Force in Nature

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What is a Force?

http://www.youtube.com/watch?v=SEBLt6Kd9EY&feature=youtube_gdata_player

A force is a push or a pull.

Forces can push or pull on an object without actually touching the object.

When Forces balance out we mostly don't notice that theyare acting on us.

When was the last time you noticed gravity?

Classification of Forces

Contact Force -Those forces that result when two interacting objects are percieved to be physically connected to each other.

Action at a Distance (or Non-contact)

Forces - Those forces that result even when

two interacting objects are not in physical

contact with each other, but still exert a push

or pull despite their physical seperation.

The Four Non-Contact Forces

- Current theory says there are four non-contact forces called Fundamental Forces.

- Each Force can be visualized as a field surrounding the source of the force.

- According to Quantum Mechanics everything is both a wave and a particle.

- therefore there must be particles associated with each of the four forces. (Called gauge bosons)

Very Basic Diagram of the 4 Forces

Fundamental Forces in Nature

The long range pair of the 4 fundamental forces.

1) Gravity- force that attracts object to each other.

-gravity is proportional to mass (not weight!)the greater the mass the greater the gravitational force.

-the weakest of all of the forces.

-gravitational force is only an attractive interactionhas a very long range effect.

-is believed to be mediated by a particle called a graviton

-are believed to be essentially massless

The long range pair (con't)

2) Electromagnetic Force

-electric currents and magnetic fieldshas a very long range

-is responsible for much of what we experience in our daily lives (chemical reactions, friction, light, electricity)

-either attractive or repulsive interaction

-is mediated by a particle called a photon, photons are believed to be massless

3) The Strong Force

-called the "Strong Force" because there is a strong possibility that these sub-atomic particles will interact

-only becomes important at the sub-atomic level

-holds the quarks that make up the protons together.

-range is about the size of a proton (10 -15 m)mediated by gluons

-there are currently believed to be 8 different gluons

-gluons are either massless or nearly massless

The Short Range Pair of Fundamental Forces

The Short Range Pair (cont.)

4) The Weak Force

-called the "Weak Force" because they very rarely interact with each other.

-has a range of 10 -18 m

-is responsible for radioactive decay and fusion in the sun.

-is mediated by W and Z bosonsW and Z bosons have mass (Discovered 1983)

Contact forces • Those forces that result when the two interacting objects are perceived to be physically contacting each other. • Generally contact forces are the macroscopic manifestation of microscopic electromagnetic interactions.

Examples of contact forces:

frictional forces, tensional forces,normal forces, air resistance forces, applied forces.

Types of Forces

Applied Force (Fapp)

- the force that results from one object pushing of pulling on another object

Normal Force (FN)

The force exerted between and object and the surface it is in contact with.

-The force exerted by a surface as an object moves across it or makes an effort to move across it. - Fµ is opposite to the direction of the applied force.

There are at least two types of friction force - kinetic and static friction.

Friction Forces (Ff or Fµ)

The maximum amount of friction that a surface can exert on an object can be calculated by

Fµ = µ • FN

where

µ = the coefficient of friction

Force of Air Resistance (Fair)

-the force that act on an object as it travels through air.

-for objects with small surface area or slow speed this effect is usually negligible (ie. Fair = 0 )

-the force is the force that is transmitted through a string, rope, cable or wire when it is pulled tight by forces acting from opposite ends.-is directed along the length of the wire and pulls equally on the objects on the opposite ends of the wire.

Tension Force (FT)

• The spring force is the force exerted by a compressed or stretched spring upon any object that is attached to it. • An object that compresses or stretches a spring is always acted upon by a force that restores the object to its rest or equilibrium position.

• For most springs the magnitude of the force is directly proportional to the amount of stretch or compression of the spring.

Spring Force (Fs )

Gravitational Force (Fg)

-The force of gravity is the force with which large object (Such as earth, moon etc.) attracts another object towards itself.

Note: This is the definition of weight.

The force of gravity on earth is always equal to the weight of the object as found by the equation:

Fg = m g

where g = 9.8 N/kg (on Earth) and m = mass (in kg)

Assignment

Read Page 4 in Dynamics Workbook