newton’s third law and momentum

Newton’s Third Law and Momentum3.1 pp 58-64

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)

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

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] = [kgm/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