PHYS16 – Lecture 17 Energy Conservation: Springs October 18, 2010

Review

Practice Question 1 If the momentum of an object increases by 4 fold, how much does the kinetic energy of the object change by? A)1/16 B)¼ C)2 D)4 E)16

Practice Question 2 If the potential energy of a 5 kg book is 100 J, what is the height of the book? A)2 m B)1 m C)20 m D)10 m E)There is not enough information

Practice Question 3 If a car engine for a 1400 kg car outputs 1000 kW of average power, how much time does it take to accelerate from 0 to 25 m/s? A)0.4 s B)1 s C)0.2 s D)0.8 s E)10 s

This Week Spring Force and Energy Conservative vs. Nonconservative Forces Conservation of Energy – Sleds and Rollercoasters – Springs – Pendulums

Springs

Spring Force and Energy Spring Force Spring Energy

What is the Energy of this system? Assume x=0 at Equilibrium point then…

What is the Energy of this system? Approach #1 Assume x=0 at Equilibrium point then… Approach #2 Assume x=0 at ground then… Accounting for zero point not at equilibrium

Zero of Potential Energy Wherever you set your zero point for potential energy will determine your coordinate system – U=0 J at y=0 point for gravitational pot. energy – U=0 J at equilibrium point for spring pot. Energy All calculations of ΔU will be relative so it won’t matter where you put 0 J just make sure not to switch back and forth!!!

Energy Conservation

Conservative vs. Nonconservative Forces Conservative Forces– force where work done over ANY closed path is zero W AtoB = -W BtoA W AtoB, path 1 = W AtoB, path 2 Nonconservative force – not conservative

Is Gravity a conservative Force? Let’s assume two different paths and see if we get the same work… Path #1 Path #2 Yes, so gravity is conservative!

Is the spring force conservative? Let’s assume two different paths and see if we get the same work… Path #1 Path #2 Yes, so spring force is conservative!

Is friction conservative? Let’s assume two different paths and see if we get the same work… Path #1 Path #2 No, so friction is nonconservative… FfFf FfFf FfFf

Conservative – Spring Force – Gravity – Buoyant Force Nonconservative – Friction – Air resistance/Drag – Fluid resistance – Applied Forces Conservative vs. Nonconservative Forces Situation Dependent Tension Normal Force