1. Newton’s 3rd

2. Remember the balloon rockets?
What two things were acting in this system?
balloon and air
Which way did the air push on the balloon?
air pushed the balloon forward
Which way did the balloon push on the air?
balloon squeezed the air out
backward

3. Newton had a law for this…
Newton’s 3rd Law says:
Whenever Thing 1 pushes on Thing 2,
Thing 2 pushes back on Thing 1
in the opposite direction
with the same force
Write this down!

4. Take notice: (write these down)
There are always 2 objects in a SYSTEM of forces
(not the same as tug-o-war forces on the same rope.)
The opposing forces always come in pairs. (Sometimes called an action-reaction pair)
Even though the forces are equal, if the MASSES of the objects are not the same, one will move and the other will not. (Remember F= ma?)

5. Click on the picture to launch the video
A video explanation…
Click on the picture to launch the video

6. Newton on the International Space Station
Click on the picture to get a good visual explanation of Newton’s 3rd law.

8. Now test your understanding…
For each scenario
Make a quick sketch in your notebook
Label the two objects in the system (just pick two if there are more than two)
Draw arrows and label the name of the force in each direction
Indicate which direction an object will move if the objects have different mass
If objects have similar mass (or very little friction) indicate the directions of motion for both objects

9. Getting off of a Boat Take 1 minute to: *sketch a quick model
*label the 2 things interacting in the system
*draw arrows representing the forces
*describe which direction thing 1 and thing 2
will move

10. Person-Boat System Model
Collision System: Body and Boat
motion
motion
Body pushes against boat and boat moves backward
Boat pushes against body and body moves forward.

11. At the Ice Rink Take 1 minute to: *sketch a quick model
*label the 2 things interacting in the system
*draw arrows representing the forces
*describe which direction thing 1 and thing 2
will move

12. Skater-Skater System Model
Collision system = skater-skater
motion
motion
Small skater’s hands push against large skater. Large skater does not move very much.
Large skater’s hands push against small skater. Small skater moves backward because the small skater has much less mass.

13. Newton and Serena Take 1 minute to: *sketch a quick model
*label the 2 things interacting in the system
*draw arrows representing the forces
*describe which direction thing 1 and thing 2
will move

14. Tennis Racket-Ball System Model
Collision system = racket-ball
The racket pushes on the ball and makes the ball move a lot since it is much smaller than the racket+Serena.
The ball pushes on the racket and makes the racket move backward just a little.
motion
motion

15. Lean on Me Take 1 minute to: *sketch a quick model
*label the 2 things interacting in the system
*draw arrows representing the forces
*describe which direction thing 1 and thing 2
will move

16. Body-Wall System Model
Since there was no motion to start with (the girl was just chillin’) and the forces are balanced, there will be no ending motion either.
The body pushes on the wall, but the wall does not move
The wall pushes on the body, but the body does not move.

17. What evidence is there that the nail is pushing back on the hammer?
The hammer bounces
back upward after
hitting
Why does the nail move LOTS more than the hammer?
The hammer has much
more mass than the
nail, so the nail moves
Hitting a Nail
Click on the picture to launch the video

18. Football
There are two pairs of action/reaction systems, can you identify them?
Foot-ground
Arm-shoulder pads

21. Exit Ticket:
Draw a model that shows the action-reaction system of a person kicking a soccer ball.