PHY 113 A Fall 2012 -- Lecture 13 19/28/2012

PHY 113 A General Physics I9-9:50 AM MWF Olin 101

Plan for Lecture 13:

Chapter 8 -- Conservation of energy

1. Potential and kinetic energy for conservative forces

2. Energy and non-conservative forces

3. Power

PHY 113 A Fall 2012 -- Lecture 13 29/28/2012

PHY 113 A Fall 2012 -- Lecture 13 39/28/2012

Note: Because of PHY 114 exams on Sunday and Monday, the tutorials those nights will be moved from Olin 101 – check for signs – this week only.

PHY 113 A Fall 2012 -- Lecture 13 49/28/2012

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PHY 113 A Fall 2012 -- Lecture 13 59/28/2012

ExampleA block, initially at rest at a height h, slides down a frictionless incline. What is its final velocity?

hh=0.5m

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f

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PHY 113 A Fall 2012 -- Lecture 13 69/28/2012

Energy diagram

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PHY 113 A Fall 2012 -- Lecture 13 79/28/2012

Energy diagrams 22

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PHY 113 A Fall 2012 -- Lecture 13 89/28/2012

Example: Model potential energy function U(x) representing the attraction of two atoms

PHY 113 A Fall 2012 -- Lecture 13 99/28/2012

Example:Simplified version:

(Assume that the sandbag is massive enough to provide the tension in the rope and R=3m.) If the actor starts at vi=0 and q=60o, what is vf?

m/s 5.4

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)cos1(21

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gyv

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PHY 113 A Fall 2012 -- Lecture 13 109/28/2012

Example: Mass sliding on frictionless looping track

i

iclicker exercise:In order for the ball completes the loop at A, what must the value of h?

A. h=RB. h=2RC. h>2RD. Not enough

information.

PHY 113 A Fall 2012 -- Lecture 13 119/28/2012

iExample: Mass sliding on frictionless looping track

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PHY 113 A Fall 2012 -- Lecture 13 129/28/2012

Another example; first without friction

Mass m1 (=0.2kg) slides horizontally on a frictionless table and is initially at rest. What is its velocity when it moves a distance Dx=0.1m (and m2 (=0.3kg) falls Dy=0.1m)?

iclicker exercise:What is the relationship of the final velocity of m1 and m2?

A. They are equalB. m2 is faster than m1.C. m1 is faster than m2.

PHY 113 A Fall 2012 -- Lecture 13 139/28/2012

Another example; first without friction

Mass m1 (=0.2kg) slides horizontally on a frictionless table and is initially at rest. What is its velocity when it moves a distance Dx=0.1m (and m2 (=0.3kg) falls Dy=0.1m)?

T

T

m2 g

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2

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1

PHY 113 A Fall 2012 -- Lecture 13 149/28/2012

Another example; now with friction

Mass m1 (=0.2kg) slides horizontally on a table with kinetic friction and is initially at rest. What is its velocity when it moves a distance Dx=0.1m (and m2 (=0.3kg) falls Dy=0.1m)?

T

T

m2 g

21

21 2

121

if vmvmxfTW D

0

f

PHY 113 A Fall 2012 -- Lecture 13 159/28/2012

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PHY 113 A Fall 2012 -- Lecture 13 169/28/2012

iclicker exercise:Assume a mass m starts at rest at A and moves on the frictionless surface as shown. At what position is the speed the largest?

A. AB. BC. CD. none of these.

PHY 113 A Fall 2012 -- Lecture 13 179/28/2012

Power

Watt(s)

(Joules) : Units

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P

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rF

PHY 113 A Fall 2012 -- Lecture 13 189/28/2012

Watts106/3102

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