Newton’s Laws of Motion Beginning with: The Law of Inertia

Force  A force is a push or a pull on an object.  Forces are measured in Newtons (N).  1 N = 1 kg  m/s 2

Net Force  External force – acts from outside the system or object  Internal force – acts from inside the system  A net external force is the sum of the external forces acting of the object. Simply, the net force is the overall force on an object.

Balanced vs. Unbalanced Forces  If the net force on an object is zero, the forces are balanced. (This condition is called equilibrium.)  If the net force is not zero, then the forces are unbalanced.

A Historical Perspective

Aristotle – 4 th century BC, Greece  The proper state of objects is at rest.  Unless an object is being pushed or pulled (violent motion), it is seeking its natural resting place.  Earth did not move, it was in its natural resting state, of course. 

Copernicus – 1540 AD  The Earth and planets move around the sun.  The idea was very controversial and Copernicus was reluctant to publish his theory, in fear of being prosecuted.  On the day of his death, the first copy was delivered to him.

Galileo on Motion AD Galileo asked how things move, not why they moved. An object's velocity will not change all on its own. Pushes, or pulls, are necessary to change an object's velocity.

Galileo’s Contributions  Inertia is a property of matter. It is that property of matter which opposes changes in velocity. Simply stated, a common object will not change its velocity spontaneously.  Friction is the name given to the force that acts between materials that touch as they move past one another.  Argued that when friction is present a force is needed to keep an object moving. 

Other Contributing Scientists  Ibn al-Haytham (965 – 1039) lived in Persia (now Iraq)  Mo Tzu (circa BCE) a collection of philosophical texts originated in China

Isaac Newton ( ) Issac Newton’s monumental work, Philosophiae Naturalis Principia Mathematica, was published in It proposed scientific laws that are still used today to describe the motion of objects. (credit: Service commun de la documentation de l'Université de Strasbourg)

The Law of Inertia  First Law of Motion: Every object continues in a state of rest, or continues moving in a straight line at constant velocity, unless it is compelled to change that state by forces exerted upon it.  Main Idea: There must be a cause (a net force) for there to be a change in velocity.

Equilibrium  The Law of Inertia describes objects at equilibrium. For an object to be at equilibrium it must: AND Have balanced forces (net force = 0) Be at rest Move at constant velocity OR

First law in Motion us/gbssci/phys/mmedia/newtl aws/mb.html

MASS A Measure of Inertia

Mass vs. Weight  They are NOT the same!  Mass = amount of stuff in an object, measured in kilograms.  Weight = FORCE of gravity on an object, measured in Newtons.

Mass is not Weight  The mass of an object does not change.  Weight can vary with location.  No gravity = no weight.  An object with a mass of 1 kg will have a weight of 9.8 N. W = F g = m g

The Second Law (Normal Words)  Force causes acceleration.  For the same mass, More force = more acceleration  For the same force, Smaller mass = more acceleration More mass = less acceleration

Newton’s Second Law of Motion The fancy words version: The acceleration produced by a net force on an object is directly proportional to the magnitude of the net force, is in the same direction as the net force, and is inversely proportional to the mass of the object.

The 2 nd Law Equation F = ma The units are: Force (F) is in Newtons (N) Mass (m) is in kilograms (kg) Acceleration is in meters/second 2 (m/s 2)

Newton’s Third Law Fancy words:  Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first. Normal words: For every action, there is a equal and opposite reaction.

3 rd Law  The 3rd law always involves two objects.  The force are equal in size.  The direction of the force is opposite. Example:  An angry person punches the wall. (action force)  The wall punches back. (reaction force) Result = sore knuckles