Today's Concepts: a) Energy and Friction b) Potential energy & force Physics 211 Lecture 9 Today's Concepts: a) Energy and Friction b) Potential energy & force

Hour Exam One on THIS Thursday This is a 50 minute exam. Most of the practice exams online were 90 minute exams. Don’t wait until Wednesday. But bring questions to Wednesday’s review. Wednesday’s review will be working on homework questions that you raise.

Macroscopic Work done by Friction

Macroscopic Work: do not understand the concept of macroscopic work. Kindaa confused. Help me please??/ This is not a new idea – it’s the same “work” you are used to. Applied to big (i.e. macroscopic) objects rather than point particles (picky detail) We call it “macroscopic” to distinguish it from “microscopic”. You will deal with this in Physics 213 Macroscopic work changes only macroscopic (mechanical) energy, not heat.

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“Heat” is just the kinetic energy of the atoms! Demo: cork popper

Checkpoint/ACT A block of mass m, initially held at rest on a frictionless ramp a vertical distance H above the floor, slides down the ramp and onto a floor where friction causes it to stop a distance D from the bottom of the ramp. The coefficient of kinetic friction between the box and the floor is mk. What is the total macroscopic work done on the block by all forces during this process? A) mgH B) –mgH C) mk mgD D) 0 m H D

CheckPoint What is the total macroscopic work done on the block by all forces during this process? A) mgH B) –mgH C) mk mgD D) 0 A) the only work there is the potential energy initially B) work= change in potential energy C) The only work being done on the object is by the friction force. D) total work is equal to the change in kinetic energy. since the box starts and ends at rest, the change in kinetic energy is zero. m H D

CheckPoint/ACT A block of mass m, initially held at rest on a frictionless ramp a vertical distance H above the floor, slides down the ramp and onto a floor where friction causes it to stop a distance D from the bottom of the ramp. The coefficient of kinetic friction between the box and the floor is mk. What is the macroscopic work done on the block by friction during this process? A) mgH B) –mgH C) mk mgD D) 0 m H D

CheckPoint/ACT What is the macroscopic work done on the block by friction during this process? A) mgH B) –mgH C) mk mgD D) 0 B) All of the potential energy goes to kinetic as it slides down the ramp, then the friction does negative work to slow the box to stop C) Since the floor has friction, the work done by the block by friction is the normal force times the coefficient of kinetic friction times the distance. Work done by friction is negative. m H D

Ouch! They’re perpendicular. I don’t understand this. no Any better understanding by now? What forces?

must be negative N1 mg m N2 mg m mg H

Potential Energy vs. Force

Potential Energy vs. Force Demo: car on track

ACT Suppose the potential energy of some object U as a function of x looks like the plot shown below. Where is the force on the object zero? A) (a) B) (b) C) (c) D) (d) U(x) x (a) (b) (c) (d)

ACT Suppose the potential energy of some object U as a function of x looks like the plot shown below. Where is the force on the object in the +x direction? A) To the left of (b) B) To the right of (b) C) Nowhere U(x) x (a) (b) (c) (d)

CheckPoint – BAD WORDING Suppose the potential energy of some object U as a function of x looks like the plot shown below. Where is the force on the object biggest in the –x direction? A) (a) B) (b) C) (c) D) (d) U(x) x (a) (b) (c) (d)

ACT – BETTER WORDING Suppose the potential energy of some object U as a function of x looks like the plot shown below. Where is the force on the object in the –x direction & of the largest magnitude? A) (a) B) (b) C) (c) D) (d) U(x) x (a) (b) (c) (d)

ACT – BETTER WORDING Suppose the potential energy of some object U as a function of x looks like the plot shown below. Where is the force on the object in the –x direction & of the largest magnitude? A) (a) B) (b) C) (c) D) (d) U(x) x (a) (b) (c) (d)

Your Comments Careful! A negative slope means a positive force! good At A) slope is neg so force is positive. At A) slope is neg so force is positive. Careful: F = - dU/dx At C) slope is pos so force is negative.

More Comments from this Morning

L v Conserve Energy from initial to final position

mg T L

mg T

h h Conserve Energy from initial to final position

h mg T r