

The laws might be simple and seem like common senses per se.

However, the connections they have with the real world make it much more valuable and essential.

The laws explain the scientific view of the world: how things move, where things move to, and how far things move.

Conclusion (Cont.) I look at my desk and see everything is at rest and now, I

know it is because there is no net external force such as me pushing them or wind blowing onto them--Newton’s first law of motion.

Now, I push a ball. It moves and stops. It would move for-ever in the same direction at a constant speed if there was no other force such as friction---Newton’s second law.

I am currently sitting on a chair. I would be falling down to the center of the Earth if the chair and the ground were pushing me back up---Newton’s

third law.

As I now notice, Newton’s law of mo-tion is everywhere.

Citation

http://csep10.phys.utk.edu/astr161/lect/history/newton3laws.html

  http://hyperphysics.phy-astr.gsu.edu/hb

ase/newt.html   http://www.grc.nasa.gov/WWW/K-12/air

plane/newton.html   http://en.wikipedia.org/wiki/Philosophi%

C3%A6_Naturalis_Principia_Mathematica