Good morning! Prepare to carefully record information on this very important lesson. 

Newton’s First Law and Free Body Diagrams A phabulous tale chronicling the lives of phorces

Part i: The Legend Newton’s 1st Law states If FNET = 0, then a = 0! This means two things…. First: If all forces are balanced, an object at rest will remain at rest. Secondly: If all forces are balanced, an object in motion will continue moving with a constant velocity. Take a quick look at balanced vs. unbalanced forces….on the board.....free fall with air resistance for example. Car moving at constant velocity.

More mass ==> more inertia Newton’s 1st Law is also known as the law of inertia Inertia is an object’s resistance to a change in its motion Inertia is an inherent quality of an object More mass ==> more inertia

Part II: The Characters A force is any push or pull on an object. All forces are interactions between two objects. The unit for force is the Newton (N) Types of Forces: Weight (Fg): the force due to gravity; defined as Fg = mg MASS (kg) IS NOT THE SAME AS WEIGHT (N)! Normal (FN ): the force perpendicular to a surface (usually a supporting force, but not necessarily) Friction (FFR ): a force that resists motion resulting from two surface in contact Tension (FT ): the force resulting from a string, rope, etc. acting on an object Applied (FA): a force applied by another object Interesting fact: in the English system, Weight is measured in Pounds and mass is measured in slugs! Normal is a term used to indicate something is perpendicular, not ordinary

Part iII: The Hero Whoa! We need to have a way to organize our information about forces acting on an object! Never fear, Free Body Diagrams are here! A Free Body Diagram (FBD) is used to identify all the forces acting ON an object. A FBD is used to: LABEL the forces acting on an object DETERMINE which forces cancel out FIND the remaining force on an object, which is called the net force

Part iV: The Decree Here are some important rules for using FBDs: Draw the object as a box or large dot (generally) Indicate all forces acting on the object with arrows going away from the object’s center. Arrow length represents the relative magnitude of the force ALWAYS label the types of force Remember: Forces are measured in NEWTONS (N)!! Here, ask students to draw one for the AIR TRACK CAR SITTING ON THE TABLE, then another one for the AIR TRACK CAR MOVING ALONG THE AIRTRACK. (Same!) FT Fg

Interlude Draw a Free Body Diagram of the forces acting on a 3 kg physics book sitting on a desk with you pushing on it with a 2 N force horizontally, if the book is moving with a constant velocity. Compare your FBD with your table partners We will come back to this one in a few slides to go over it.

Let’s check: FBD for a 3 kg physics book sitting on a desk with you pushing on it with a 2 N force horizontally , where it is moving with a constant velocity. (a = 0, FNET = 0 N) FN Fg FA = 30 N Ffr=2 N = 2 N = (3 kg) (10 m/s2) = 30 N

Epilogue Draw a FBD for YOU sitting in your seat FN Fg

More Free Body Diagrams! Look at the 1 kg mass hanging on a string. I pull down with a force of 12 N. Draw the resulting FBD. FT Fg = Fg + FT = 10 N + 12 N = 22 N = (1 kg) (10 m/s2) = 10 N = 12 N

Ask yourself: Are forces balanced? If so, no change in motion. Are forces unbalanced? If so, object speeds up or slows down. Speeding up? Net force in direction of motion. Slowing down? Net force opposite direction of motion. We will look at turning later…...just looking at linear motion for now. Can ask these questions in reverse order.

If FNET = 0, a = 0

True or False? See the Free body diagram. This object must be moving to the right. False! It could be an object moving leftward, slowing down (friction force is rightward). Example: Sliding in your socks on a wood floor!

Riding an escalator! You are riding an escalator (in the normal fashion!) moving at a constant speed. Draw the quantitative FBD (use your mass).

The Story Continues… Fun with Free Body Diagrams will be a big part of your Physics Life! Always draw the FBD even if the problem doesn’t ask you to.