Forces

• Force = a push or pull that results from an interaction between two objects

• Evidence of Interaction = change in:– Volume -- Sound -- Motion – Shape -- Color -- Tempo– Size -- Pitch -- Brightness– Texture -- Speed -- Temperature– Illumination -- etc.

Two Categories of Force

Contact forcesWhen objects physically touch

– Applied (push/pull)– Friction/Air Resistance– Normal– Tension– Elastic

Non-contact forcesAction occurs at a distance

– Gravitational– Electrostatic– Magnetic

2

Applied Force

• When one object pushes or pulls on another• Direction: Depends on application

Force of Friction/Air Resistance

• When one object rubs against another• Direction: opposite of motion

Normal Force

• The force pushing up on an object from the contact surface.

• Direction: perpendicular to surface

Tension Force

• The force exerted by a loaded string, rope, wire, cable, etc.

• Direction: depends on the position of the load

Elastic Force

• The force exerted by an object returning to its original shape.

• Direction: depends on the position of the load

Gravitational Force

• Attraction between two objects (depends on mass)

• Direction: pulls toward the Earth

Electrical Force

• The attractive or repelling force between two charged objects

• Direction: depends on the relative position of objects.

Magnetic Force• The attractive or repelling force between two

objects with a magnetic field• Direction: According to magnetic field (+ is

attracted to -)

Representing Forces

• Drawing Force Diagrams:– Use a dot to represent the center of the object

forces are acting on. – Use arrows to represent• Direction• Relative amount/strength

– Label Type of Force (see notations on the next slide)

– Label the “on ____ , by ____” to give your understanding of how the objects are interacting

Label Notations

Contact forces• Fa: FApplied

• Ff: FFriction

• Fn: FNormal

• Ft: FTension

• Fel: FElastic

Non-contact forces• Fg: FGravitational

• FE: FElectrostatic

• Fm: FMagnetic

12

Example of a Force Diagram