Forces and Newton’s Laws of Motion Everyday Forces

Weight Weight – magnitude of the force of gravity acting on an object Fg or w Fg = mg Force of gravity = mass * free-fall acceleration Changes based on gravity On Earth -9.81m/s2 Local variations of gravity Gravity decreases as altitude increases Gravity decreases as latitude decreases scalar, magnitude of force only

The Normal Force Normal force – a force exerted by one object on another in a direction perpendicular to the surface of contact Often, but not always, opposes gravity Fn For objects at an angle, Fn can be calculated by the following equation Fn =-mg(cosθ)

Force of Friction Friction opposes motion Friction also opposes any applied force Static friction – the resistive force that opposes the relative motion of two contacting surfaces that are at rest with respect to one another Friction involving stationary objects Fs As long as the object that the applied force is acting on does not move Fs = -Fapplied When the applied force is as great as it can be without causing movement, static friction is at its maximum value (Fs,max)

Force of Friction Kinetic friction – the resistive force that opposes the relative motion of two contacting surfaces that are moving past one another Friction involving moving objects Fk Net external force acting on an object is equal to Fapplied-Fk Caused by adhesion due to electrostatic forces between molecules

Force of Friction Friction is proportional to normal force Easier to over come friction for less massive objects than more massive objects Friction depends on the surfaces in contact The more smooth the object, the less friction Coefficient of friction – the ratio of the force of friction to the normal force acting between the two objects Greek letter mu (µ)

Force of Friction To calculate coefficient of kinetic friction µk=Fk/Fn To calculate coefficient of static friction µs=Fs,max/Fn To calculate frictional force Ff= µFn

Force of Friction Air resistance is a form of friction Act on an object in the opposite direction of movement Falling objects also experience air resistance As velocity increases, resistance also increases Terminal speed - when the upward force of air resistance balances the downward force due to gravity

Fundamental Forces The universe has four fundamental forces or interactions Gravitational force, electromagnetic force, weak nuclear force (or weak interaction), strong nuclear (or just strong force) (in order of weakest to strongest) They are all field forces At the macroscopic we can only observe gravitational and electromagnetic forces

Fundamental Forces Strong force holds the nucleus together. It also holds the quarks together that makeup protons and neutrons Weak interaction produces nuclear instability Too much weak interaction causes a nucleus to undergo beta decay Electromagnetic force – binds the electron to the nucleus as well as binds atoms and ions together to form compounds and molecules Gravitational force – attracts masses together Really only noticeable if one object is planet or moon sized, and even then, it is only noticeable at short distances