# Newton’s Laws of Motion

Newton’s Laws of Motion

By Eugene Toth

Aristotle believed that everything had a natural state.  Water would stay in the hydrosphere.  It always flowed to the hydrosphere or water bodies.  Rocks fell to the geosphere or Earth.  Air entered the atmosphere or the rest of the air around us.  Fire rose in the form of smoke to a place above the atmosphere. People believed Aristotle’s theory until Galileo started studying physics.  Galileo’s studies of falling objects proved Aristotle wrong.  Then, in 1687, Sir Isaac Newton published his book Philosophiae Naturalis Principia Mathematica, or ,in English, Mathematical Principles of Natural Philosophy. People more commonly know Newton’s book as “The Principia.”

Sir Issac Newton Painted by Godfrey Kneller

Newton’s Laws of Motion

Newton made his image of the universe from generalizations which people called “Newton’s Laws of Motion.” Most of Newton’s laws are assumptions.  No environment could fit Newton’s laws.  Newton based these principles on assumptions. Nature made the laws billions of years before humans existed. They are not the same as man made judicial laws.

Newton’s First Law of Motion

Newton proposed three laws of motion. People know the first law as “An object that is in motion stays in motion.  At object that is at rest stays at rest.”  We may be confused by this law.  When we roll a wheel, won’t the wheel stop eventually? “A body remains at rest or in motion, remains at constant speed, traveling in a straight line unless the object is unbalanced by an external force”  On Earth, when we roll a ball, friction stops the ball.  Friction is therefore an external force. Newton disagrees with Aristotle. Newton argued there is no “Natural place” for an object.  If it is at rest, it remains at rest unless another force acts on it.  Objects do not change their state of rest or state of motion unless “forced to do so” Newton called a state of “inertia” from the Latin word for “idleness.”  In the picture below, the boy on the skateboard will continue rolling until he collides with the rock.

Newton’s Second Law of Motion

Newton wrote “f=ma.”  In other words, the relationship between an object’s mass “m”, its acceleration “a”, and the applied force “F” is F = ma. Acceleration and force are vectors.¹  For example, to calculate the force one exerts on a table when pushing it is equal to the mass of the table multiplied by the acceleration the table is moving at.  The higher the acceleration, the more force you have exerted on it.  In algebra, force  exerted on the object divided by the mass of the object equals to the acceleration of the object (a=F/m)

Newton’s Third Law of Motion

“When one body exerts a force on a second body, the second body simultaneously exerts a force equal in magnitude and opposite in direction on the first body.”  When we push an apple, the apple pushes us.  Newton contends that the second body simultaneously exerts a force equal in magnitude. Then why can we push the apple forward a certain distance while the apple can only push us a distance we can not notice?  The apple is small.  We do not notice the apple’s push because we are much larger than the apple.  The apple exerts a force on us proportional to the force we exerted on it depending on our size.  If we are 100x the size of the apple, the apple exerts a force 1/100 of that which we exerted on the apple.  Equal in magnitude does not mean directly equal.  It means proportionally equal.  In the picture below, a 200 pound man must exert a force of 52 Newtons to move a 252 pound rock.

Conclusion:

Newton’s Principia stated three universal laws of motion. The Principia contributed to the Industrial Revolution. For more than 200 years, no one could improve on Newton. In sum and substance, Newton’s First Law states that an object in motion stays in motion. An object at rest stays at rest.  Simply stated, Newton’s Second Law defines force. F=ma. Force equals mass times acceleration.  Newton’s Third Law states that when one body exerts a force on a second body, the second body exerts a proportionally countervailing force.