You have completed Motion 101: HOW things move Welcome to Motion 102: WHY things move

Aristotle ( BC)  Ancient Greek philosopher  Divided motion into two types: natural and violent  Natural motion: straight up or straight down; objects have natural resting places  Violent motion: imposed motion; result of forces that pushed or pulled  No force = no motion

Nicolaus Copernicus ( )  Renaissance astronomer and mathematician  Poland, Italy  Earth and other planets move around the Sun.

Galileo Galilei ( )  Italian physicist, astronomer, and philosopher  Force is NOT necessary to keep an object moving.  Force is any push or pull.  Friction is a force that acts between materials.  “Inertia” – Every material object resists change to its state of motion.

Sir Isaac Newton ( )  English physicist, mathematician, astronomer, and philosopher  Invented calculus  Law of gravity  Binomial Theorem  Light from prism  Reflecting Telescope

Sir Isaac Newton (continued)  Philosophiae Naturalis Principia Mathematica  Newton’s laws of motion  Kepler’s laws  Theory of tides  Wave motion  Fluid dynamics

Newton’s First Law An object at rest remains at rest, and an object in motion continues in motion with constant velocity (that is constant speed in a straight line) unless it experiences a net external force. “Law of Inertia”

Two types of forces  A contact force arises when one object touches another.  A field force can exist between objects, even in the absence of physical contact between two objects. For example, gravity or the electromagnetic force.

What is a net external force?

Inertia  Inertia is the tendency of an object to maintain its state of motion unless acted on by a net force.  An object that is either at rest or moving with constant velocity is said to be in equilibrium (no net force is acting on it).  Mass is a measurement of inertia.

Newton’s Second Law The acceleration of an object is directly proportional to the net external force acting on the object and inversely proportional to the mass of the object. ΣF = ma

Applying a larger net force to an object results in a larger acceleration.

Applying equal net forces to two different objects will accelerate the smaller mass more.

If you want to cause two different objects to have the same acceleration, the object with the larger mass will require a larger force.

Special Forces  weight – the magnitude of the force of gravity on an object  normal force – a contact force exerted by one object on another in a direction perpendicular to the surface of contact  tension – a force generated when a string is attached to a body and pulled taut; the direction of the force is away from the body along the string at the point of attachment  friction and air resistance (later!)

Newton’s Third Law in his words  From Principia: Lex III: Actioni contrariam semper et æqualem esse reactionem: sive corporum duorum actiones in se mutuo semper esse æquales et in partes contrarias dirigi.  “To every action there is always opposed an equal reaction; or, the mutual actions of two bodies upon each other are always equal and directed to contrary parts.”

Newton’s Third Law If two bodies interact, the magnitude of the force exerted on object 1 by object 2 is equal to the magnitude of the force simultaneously exerted on object 2 by object 1, and these two forces are opposite in direction. “Law of action – reaction” “Forces come in pairs”

Tug-of-war  Work with a partner.  Hook the two spring scales to either end of the string.  With each of you pulling on each spring scale, try to get the largest difference in the readings on the scales. If both scales read the same, the difference is 0. Try to get one scale to read a large value for the force and the other scale to read a small value.  Record the results of your efforts in your notebook.

Walking  Get up and walk normally.  Pay attention to what your body is doing.  Pay special attention to how you go about starting from a standstill. In going from a standstill to walking, you’re accelerating, right? What force is causing that acceleration?  Remember that when you accelerate, that’s caused by something other than you exerting a force on you – NET EXTERNAL FORCE!

Rifle

Horse and Cart