Net Force & Newton’s 1st Law Learning Goal 4.2: Use free body diagrams to solve for net force and/or individual component forces. Learning Goal 4.3: Use Newton's 1st law to distinguish between equilibrium and non- equilibrium situations

Review A force can be thought of as a push or a pull It causes an acceleration, so it causes a change in speed and/or direction of travel Free body diagrams (FBDs) show all the force vectors acting on an object

Why Draw FBDs? Once we’ve drawn a FBD, we can calculate the net force on the object Net force = the overall unbalanced force on an object… it’s the same as the resultant sum of all the different force vectors

Calculating Net Force from FBDs What is the net force on a kitten being pushed with 6 N east and 2 N west? What is the net force on a puppy being pushed with 4 N west and 3 N north? 6 N2 N 3 N 4 N Redraw the vectors head to tail! F net = 4 N east F net = 5 N, 36.9  N of W

Calculating Net Force from FBDs Two mice are fighting over a piece of cheese. Mouse A pulls to the north, and Mouse B pulls to the east. If the overall net force on the cheese is 8 N, 40  N of E, how hard is each mouse pulling? 8 N 40  Force B Force A 8 sin(40  ) = 5.14 N 8 cos(40  ) = 6.13 N

Why Does Net Force Matter? The net force on an object determines how it will move Newton’s 1 st law: an object’s state of motion (velocity) won’t change unless it is acted upon by a nonzero net force

Zero Net Force When the net force on an object is zero, we say that it is in equilibrium ◦ An object in equilibrium will be at rest or moving at a constant velocity The reason for this is that all objects tend to resist changes in motion; this property is called inertia ◦ The more mass an object has, the more inertia it has

Nonzero Net Force If an object has a nonzero net force acting on it (in other words, the forces don’t all balance out), it will accelerate in the direction of that nonzero net force. We’ll talk more about this next class!

Equilibrium and Net Force What is the net force on a book sitting on a table? F net = 0 What is the net force on a puck sliding at a constant velocity of 5 m/s to the east? F net = 0 What is the net force on the object at right? F net = 2 N north If it’s moving south, what will happen to it due to this force? it will slow down 5 N 3 N