Newton’s Third Law of Motion Ch. 2 Lesson 4

Newton’s Laws 1 & 2 Newton’s first two laws of motion describe the effects of balanced and unbalanced forces on one object What was Newton’s First Law? What was Newton’s Second Law?

Newton’s Third Law of Motion When one object exerts a force on a second object, the second object exerts an equal force in the opposite direction on the first object Or for every action there is an equal and opposite reaction

An Example of Newton’s 3rd When the gymnast pushes against the vault, the vault pushes back against the gymnast Notice that the lengths of the force arrows are the same, but the directions are opposite

The forces two objects apply to each other Force Pairs The forces two objects apply to each other

Why Don’t Force Pairs Cancel? If the forces of a force pair always act in opposite directions and are always the same strength, why don’t they cancel each other? Think about it silently

Why Don’t Force Pairs Cancel? Because each force acts on a different object!

The girl’s feet act on the boat. The force on the boat acts on the girl’s feet. The forces do not result in a net force of zero because they act on different objects. Adding forces can only result in a net force of zero if the forces act on the same object

Action and Reaction In a force pair, one force is called the action force The other force is called the reaction force. The girl applies the action Force against the boat The reaction force is the Force that the boat applies To the girl

Action and Reaction For every action force, there is a reaction force that is equal in strength but opposite in direction

Using Newton’s Third Law of Motion When you push against an object, the force you apply is called the action force The object then pushes back against you. (reaction force)

Using Newton’s Third Law of Motion According to Newton’s second law, when the reaction force results in an unbalanced force, there is a net force and the object… -----ACCELERATES!----- Newton’s third law explains how you can swim and jump. -Pg. 72

Pg. 72 Action Reaction Forces For each scenario (swimming, jumping, rocket motion) explain how Newton’s Third Law of Motion makes it possible.

A measure of how hard it is to stop a moving object Momentum Because action and reaction forces do not cancel, they can change the motion of objects. A useful way to describe changes in velocity is by describing momentum A measure of how hard it is to stop a moving object

Momentum

Momentum Comparisons Bullet vs. large ship Bullet may not have a lot of mass, but large velocity Large ship might be moving slowly, but lots of momentum

Interpret Silently read the following:

Newton’s 3 Laws and Momentum pg. 73 How does Newton’s First Law of Motion relate to momentum? How does Newton’s Second Law of Motion relate to momentum? How does Newton’s Third Law of Motion relate to momentum?

Law of Conservation of Momentum The total momentum of a group of objects stays the same unless outside forces act on the objects What is an example of an outside force? Friction

Law of Conservation of Momentum When the first ball (cue ball) has mass and velocity When the cue ball hits the triangle of balls the cue ball’s velocity and momentum decrease. But the other balls start moving. This is because these balls have mass and velocity, they also have momentum!

Types of Collisions When colliding objects bounce off each other, it is elastic collision If the objects collide and stick together, the collision is inelastic when one football player tackles another, No matter the type of collision, the total momentum will be the same before and after the collision

Types of Collisions http://www.teachertube.com/video/elastic-and-inelastic-collisions-30870