Newton’s Third Law and Momentum 3.1 pp Mr. Richter

Agenda  Warm-Up  Notes  Newton’s Third Law (N3L)  Action-Reaction Pairs  Momentum  Test Grades are Online

Objectives: We Will Be Able To…  Use Newton’s third law to explain various situations  Define and calculate momentum.

Warm-Up:  Imagine that a man and an elephant are standing on skateboards next to each other. Now imagine that they push against each other. What will happen?  Will the elephant move? The man? Both?  Which will move faster, if at all?  Discuss at your table, and we will discuss as a class in a few minutes.

Newton’s Third Law (N3L)

 In your head, complete the following sentence:  “For every action there is…  “an equal and opposite reaction.”  This is essentially Newton’s Third Law.  For every action force, there is a reaction force equal in magnitude (strength) and opposite in direction.

Newton’s Third Law (N3L)  Forces only come in action-reaction pairs  There are no solo forces!  It doesn’t matter which force is the action and which is the reaction, because they are equal.

Newton’s Third Law (N3L)  Action-reaction forces exist even when objects aren’t moving!

Newton’s Third Law (N3L)  If action and reaction forces are equal, how does anything move?  Action-reaction forces DO NOT cancel each other out.  They act on different objects. Look at the force diagram of just the skateboarder.

Back to the Warm-Up  The man and the elephant push on each other with equal force.  Why does the elephant move less?  The elephant has more mass, so it will accelerate less than the man, who has less mass, even though the forces are equal. Follow up question: When you jump, does the earth move?

Momentum

 We already know that objects with more mass (and therefore more inertia) are harder to stop than objects with less mass.  But what about objects with the same mass?  Which skater is more difficult to stop?  What other quantity (besides mass) makes a difference in how easy an object is to stop?

Momentum  Momentum measures a moving object’s resistance to changes in motion.  Inertia only relates to an object’s mass, whether it is moving or not.  Momentum (p) depends on an object’s mass and velocity.

Calculate Momentum  The units of momentum are units of mass time units of velocity.  mass x velocity  [kg] x [m/s] = [kg  m/s]  Calculate the momentum of both skateboarders.

Wrap-Up: Did we meet our objectives?  Use Newton’s third law to explain various situations  Define and calculate momentum.

Homework  p 64 #1,2