# Physics 211 - PowerPoint 2

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

Physics 211 Lecture 9, Slide 1
Can I get extra credit for saying it's awesome? Seriously, though, it's a great
class to be in and the preflights/prelectures coupled with the in-class
examples make it an enjoyable experience.
would like to see more examples on how to do different types of problems.
I enjoy the course. I really wish there was a coursebook. It would make
studying easier rather than going back and skipping around in the prelectures.
Mainly, this course is a tad confusing for me because I never took physics in
high school. Despite this, I feel I am catching up alright and understand the
material pretty well. I just wanted you to know that there ARE some (or maybe
I'm the only one) students who are taking this class for the first time. :)
I keep saying in all my responses to "What topics would you like us to discuss
in lecture?" that I would like to learn about the calculus aspects of the topic we
are on.
One of these days I'm going to write something in here that is so interesting
that it will get posted up on the PowerPoint in class... that's my goal. Oh and
also to get an A... but getting on the PowerPoint is more important.
Whoever wanted to throw the pudding/pizza party, can you tell me what
time and where you'll do it? It is sort of hard for me to go there if I don't
know where it is. Love you lots!                             Physics 211 Lecture 9, Slide   2
Physics 211 Lecture 9, Slide 3
Macroscopic Work:

This is not a new idea – it’s the same “work” you are used to.

b
W   F  dl      Applied to big (i.e. macroscopic) objects
a

We call it “macroscopic” to distinguish it from “microscopic”.

You will deal with this in Physics 213

Physics 211 Lecture 9, Slide 4
Physics 211 Lecture 9, Slide 5
Physics 211 Lecture 9, Slide 6
f
f

Physics 211 Lecture 9, Slide 7
“Heat” is just the kinetic energy of the atoms !

Do Stupid Spinning Heat Demo
Physics 211 Lecture 9, Slide 8
K  Wtot  Wgravity  W friction
0  Wgravity  W friction
0  mgH  W friction
must be negative

N1

m
N
2
mg
H
mmg

mg       Physics 211 Lecture 9, Slide 9
Preflight
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) mkmgD      D) 0

m

H

D Physics 211 Lecture 9, Slide 10
… What is the macroscopic work done on the block by friction
during this process?

B) –mgH
Gravity invests mgH Joules of energy as the box goes down the ramp, and so
the box has mgH Joules of kinetic energy at the bottom. Therefore, the friction
force must do -mgH Joules of work on the box to bring it to a stop.

since the block slows down, the work done by friction must be negative. B is the
only negative choice; C would also be correct if it were negated.

H

D Physics 211 Lecture 9, Slide 11
Preflight
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) mkmgD      D) 0

m

H

D Physics 211 Lecture 9, Slide 12
… What is the total macroscopic work done on the block by all
forces during this process?

D) 0

The kinetic energy is the same at both the beginning and end of its journey.

delta K = Work = 0

I feel like there is some sort of trickery where mgh-ukmgD cancels out.
Otherwise I'm not too sure.

m

H

D Physics 211 Lecture 9, Slide 13
Potential Energy vs Force

b
dU ( x)
U b  U a  Wab    F  dl    F ( x)  
a
dx

Demo        Physics 211 Lecture 9, Slide 14
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)              Physics 211 Lecture 9, Slide 15
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)              Physics 211 Lecture 9, Slide 16
Preflight
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)                Physics 211 Lecture 9, Slide 17

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