Newton's Third Law of Motion

John E. McCoy                  Douglass Math and Science Academy 
                               543 N. Waller
                               Chicago IL 60644
                               (312)534-6176

Objectives:

This lesson is for 7th and 8th graders.
Students will be able to do activities to demonstrate Newton's Third Law of 
  Motion. 
Students will be able to explain Newton's law.

Materials Needed:

1. dynamic carts
2. 100g weights-6
3. spring balances-3
4. fish line
5. rope
6. clamps-4
7. pulleys-2
8. meters sticks-10
9. scissors,tape,paper
10. blueprint of rocket

Strategies:

Using the PHENOMENOLOGICAL approach, the teacher demonstrates Newton's Third Law 
of Motion by placing a firecracker between two pop cans and lighting it.  Also a 
student is used to demonstrate force by having the student press against a wall 
(action) and the wall presses against the student (reaction).  Also the teacher 
pushes down on the student's arm (action) and the student's arm moves upward. 

Students move to the various stations to do the activities.
 Station 1-Balloon Race
 A fish line is extended from two ring stands for the race.  The students use 
tape to attach the balloons to the straw.  The fish line is pushed or threaded 
through the straw.  The balloon(s) is/are blown-up and the end is pinched or 
closed.  The students release the balloons.  The air comes out the end of the 
balloon (action) and the balloon moves forward (reaction).  The distance 
traveled is measured in meters. 

 Station 2-Spring Balances and Weights.
 A demonstration of equal force being exerted on a spring balance is set-up for 
the students.  There are two 200g weights attached to each end of the balance.  
The students are asked to guess the weight on the balance. Later the students 
will vary the number of weight and check the reading on the balance to see if 
the amount of force is the same (equal).  The weights are pulling downward on 
the balance scale (action) and the scale is pulling on the weights (reaction).

 Station 3-Dynamic Carts
 Two carts are placed end to end and a trigger is released where one cart hits 
the second cart (action) and the second cart moves backward (reaction).  Both 
carts move backward equal distances because the masses are the same.  However, 
if the masses are varied 2x, 3x or 4x, the students will get some interesting 
results about the distance that each cart will travel. 

 Station 4-Roller Skates
 Two students, both on roller skates, will pull on a rope and the other students 
will observe which student moves the greater distance. 

Station 5-Rocket Launching
Using blueprints, the students make rockets for launching.  The pieces are put 
together with tape.  The rockets are launched by using a straw to blow into the 
open end.

Conclusions:

1. Students will discover that the activities dealt with forces.
2. Students will discover that the two forces, action and reaction, are equal in 
   magnitude and opposite in direction.
3. The student will discover that the motion of the forces must be in a straight 
   line. 
4. Students would have discovered Newton's Third Law of Motion using the 
PHENOMENOLOGICAL approach.