Aim How can we describe Newtons Third Law

Aim: How can we describe Newton’s Third Law? Do Now: Quiz HW: Problem Set

Review What are Newton’s First and Second Law? 1. 1 st Law: Every object continues in it’s state of rest or of uniform velocity in a straight line as long as no net force acts on it. 2. 2 nd Law: The sum of the vector forces is equivalent to the mass of the body that the forces are acting on times the object’s acceleration, resulting in a NET force.

How can we best describe Newton’s Third Law? 1. What is the action and reaction force during the shuttles take off?

Examples of Newton’s Third Law 1. 2.

What is a formal definition for Newton’s Third Law? • Simply put… For every action there is an equal and opposite reaction! • AP Style definition: For every force applied to one object, an equal and opposite force is applied by ANOTHER object! • F 12 = F 21 • Newton’s Third Law force pairs are equal and opp.

Demo & Question I slam a book on the table… What is F 12? What is F 21? How do you know? ? ? • Table shakes and book’s motion changes!

Question A block of wood sits at rest on a table. A free body for the block is shown. State two actionreaction force pairs in accordance with Newton’s 3 rd Law involving the wood. Did you say mg = FN? ? • WRONG!!!!! Remember when I said to label all the forces in your FBD? FN mg

Third Law Rules 1. Forces must me equal and opposite! • Can “mg = FN” ALWAYS if FN can be less or more than “mg” due to a Net acceleration, such as in the elevator? 2. Opposite Forces come from another object! Piece of wood: 1. Force of wood on table and table on wood. 2. Force of earth on wood and force of wood on earth!

Summary 1. Describe Newton’s Third Law 2. Why is mg = FN NOT a Newton’s 3 rd Law force pair? 3. How can we easily determine the Newton’s 3 rd law force pairs?

Aim: Fnet for the system Up to this point we have always looked at the net force for individual parts of a system. How have we done this?

Fnet for the System In the diagram at right a 1 kg block pulls a 2 kg block over a smooth table. Determine the Net force for the system. What is the system mass?

Fnet for the System What is the system’s acceleration if the FF is now 5 N? How about if the top block is pushed to the left initially?