2/24/17 Today we will identify action/reaction pairs Warm Up: Identify the action/reaction pairs below: 1. A foul ball hits a fan in the stands. 2. Mrs. Nairn drops her computer out the window

Identifying Action and Reaction Action/Reaction Pairs are not always the most obvious forces

Newton's Third Law of Motion: Astronauts in Outer Space

Falling Boulder Reaction Force: Boulder exerts a force on the Earth Interaction: Gravitational attraction between the boulder and the Earth Action Force: Earth exerts a force on the boulder Reaction Force: Boulder exerts a force on the Earth

Earth’s mass so large that it has a very small acceleration; Rock small mass big acceleration

Example Compare force, mass and acceleration of cannon and cannonball

What is the action? What is the reaction? Cannon pushes cannonball. Cannonball pushes cannon.

How do the forces acting on each compare? Equal but opposite

How do the masses compare? Cannon has a much larger mass than cannonball

Do both the cannon and cannonball accelerate? Yes! (we know cannonball does…we see it fly from the cannon…) (cannon has a recoil/kickback)

How do their accelerations compare? Cannonball will accelerate more due to its small mass compared to the cannon

Compare the accelerations if the cannon and cannonball had the same mass. Acceleration would be the same

A moving car hits a bug flying across a road. What is the action-reaction pair? Which feels the greater force? Why does the bug have a bad day, but the car does not?

How does one team win a game of tug-of-war? It can’t be that one team pulls harder than the other, why can’t it be that? Forces would be equal and opposite Example: piece of paper The team that wins has a greater force with the ground! (more friction) Think back to the video with the astronaut

A rocket does not need the air around it to move. Why? The rocket and the fuel inside that is being released are part of the action, reaction pairing. Example: a Balloon Why does a balloon fly around the room when you let go of it?

Do action and reaction pairs cancel? Since action and reaction pairs are equal but opposite, why don’t they cancel? Simple….they are acting on two different objects!

Look back at the cannon and cannonball Cannon exerts a force on the cannonball Cannonball exerts a force on the cannon In order to be considered an action-reaction pair, the forces must be on each other!

Back to the cannon… So both the cannon and the cannonball have an external force on them Remember Newton’s first law – object will accelerate when there is an external force on it

The Horse-Cart Problem

Book Work Page 83: 1-8, 12-26, 29-30.