1. PHYSICS 50: Lecture 11.1
RICHARD CRAIG

2. Homework #10 Chapter 10 Chapter 11 Exercises and Problems
We will skip 10.7
Chapter 11
Sections 1,2,3 only
Exercises and Problems
10.1, 10.16, 10.41, 11.12, 11.14, 11.56
Due Tuesday April 14

3. Plan for the Day Some thoughts on Quiz of Tuesday
Center of Mass and Center of Gravity
Equilibrium
Static Problems (things don’t move)
Simple: Board on benches
Medium: Hammer
Hard: Hanging sign
No Quiz (yeah!) but have short exit test

4. Center of Mass Formal Definition:
If density constant center of mass is just the geometric center of a body
If gravity constant center of gravity and center of mass the same point
For torque calculations a body acts as if all the weight is at the center of gravity (linear function unlike moment on inertia)

5. Conditions for Equliubrium
Sum of forces must be zero
Sum of torques must be zero

9. Simple Example: Bench
What are the forces from each support on the bench?

10. Idea problem Q11.2 A. less than 0.25 m B. 0.25 m
A rock is attached to the left end of a uniform meter stick that has the same mass as the rock. How far from the left end of the stick should the triangular object be placed so that the combination of meter stick and rock is in balance?
A. less than 0.25 m B m
C. between 0.25 m and 0.50 m D m
E. more than 0.50 m

11. Medium Example: Hammer
If I pull the hammer with 50lbs, how much force is on the nail?

12. Hard Example: Hanging Weight
Bar weighs 150N, Wire is massless, What is the tension in the wire

13. C. T = w/(sin q) D. T = w/(cos q) E. none of the above
Tell me the answer to leave today………
A metal advertising sign (weight w) is suspended from the end of a massless rod of length L. The rod is supported at one end by a hinge at point P and at the other end by a cable at an angle q from the horizontal.
What is the tension in the cable?
A. T = w sin q B. T = w cos q
C. T = w/(sin q) D. T = w/(cos q)
E. none of the above