5Net 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.
6Free Body DiagramsA 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
7The book is drawn as a ball Free Body DiagramsHere 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 accelerationFNThe book is drawn as a ballFg
8Free Body DiagramsHere 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.FNFfFTFgObject is in motion, but not accelerating
10Free Body DiagramsHere 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.FgObject is in motion and accelerating
12Questions 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.)