Forces Diagrams and Types

By the end of this lesson you will be able to:Model verbally or visually the properties of a system based on its substructure and to relate this to changes in the system properties over time as external variables are changed.

Apply F = mg to calculate the gravitational force on an object with mass m in a gravitational field of strength g in the context of the effects of a net force on objects and systems.

By the end of this lesson you will know/understand:• Forces are described by vectors, detected by their influence on the motion of an object,

Forces have magnitude and direction.• Free­body diagrams are useful tools for visualizing forces being exerted on a single

object and writing the equations that represent a physical situation, An object can be drawn as if it was extracted from its environment and the interactions with the environment identified, A force exerted on an object can be represented as an arrow whose length represents the magnitude of the force and whose direction shows the direction of the force.

• Contact forces result from the interaction of one object touching another object and they arise from interatomic electric forces. These forces include tension, friction, normal, spring

• Gravitational mass is the property of an object or a system that determines the strength of the gravitational interaction with other objects, systems, or gravitational fields. The gravitational mass of an object determines the amount of force exerted on the object by a gravitational field.

Essential Knowledge 3.A.2 ; Essential Knowledge 3.B.2 ; Essential Knowledge 3.C.4 ; Essential Knowledge 1.C.2 ; Learning Objective (1.A.5.1) ; Learning Objective (2.B.1.1)

Forcepush or pull on some object

Types of Forces

Contact Forces Field ForcesFrictionTensionNormalAir Resistance ­ usually ignoredAppliedSpring

GravityElectricalMagnetic

Symbol: FUnit: Newtons (N) = 1 kg*m/s2

must be in contact!!!! ... no contact, no force ... think about what is touching it

is it near ... large mass, charges, magnet

Applied Force - Contact

FA FBOY F1

An applied force is a force that is applied to an object by a person or another object

must be in contact!!!! ... no contact, no applied force

ex: you kick a soccer ball it sails through the air...not feeling applied force while in the air!!!

Normal Force - Contactin math normal means perpendicular

Force exerted by a surface, the force is always perpendicular to the surface

must be in contact!!!! ... no surface contact, no normal force

Symbol: N ... sometimes FN

Frictional Force - ContactThe resistance an object feels while moving across a surface to through air or a viscous (liquidy) substance

• ALWAYS acts in the OPPOSITE direction of movement or desired movement

• Acts Parallel to the surface/movement

Symbol: Ff

Air Resistance: subcategory of friction... same rules

Reference Table:

Tension Force - Contact

force along a rope or a stringSymbol: T of FT

Spring Force - Contact

- because force is a vector and the force acts in the opposite direction of the stretch/compress

force from a stretched or compressed spring

Symbol: Fs

distance from equilibrium

Field Forces

Gravitational Force - NONContact

WEIGHTSymbols: Fg or often W

• ALL OBJECTS FEEL GRAVITY!!!!

• Weight is different from mass!!!

• Mass is the same everywhere in the universe, weight however can change depending on location (less weight on moon)

• Always acts center of mass to center of mass (on Earth that means straight down)

• On Earth you may sometimes feel heavier but, if your mass and location (Earth) didn't change then you weight didn't change

W = Fg = mgg = 9.8 m/sm = mass ON EARTH

Electrostatic Force - NONContact

Symbol: Fe or FE

The force of attraction or repulsion from a charged object

opposites attract ... likes repel

Free Body Diagram

• A diagram which shows all the forces acting on an object• The object is represented by a dot or box.• Each force is represented using an arrow pointing in the correct direction starting from the dot/box• Bigger arrow = Bigger Force

Free Body DiagramWhen preparing to start a forces problem

get into the IMMEDIATE habit of sketching a free-body diagram

An egg is free­falling from a nest in a tree. Neglect air resistance

FBD:

FBD:

A car is coasting to the right and slowing down.

A box is pulled horizontally across a rough surface with a rope.

FBD:

A box is pulled across a rough surface with a rope angled 30o above the horizontal.

FBD:

relationships ‐ double mass = double weight, half x = have spring force

connections ‐ acceleration due to gravity = 9.8 m/s/s from unit 1, only force acting in the vertical during free fall so acceleration during free fall, vector like displacement, velocity, acceleration

real world examples ‐ all fbd examples ; ideal springs vs. real world springs