Forces and Free
Body Diagrams
Kinematic Forces
Force
Description
Applied Force (FA)
Result of a push, pull or thrust
Force of Gravity (FG)
Attraction between objects (ex. between an object and Earth)
Normal Force (FN)
Perpendicular to surface of object. Balancing force.
Tension (FT)
Exerted by materials that can strech (ex. rope, spring, cable)
Friction
Resists motion/attempted motion between objects in contact. Acts
in opposite direction to motion/attempted motion
Static Friction (FS)
Prevents stationary object from starting to move
Kinetic Friction (FK)
Acts against object’s motion
Air Resistance (FAIR)
Frictional force opposing object’s motion through air
Example 1
Draw a FBD of the following scenarios:
1. A pen sitting on a table.
2. A rope connected to a crane raising a piano vertically
upward at a constant speed.
3. A suitcase being rolled across the floor at a constant
velocity with a force applied at 20° above the horizontal.
4. A dresser that is being pulled to the right up a ramp into
a delivery truck by a cable parallel to the ramp; the
ramp is 14° above the horizontal.
Example 2
Draw three FBDs depicting the forces of an object as it
changes from free-fall to terminal velocity.
Net Force
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Often there is more than one force acting on an object. To describe the
effect of all the forces acting on an object, physicists determine the net
force. Sometimes the net force is referred to as the total or resultant force.
Fnet = Σ Facting on object
Where ΣF1 = F1 + F2 + F3 + …
•
•
•
Where F1 , F2 ,F3 are all the forces acting on a body.
Remember that an appropriate vector method will be required to solve this
force problem.
Note that if the net forces acting on an object equals zero, we say that the
object is in static equilibrium
Example 3
Two tractors pull a large rock on a construction site with a
net force of zero on the rock. Tractor 1 exerts a force of
1.2 x 104 N [E 120 N] on the rock, and tractor 2 exerts a
force of 1.2 x 104 [E 120 S]. Calculate the force of friction
acting on the rock. Assume the rock is at rest.