2007-10-17

Today's nonsense
- Donuts! (Thanks Brook.)
- PSAT testing for sophomores and juniors today.
- Homework due tomorrow: pg 105 #....
- Spiller wants the administration to start requiring me to write down in the pass-book every time I use an electrical outlet.
- The autotech guys don't even know what's wrong with the car.
-

The SI unit of mass is the kilogram.
SI unit of force is the Newton or kg-m/sec^2
US customary system unit of force is the pound
US unit of mass is the slug which is the mass of an object that weighs 32.2 lb

Weight - the force of gravity on a object causing it to accelerate. Your weight is going to change from planet to planet ... during your career? Whatever. Your mass is constant. What's the relationship between weight and mass?
Weight is a force. So, force=ma, so ... according to the book, g = 10 m/s^2.

1 pound (lb) = 4.448222 Newton.

Hence weight is a force.

ex 1) What is the weight in pounds of 1 kg?

2) The net force acting on a 130 lb student is 25 lb. What is her acceleration?

ex 3) What force is needed to give an acceleration of 3 ft/sec^2 to a 5 lb block? The force is 0.4658 pounds (lbs).

Newton's Third Law

Forces always occur in equal and opposite pairs. If object A exerts a force ofn an object B, an equal but opposite force is exerted by object B on object A.

ex) A 5.6 kg object hangs at rest from a rope-like substance attached to a beam.
A) Draw a diagram showing all forces and reaction forces acting on the object (and indicate each reaction force). So you have an object hanging, so here's the string, here's the object, The force in the rope is tension.
B) So, in the string and the ceiling, you have a tension going down, and an equal tension pulling up.