1. Forces and Free Body Diagrams (FBD)

2. Forces
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

3. Forces 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!

4. Type of Force
Symbol
Description
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

5. 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 box
2. The larger the force the longer the vector arrow should be
3. The direction of the arrow’s tip should show the
direction that the force is acting
4. Always label your force vectors
5. With constant velocity vectors are equal and
with acceleration/deceleration vectors are unequal

6. FBD Example
A book on a table is being pushed to the right at a constant velocity

7. FBD Example
A parachutist jumps out of an airplane; include air resistance

8. How would I draw the FBD?

9. How would I draw the FBD?
This is what the FBD should look like!

10. How would I draw the FBD?

11. How would I draw the FBD?

12. How would I draw the FBD?