Free Body Diagrams Notes

Force A force is a push or a pull. Force is not a thing in itself, but rather an interaction between two objects. Force is a vector quantity… direction matters in the answer!

Common Forces

Common Forces

Net force is the vector sum of ALL forces acting on an object. If there is zero net force, then there is zero acceleration (constant velocity), this is a special case called equilibrium. If there is a net force, there will be an acceleration. That means that the object will be speeding up, slowing down, or changing direction.

Free Body Diagrams A Free Body Diagram is a simple drawing that shows the magnitude and direction of all of the force vectors acting on an object. The length of the arrows in relation to each other is VERY important Each arrow must point away from the “free body” and be labeled appropriately The system, the object the force is applied to, is drawn as a shaded circle

The book is drawn as a ball Free Body Diagrams Here is an example of a FBD of a book at rest on a table top. Fg is acting downward but is “balanced” by FN acting upward. Results in no net force and zero acceleration FN The book is drawn as a ball Fg

Free Body Diagrams Here is an example of a FBD of a box being pulled by a rope at a constant speed on a flat surface. Fg and FN are still opposite and equal. FT and Ff are also opposite and equal. FN Ff FT Fg Object is in motion, but not accelerating

Balanced Forces (zero net)

Free Body Diagrams Here is an example of a FBD of a ball under free fall conditions. Fg is the only force acting on this object. The net force is down and the object is accelerating. Fg Object is in motion and accelerating

Unbalanced Forces (non-zero net)

Questions to Try Draw a FBD for the following situations: A flowerpot falls freely from a windowsill. (Ignore any forces due to air resistance.) A sky diver falls downward through the air at constant velocity. (The air exerts an upward force on the person.) A cable pulls a crate at a constant speed across a horizontal surface. The surface provides a force that resists the crate’s motion. A rope lifts a bucket at a constant speed. (Ignore air resistance.) A rope lowers a bucket at a constant speed. (Ignore air resistance.)

Answers #1 #2 Fg Ff Fg

Answers to HW #3 #4 Fg FN FT Ff Fg FT

Answers to HW #5 Fg FT

Project Complete the practice FBD that I provided for you. Do Not forget your Poster Board on Friday