Physics

Newtons three Laws of Motion



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Sir Isaac Newton was one of those renaissance men who made contributions in a number of fields.  What separates him from the rest of the scholars of history is that so much of his teaching has endured with minimum change.

Before his time, no one understood gravity nor did anyone understand how and why objects moved.  Astronomers struggled with fantastic theories to explain the motion of planets and moons.  Galileo had to drop objects off the leaning tower of Piza to demonstrate that all objects accelerate at a constant rate in free fall regardless of weight.

Newton defined gravity as a force caused by the attraction of two objects.  In 1686 Newton introduced what became known as Newton's three laws of motion.  When these three laws are applied along with Newton's gravitational theory, almost all motion becomes systematic.  With the aid of a new form of mathematics, calculus, Newton was even able to make the calculation of orbits practical.  Everything in the universe follows Newton's three laws of motion.

1st law:  An object will stay at rest unless and until it is acted upon by an outside force.  When acted upon by an outside force, the objects accelerates in proportion to size of the force and in inverse proportion to the mass of the object.  In practical terms we use the equation F=ma where F is the force, m is the mass of the object and a is the acceleration that results.  Acceleration is defined as the rate of change of velocity with respect to time.  In calculus terms it is the derivative of velocity with respect to time.

The most obvious example of this is a golf ball.  It sits on the fairway without moving until the golfer hits it with his club.  The club applies a force to the ball and the ball accelerates.  It moves.  The harder it is hit, the faster it moves.

2nd law:  An object in motion will travel on a linear path with a constant velocity unless and until it is acted  upon by an outside force.  Again, the equation F=ma.  If the force directly opposes the path of the object, the resulting acceleration will slow or stop the object.  If the force is acting directly along the path of movement, the object will accelerate to a faster velocity.  As before, acceleration is defined as the rate of change of velocity with respect to time.  If the force acts at an angle to the path of the moving object, the object will change direction.  Thus under the second law, we also define acceleration as a change in direction.  The equation F=ma in conjunction with the principles of trigonometry can be used to calculate the new direction and velocity.

Playing on a gravity free golf course in a vacuum, our golfer would hit a shot that would send the ball in a straight line forever.  Here on Earth, air resistance applies a force on the ball that begins slowing the ball the moment it leaves the golf club.  Here on Earth, gravity applies a downward force to the golf ball causing its path to be bent downward.  Thus after 170 yards, more or less, the golf ball will fall to the earth and come to rest.  The golf ball has followed both of Newton's first two laws.

A better example of Newton's second law is a baseball player.  The pitch is crossing the plate in approximately a linear path.  The batter swings the bat to apply a force to the ball.  As the result of the impact of the bat, the ball changes both direction and speed.  Newton's second law can be used to calculate if the ball will go over the fence or not.

Some people interpret Newton's first two laws with respect to momentum where momentum is mass times velocity.  An object with a large mass needs a large force to get it moving at a given speed.  A moving object needs a force proportional to the mass of the object to cause a given change in speed and/or direction.  This simply applies different mathematical tools to the same application.

When we deal with massive objects such as planets and very large forces such as the gravity of the sun, the force of the sun's gravity causes the planet to change direction.  Newton's second law tells us why planets follow elliptical orbits around the sun. 

3rd law.  Every action has an equal and opposite reaction.  Perhaps this is the most revolutionary of Newton's laws.  It is saying that while the bat is causing an acceleration of the baseball, the baseball is also causing a negative acceleration of the bat.  Because the bat and the batter are higher in mass than the ball, one does not see the acceleration of the bat, but the batter feels the force of the ball against his bat.  Consider what will happen if the batter is standing on ice.  When he hits the ball, the ball results in a force against the bat.  With nothing to hold the batter in place, the bat and batter will slide backward while the ball sails forward.

More apparent is the collision of two pool balls.  They are the same mass.  After the collision, both balls change speed.  Unless the impact is directly head on, the cue ball also will change direction.  The new speeds and directions can be calculated using either momentum calculations or by using the F=ma equation.  In either case, trigonometry is used to determine the changes in direction.

The most practical application of the third law of motion is in rocket engines.  A large amount of gas is expelled out of the rocket at a very high speed.  Newton's third law tells us that the momentum of the gas leaving the rocket engine must be balanced by the momentum of the rocket moving forward.  Thus when the exhaust shoots backwards, there is a force to move the rocket forward.

Of importance is that Newton's three laws of motion apply to everything.  The massive celestial bodies move according to Newton's laws of motion as do everyday objects in our lives.  Furthermore, molecules, atoms and subatomic particles obey Newton's laws of motion.  Astronomers, engineers, theoretical chemists and nuclear physicists are all using the same laws of motion.

Thanks to Sir Isaac Newton, we have a systematic way of explaining the motion or lack of motion of everything in the universe.  Thanks to Sir Isaac Newton's laws of motion we can also predict the velocity and direction that will result from the interaction of a force an an object with a given mass.  While this can all be done with algebra and trigonometry, calculus makes all of this easier.  No wonder Newton also invented calculus.

All of us, scientist and non-scientist alike, are indebted to Sir Isaac Newton for discovering the laws of motion and presenting them in this relatively easy form.

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