Forces and Free Body Diagrams (FBD)

ForcesForces

• A force causes an object to change its velocity, by a change in speed OR direction

• Force is a vector quantity since direction is important

• There are contact forces between objects that touch and there are field forces between objects that do not touch

ForcesForces

• SI Unit for Force is the Newton (N)

• The formula for force is F = ma

NOTE: the unit for mass is kg and the unit for acceleration

is m/s2… so a kg•m/s2 is the same thing as a Newton!

Type of ForceType of Force SymbolSymbol DescriptionDescription

Frictional Ffrict force exerted between surfaces as an object moves across it

Tension Ftens force transmitted through a string, rope, cable, or wire

Normal Fnorm force exerted on an object when it is on top of another object; ┴ to resting plane

Air Resistance Fair force exerted on object as it travels through the air; usually negligible

Applied Fapp force on an object being pushed or pulled

Spring Fspring force exerted by a stretched or compressed spring with an object attached to it

Gravitational Fgrav force of gravity that all objects on Earth experience

Free Body Diagrams (FBD)

FBDs are used to show relative magnitude and direction of all forces acting on an object in a given situation, therefore:

1. Draw the object as a box2. The larger the force the longer the vector

arrow should be3. The direction of the arrow’s tip should show

the direction that the force is acting

4. Always label your force vectors5. With constant velocity vectors are equal and

with acceleration/deceleration vectors are unequal

FBD Example

A book on a table is being pushed to the right at a constant velocity

FBD Example

A parachutist jumps out of an airplane; include air resistance

How would I draw the FBD?

How would I draw the FBD?

This is what the FBD should look like!

How would I draw the FBD?

How would I draw the FBD?

How would I draw the FBD?