1. Warm-up:
Identify the action force and the corresponding reaction force for each event:
A fish swims through water.
A rocket accelerates in outer space.
If a soccer ball pushes back on your foot just as hard as your foot pushes on the ball, how can the soccer ball go anywhere?

2. Newton’s 3rd Law
With Newton’s 1st and 2nd Laws, we only talk about the forces on a single object.
For N3L, we are now talking about the interaction between TWO objects.
Symbols:
To show the force that one object exerts on another, we use a symbol like this:
F1-2 = the force that object 1 exerts on object 2

3. Newton’s 3rd Law
Example: if you kick a soccer ball, the two forces would be written like this:
Ff-s = the force of your foot on the soccer ball
Fs-f = the force of the soccer ball on your foot
So, N3L says that Ff-s = Fs-f

4. Newton’s 3rd Law The Horse-and-Cart Problem:
Problem: If the force of the cart on the horse (Fc-h) is just as strong as the force of the horse on the cart (Fh-c), how can the horse move the cart?
Fh-c
Fc-h

5. Newton’s 3rd Law Think about each object separately! Fh-c Fc-h
First, draw a force diagram for the cart:
Then, draw a force diagram for the horse: FN
Fh-c Ff FG FN
Fc-h Ff
FAPPLIED FG

6. Newton’s 3rd Law
Answer: the horse can move because the force from the ground (forward) on the horse is even bigger than the force from the cart (backward)!!!
IMPORTANT!!! -
The “equal and opposite” forces don’t cancel each other out, because they are acting on different objects!

7. N3L: Force Diagrams
Your assignment is to finish the N3L Force Diagrams worksheet
We’ll do an example together.