Evans SPH 3U1

Physics Grade 11

Unit 2: Dynamics

Note 9: Newton's 3rd Law and Free Body Diagrams


Reference: Chapter 3.4



Balloon rocket demo - Tie one end of fishing line to a chair. Put the other end of the line through a straw and pull the line tight and tie the other end to another chair. Blow up the balloon (don't tie it). Pinch the end of the balloon and tape it to the straw. You're ready to launch!

Chairs on rollers - Two roller chairs are set up with one student in each one. Each student holds one end of a rope. HAve one student pull on the rope. Now have both students pull on the rope.

Newton’s Third Law (action-reaction): For every action force, there is a simultaneous reaction force equal in magnitude but opposite in direction.

Discuss: Car accidents (force same but compare acceleration of small car and large truck), Get students to jump on the earth (can you move it?)


Free Body Diagrams Show Teacher Resource overheads 4.11A, 4.11B. 4.16


Perform the Farmer Brown Drama Skit

Action/Reaction forces act on different objects. They can never be drawn on the same FBD.


Handout Newton's Third Law Practice Set


Pg 140 #1,2
Pg 141 #3,5,7,9