1. Aim: How do we explain Newton’s 3rd Law?

2. Explain this

3. Newton’s Third Law
When two objects interact, they exert forces on each other. Newton’s Third Law says “The force object A exerts on object B is equal and opposite to the force that object B exerts on object A” These forces form an action-reaction pair

4. Newton’s Third Law Action-Reaction Pairs

5. Newton’s Third Law Paradox
A donkey starts to pull a cart (he exerts a force in one direction). The cart therefore exerts the same amount of force on the donkey (a force in the opposite direction). (Newton's third law: For every action, there is an equal and opposite reaction.) So how can the donkey pull the cart if the cart exerts the same amount of force on the donkey in the opposite direction? Should he not move?

6. Donkey-Cart Paradox

7. Why can the Donkey move?
The cart’s force is only in reaction to the horse’s force so it does not define the direction of movement.
The forward and backward forces are equal, so it actually can’t move forward.
The forward force of the horse is just big enough to overcome the backward force of the cart and start the cart forward
The cart is rolling on wheels while the horse’s hooves have traction with the ground
There is a brief moment where the horse pulls before the reaction force kicks in

8. Donkey-Cart Paradox Resolved
Remember there are two different objects to consider here: the horse and the wagon.

9. Thought Question
Do the forces weight and normal force as shown below form an action-reaction pair? They do not because both forces act on the same object

10. Contact Force Problem 1 (Assume no friction)
A 2kg and 8kg box are pushed by a 40 N force across a frictionless surface. Calculate the contact force between the two boxes.

12. Contact Force Problem 2 (Assume no friction)
A 3kg box, 5 kg box, and 2 kg box are pushed together by a force of 54 N across a frictionless surface.
Find the contact force between the 3 kg box and the 5 kg box.
Find the contact force between the 5 kg box and the 2 kg box.

14. Multiple Block Problem
A force F=12 N is pushing three adjacent blocks across a frictionless surface of masses (m1 = 1 kg, m2=2kg, and m3 = 3kg.
What is the acceleration of each block?
What is the net force on m1?
What is the magnitude of the force m1 exerts on m2?
What is the magnitude of the force that m2 exerts on m3?