Download Newton`s Three Laws of Motion

Chapter one
Newton's Mechanics
2-Newton's Three Laws of Motion
background
Sir Isaac Newton (1643-1727) an English scientist and mathematician
famous for his discovery of the law of gravity also discovered the three
laws of motion. He published them in his book Philosophiae Naturalis
Principia Mathematica (mathematic principles of natural philosophy) in
1687. Today these laws are known as Newton’s Laws of Motion and
describe the motion of all objects on the scale we experience in our
everyday lives.
Let us begin our explanation of how Newton changed our
understanding of the Universe by enumerating his Three Laws of Motion.
Newton's First Law of Motion:
I. Every object in a state of uniform motion tends to
remain in that state of motion unless an external force
is applied to it.
This we recognize as essentially Galileo's concept of inertia ( Any
velocity once imparted to a moving body will be rigidly maintained as
long as the external causes of retardation are removed), and this is often
termed simply the "Law of Inertia".
Newton's Second Law of Motion:
II. The relationship between an object's mass m, its
acceleration a, and the applied force F is F = ma.
Acceleration and force are vectors (as indicated by
their symbols being displayed in slant bold font); in
this law the direction of the force vector is the same as
the direction of the acceleration vector.
His second law is more notably named as the law of acceleration. His law
is;
"The rate of change of momentum of an object is proportional to the
resultant force acting on the body and is in the same direction."
This law means that the force of the object moving will be equal to the
opposing force such as air resistance. Mathematically, Newton's second
law is stated as:
F=dP/dt
where F is the total force, p is the momentum, and t is the time passed. In
classical physics, where the object's mass is constant, this equation
becomes a more familiar form:
F=dP/dt = d(mv)/dt= m(dv/dt)=ma
where m is the object's mass and a is the acceleration of the
object. Example
A skydiver jumps from a plane and accelerates until he reaches the
highest velocity possible, when this happens his acceleration is equal to
nothing, this happens when air resistance is equal to the downward force
of the skydiver.
This is the most powerful of Newton's three Laws, because it allows
quantitative calculations of dynamics: how do velocities change when
forces are applied. Notice the fundamental difference between Newton's
2nd Law and the dynamics of Aristotle: according to Newton, a force
causes only a change in velocity (an acceleration); it does not maintain
the velocity as Aristotle held.
This is sometimes summarized by saying that under Newton, F = ma, but
under Aristotle F = mv, where v is the velocity. Thus, according to
Aristotle there is only a velocity if there is a force, but according to
Newton an object with a certain velocity maintains that velocity unless a
force acts on it to cause an acceleration (that is, a change in the velocity).
As we have noted earlier in conjunction with the discussion of Galileo,
Aristotle's view seems to be more in accord with common sense, but that
is because of a failure to appreciate the role played by frictional forces.
Once account is taken of all forces acting in a given situation it is the
dynamics of Galileo and Newton, not of Aristotle, that are found to be in
accord with the observations.
Newton's Third Law of Motion:
III. For every action there is an equal and opposite
reaction.
This law is exemplified by what happens if we step off a boat onto the
bank of a lake: as we move in the direction of the shore, the boat tends to
move in the opposite direction (leaving us facedown in the water, if we
aren't careful!).
Examples
1. When we push a wall, it creates an opposite amount of force, thus,
either we move or does the wall move.
This in simple terms is that to every action force there is an equal, but still
the same, reaction force. For example if one presses on a wall a equal and
same force is pushed back at them.