1. Chapter 8: Newton’s Third Law
Action/Reaction Pairs
Acceleration Constraints
Statement of Newton’s Third Law
Applications
HOMEWORK ASSIGNMENT
Chapter 8: 28, 34, 38, and 40
Chapter 9: 8, 14, 20, and 42
DUE April 8, 2005

2. Action/Reaction Pairs
When two particles interact, the force can be viewed as action/reaction pair
For example: Bat hitting a base ball, your pushing the wall, hanging onto a rope etc.
Observe that action/reaction pairs does not act on the same particle
Net force of a system of interacting particles should be worked out by identifying all the forces acting on each particle

3. Newton’s Third Law
Technically the action/reaction forces can be stated as “for every action there is an equal and opposite reaction”
Justify your definition using the examples such as normal force, frictional forces (static and kinetic)
If a ball is thrown vertically upwards, what type of Force is acting? Identify action/reaction pair and justify your answer in terms of Newton’s third Law!!!

4. Forces involved in towing a car
Forces are the same …but the acceleration would depend on particle’s mass

5. Acceleration Constraint
When two particles are connected (by a rope, say) then, the particles can’t have different accelerations!
What happens otherwise?
aAx= -aBy

6. Example:
What is the maximum force with which the truck can pull the crate without damaging the contents of the carte?

7. Ropes We assume that the mass of the rope is negligible
This approximation is popularly known as “massless string approximation”
Tension in the string (A on B) = Tension in the string (B on A)
Therefore, T(S on A) = - T(S on B)

8. Ropes and Pulleys
What would the acceleration of the system if mass of B is > mass of A ?

9. Example(2)
A person is hanging on the other end of the rope with one end holding a load as shown in figure. Find the person’s acceleration given the information 

10. Example (3)
Find the acceleration if the smaller mass is twice the bigger mass, and a force of 40 N replaces the force shown in the figure.
Given that the coefficient of kinetic friction between the blocks is 0.5, and between the lower block and the surface it is 0.3