Class Notes 31 October Pick up HW / Turn in new HW Today: #1, 2, 3: Use Kepler’s third law. #2b: T needs to be in seconds! #4a: Ratio is compared to total dist Today : Don’t leave out the cosq! Today: Kinetic and Potential Energy Non/Conservative Forces

If 1 Joule of work is required to stretch a spring 1 cm, how many Joules of work are required to stretch the spring another cm? . 1 2 3 4 other Correct answer is (d) 3 Joules.

Could you give an example where the work done by a force is positive, negative, and zero? Yes, all three, many examples. I could think of a few. Maybe one if I were awake. No I’m completely baffled.

Energy Definition: Loosely, the capacity or ability to do work Various types of energy Chemical, Electrical, Thermal, Solar, Nuclear, … Mechanical Energy (ME) Kinetic Energy (KE) Potential Energy (PE) Also list units with each type.

Kinetic Energy Define (translational) Kinetic Energy as the energy associated with motion KE  ½ (mv2) Work–Energy Principle The net work done on an object is equal to the CHANGE in its kinetic energy. Can now estimate the KE of a human being running. Wnet = DKE

Example A 10.0 N force pulls a 2.00-kg object at a 60.0° angle across a frictionless surface. Find the work required to go from 0 to 5.00 m and final speed if it starts with a speed of 3.32 m/s. For part b) give an old way: “draw FBD, sum force in y and x, find a, use a to find time, then find velocity”, or just use work-energy principle. W = 25.0 J, vf = 6.00 m/s

Which has more energy? Recall that energy is the ability to do work! The higher one, of course. External work was required to move it there. How much? PEg = mgy

Potential Energy Rather than talk about the amount of work done on an object (that may have occurred previously) let us realize that this object has the ability to do work. That is, it has stored energy!  I’ll keep this slide in here and say this, but delete before class. PEg = mgy

Potential Energy Potential energy is stored energy associated with position or configuration. This implies that you must choose a reference, a zero point! Only a DPE matters (that is has a physical significance)! By the same logic, for a spring PEspring = ½kx2

Homework Read chapter 6, sections 6-7 Do problems on WebAssign #26 Do problems using the work-energy principle where appropriate. HW#27: Test Rework due Thursday, 5:00 PM