Monday, Dec 8, 2008 - Blocks 1 and 3

Collected RA 6.1

Handed back and went over Momentum Test
Collected tests.

Started energy
Walking on beach you find a magic lamp of physics.
Feynman's Block example
Places energy can hide - for this unit, GPE = m g h, KE = 1/2 m v^2, Elastic PE = 1/2 k x^2, work = F * d where F is the component of force in the direction of motion.

Showed that if the Earth went around the Sun in a circular orbit, the Sun would not do work on the Earth. If the orbit is elliptical, when moving towards the Sun, there is a component of force along the direction of motion so the Sun does work on the Earth and the Earth speeds up. When moving away, there is a component of force opposite the direction of motion so the Earth slows down.

Showed again the work done in pulling a block if the force is parallel to the surface and if the force is at an angle. For friction, the force is opposite the direction of motion so the friction force takes away energy in the form of heat.

Introduced Energy Balance Equation.

Did example of lifting a 10 kg mass. First at 120 N for 2 seconds, then at 100 N for 4 seconds and then at 60 N for 1 sec to slow it down. Drew force vs time graph. Showed applied force, weight, and net force. Since net impulse is zero, there is no change in momentum and the block comes to rest. Calculated the work done and showed that it was equal to the change in gravitational potential energy.

Main idea: If you lift an object, the work you do is equal to the increase in gravitational potential energy.

Handed out RA 6.2 and allowed students to work on it for tomorrow.