# Forces and Interactions Mr Finn Honors Physics October 2011.

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Forces and Interactions Mr Finn Honors Physics October 2011

Where are we? Kinematics Projectile Linear Circular Simple Harmonic a = constant (free fall a = -g) a x = 0 a y = -g direction = +/  direction = vector, angle Kinematics of Uniform Acceleration dot or scalar product Projectile Motion = free fall plus relative motion Next Key Question: What causes acceleration or the motion of an object to change?

Aristotle Aristotle: Position - all objects have natural place, at rest Motion = natural vs violent Aristotle Aristotle: Position - all objects have natural place, at rest Motion = natural vs violent Explaining vs Describing Motion Kinematics variables to be explained –position (x) –velocity (v) –acceleration (a) Galileo Galileo: All motion is relative. Absolute rest has no meaning The earth is not at the center of the universe Galileo Galileo: All motion is relative. Absolute rest has no meaning The earth is not at the center of the universe Galileo Galileo: All objects accelerate at the same rate in free fall. Unexpected result - needs to be explained. Acceleration best number to describe motion. Galileo Galileo: All objects accelerate at the same rate in free fall. Unexpected result - needs to be explained. Acceleration best number to describe motion. predict What is needed to predict motion? x o = initial position v o = initial velocity a = acceleration predict What is needed to predict motion? x o = initial position v o = initial velocity a = acceleration

Key Question In free fall, all objects accelerate at the same rate of 9.8 m/s 2 Galileo observed this and argued that it makes logical sense, but … whyHe could not explain why!! Galileo identified the key question as explaining why objects change their state of motion (accelerate) and why everything responds to gravity the same.

Key Observation All objects in motion tend to remain in motion in absence of outside influences. It is change in motion that is important because uniform motion (not rest) is natural. –Changes in motion must be explained.

Fundamental Idea in Physics PhysicalobjectsInteractCause Changes Physical objects … Interact … to Cause Changes Observable effects Natural causes … via universal laws Forest & Trees!

What exists in nature? “matter” –atoms “energy” –interactions space and time –when/where interactions occur

Contemporary views - Matter “Fermion” –obey Pauli exclusion principle cannot have identical quantum # occupy space Changes –Macroscopic/Composite internal change: P, V, T external: x, v, a –Microscopic/Fundamental internal change: m, q, S, I, L, B (quantum numbers)

Modern Interactions “Bosons” –do not obey Pauli exclusion principle indefinite number with same quantum numbers exchanged during interaction Fundamental forces –gravity –electricity –magnetism –weak nuclear –strong nuclear EM EW GUT unification Contact forces friction air resistance tension normal etc

Newton’s Interpretation of Fundamental Idea MassesForcesAccelerations Masses … exert Forces on each other … to cause Accelerations Force = vector, always “action-reaction pair” Catalog interactions via “force laws”: F g = mg (near surface of Earth) f s   s N F D = 1/2 C D  A v 2 etc. “Quantity of matter” = inertia Lavoisier - 0 th Law of Motion: Mass is conserved Intrinsic Property affects change in motion PhysicalobjectsInteractCause Changes Physical objects … Interact … to Cause Changes Acceleration = change in state of motion Galileo’s kinematics Measured in “inertial frame of reference”

Surroundings Rest of the Universe (“action at a distance) Immediate surroundings (in direct contact) Object (solid) Surroundings near far Organize forces using “FBD”

Classical Interactions Solidobject Contact (local) Non-Contact (action-at-a-distance) Solids Fluids Rigid Flexible rope-like spring-like Gravitational Electric Magnetic normal frictional tensional elastic(Hooke) buoyant resistant Mass Charge “Surroundings” near far

Classes of Forces PrimaryPrimary –fundamental type of interaction –classical “action at a distance” SecondarySecondary –semi-empirical model –linked to atomic model of matter ConstraintConstraint –simplifying assumption cords, ropes … do no stretch  tensional force surfaces do not bend  normal force gravity electricity magnetism friction Hooke’s law air resistance