NEWTON RULES

Newton's Three Laws of Motion

Sir Isaac Newton (Jan. 4, 1643–March 31, 1727) was a superstar of physics, mathematics, and astronomy, even in his own time. He occupied the chair of Lucasian Professor of Mathematics at the University of Cambridge in England, the same role later filled, centuries later, by Stephen Hawking.  Newton introduced the three laws of motion in 1687 in his book entitled "Philosophiae Naturalis Principia Mathematica" (or simply "The Principia"). The same book also discussed the theory of gravity. This one volume described the main rules still used in classical mechanics today. 

Each law of motion Newton developed has significant mathematical and physical interpretations that help us to understand motion in our universe. The applications of these laws of motion are truly limitless. Essentially, Newton's laws define the means by which motion changes, specifically the way in which those changes in motion are related to force

Newton's First Law of Motion

Newton's First Law of Motion states that an object in motion tends to stay in motion unless an external force acts upon it; an object at rest will remain at rest unless an unbalanced force acts upon it. 

Basically, what Newton's First Law is saying is that if a ball is sitting on a table, it isn't going to start rolling or fall off the table unless a force acts upon it to cause it to do so. Moving objects don't change their direction unless a force causes them to move from their path.

Newton's First Law of Motion is also known as the Law of Inertia. Inertia is the name for the tendency of an object in motion to remain in motion, or an object at rest to remain at rest unless acted upon by a force.  Inertia is a quality of all objects made of matter (and so possess mass). They keep doing what they are doing until a force changes their speed or direction. A ball sitting still on a table won't start rolling around unless something pushes on it, a hand, a gust of air, or vibrations from the surface of the table. If you slide the ball across a table, it eventually stops rather than continuing on forever. This is because the friction force opposes the continued movement.  If the ball  is thrown in the frictionless vacuum of space, it would travel on at the same speed and direction forever unless acted on by gravity or another force such as a collision. 

Mass affects inertia. Objects of higher mass resist changes in motion more than objects of lower mass. A more massive ball, such as a basketball, will take more of a push to start it rolling. A styrofoam ball of low mass might be set in motion by a puff of air. 

Newton's Second Law of Motion

Newton's Second Law of Motion states that when a force acts on an object, it will cause the object to change its speed, ie to accelerate. The larger the mass of the object, the greater the force will need to be to cause it to accelerate. The Second Law may be written as: 

force = mass x acceleration

F = m x a

Another way to state the Second Law is to say it takes more force to move a heavy object than it does to move a light object. The law also explains negative acceleration, or slowing down. 

A ball rolling down a hill moves faster or accelerates as gravity acts on it in the same direction as the motion (acceleration is positive). If a ball is rolled up a hill, the force of gravity acts on it in the opposite direction of the motion (acceleration is negative or the ball decelerates).

Newton's Third Law of Motion

Newton's Third Law of Motion states that for every action, there is an equal and opposite reaction.

What this means is that pushing on an object causes that object to push back against you, the exact same amount, but in the opposite direction. For example, when you are standing on the ground, you are pushing down on the Earth with the same magnitude of force that it is pushing back up at you.

NOTE:

• Force is a quantitative (numerical, calculation) description of an interaction that causes a change in an object's motion. An object may speed up, slow down, or change direction in response to a force. Put another way, force is any action that tends to maintain or alter the motion of a body or to distort it. Objects are pushed or pulled by forces acting on them.

• Contact force is defined as the force exerted when two physical objects come in direct contact with each other. Other forces, such as gravitation and electromagnetic forces, can exert themselves even across the empty vacuum of space.

Gravity

Newton defined the force of gravity in the following way (translated from the Latin):

Every particle of matter in the universe attracts every other particle with a force that is directly proportional to the product of the masses of the particles and inversely proportional to the square of the distance between them.

Mathematically, this translates into the force equation (slightly simplified):

Fg = m1m2 / r2

In this equation, the quantities are defined as:

• F = The force of gravity (typically in newtons)

• m1 and m2 = the masses of the two particles (typically in kilograms)

• r = The straight-line distance between the two particles (typically in metres)

This equation gives us the magnitude (size) of the force, which is an attractive force and therefore always directed toward the other particle. As per Newton's Third Law of Motion, this force is always equal and opposite. The particle with less mass (which may or may not be the smaller particle, depending upon their densities) will accelerate more than the other particle. This is why light objects fall to the Earth considerably faster than the Earth falls toward them. Still, the force acting on the light object and the Earth is of identical magnitude, even though it doesn't look that way.

The force is inversely proportional (as one factor increases eg distance, the other decreases eg force) to the square of the distance between the objects. As objects get further apart, the force of gravity drops very quickly. 

At 2 metres apart the force is 1/4 of the force at 1 metre apart.

At most distances, only objects with very high masses such as planets, stars, galaxies, and black holes have any significant gravity effects.

Contact forces can be classified according to six different types:

• Tensional: such as a string being pulled tight

• Spring: such as the force exerted when you compress two ends of a spring

• Normal reaction: where one body provides a reaction to a force exerted upon it, such as a ball bouncing on a court

• Friction: the force exerted when an object moves across another, such as a ball rolling over a court

• Air friction: the friction that occurs when an object, such as a ball, moves through the air

• Weight: where a body is pulled toward the centre of the Earth due to gravity

Noncontact forces can be classified according to three types:

• Gravitational: which is due to the gravitational attraction between two bodies

• Electrical: which is due to the electrical charges present in two bodies

• Magnetic: which occurs due to the magnetic properties of two bodies, such as the opposite poles of two magnets being attracted to each other

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