Announcements

• Reading Week 10:• Gregory, Chapters 15; 16, pp. 326-331, 339-end; and

Chapter 20, pp. 419-25. Conservation of Mechanical Energy

• HW7 due 3 November • Midterm mean was 85• Grades posted on canvas later today• Midterm solutions and essay grading remarks

posted on Tests page.• Acceleration problem

Last time18th cent. vis viva controversy: does the universe run down?

Descartes: universe consists of many parts collidingwith each other, but in each collision God ensures that “no motion is lost”

His guess for what physical quantity stayed the samein a collision: “force of motion” mv

v v

Inelastic collision:before after

m m 2m

+mv – mv = 0 2m(0) = 0

[today: mv=momentum]

Huygens: yes, but remember to include the sign of v!

Last time

Leibniz: doesn’t like Cartesian proposal, since inelastic collisions will still run universe down.

Proposed instead vis viva, mv2

Vis viva survives inelastic collisions, since clay particles move afterwards (clay heats up)

‘sGravesande corrected to ½ mv2

[today: ½ mv2 =kinetic energy]

Clicker questionWhich of the following was a challenge to his 1/r2 law of gravitation that

Newton survived in the 18th century?

• shape of the Earth: prolate or oblate• date of return of Halley’s comet• influence of sun on moon’s motion• slowing down of moon’s orbit• all the above

Flammarion engraving 1888

The infinite universe that Laplace showed was stable and eternal

We may regard the present state of the universe as the effect of its past and the cause of its future. An intellect which at a certain moment would know all forces that set nature in motion, and all positions of all items of which nature is composed, if this intellect were also vast enough to submit these data to analysis, it would embrace in a single formula the movements of the greatest bodies of the universe and those of the tiniest atom; for such an intellect nothing would be uncertain and the future just like the past would be present before its eyes.— Pierre Simon Laplace, A Philosophical Essay on Probabilities

It was a mechanical clockwork universe that had and would continue to tick along

As Halley had shown in the problem sof the shrinking of the Moon’s orbit and the prediction of his comet’s reappearance, you could run the Newtonian mechanism of the heaven backwards as well as forwards

Newtonian celestial mechanics was, in other words, reversible

This week we’re going to give you the facts of life

We’ll replace Laplace’s well-ordered, stable, eternal clockwork universe with one that ends with a whimper

This is because there are irreversible processes at work

“This is the End”

From Camille Flammarion (1842 -1925)

Goals for today

1. To lay the groundwork for the undoing of Laplace’s universe

2. To do that we have to look at some of the other forces of nature than just the contact forces we have been considering

Electrical Magnetic Chemical Optical Thermal

3. To see some examples of how these “forces” were interconvertible

4. To begin to see how there was something special about heat

Descartes’ cosmos underscored the central importance of matter in motion

It was “the name of the game”

Natural philosophers wanted to learn all they couldabout how matter comes to be in motion

Descartes was concerned with contact motive forces – collisions of masses already moving

In the wake of debates about vis viva, natural philosophers became interested in other forces that were a counterpart to “living forces”

These “dead” forces were exerted on matter but did not result in the motion of matter unless they were converted into motive force

As they investigated these forces they discovered that there were numerous ways in which they were interconvertible

What were these forces?

Electrical force

Magnetic force

Chemical force

Thermal force

Optical force

What was electricity?

Franklin thought of it as a weightless fluid that repelled itself but was attracted to normal matter

Invention of the Leyden Jar

Lucia Galeazzi

Force conversions ??

Force conversion??

Dissociated water into two gases using current from a battery, 1800

William Wollaston

Herschel experimented on the temperature of colored light

Noticed that region below red was hottest of all

Force conversion ??

Ritter experimented with darkening of the muriate of silver (AgCl) by colored light

Prevented darkening DarkenedNo effect

Darkened most of all

Force conversion ??

Heat

The transformation of heat into motive force was a major factor of the Industrial Revolution

Newcomen steam engine

Sadi Carnot

Carnot noted that to use heat to produce mechanical force required that something at a higher temperature fell to a lower temperature. Without a temperature difference the heat was “useless”

He also thought that heat was conserved

Carnot imagined that heat was merelyused to create the motion of the pistonlike water is used in a water wheel(so the water is not used up but can be used again)

Others said Carnot was wrong -the heat actually turned into mechanical force

James Joule

In England James Joule determined experimentallyhow much heat corresponded to how much mechanical force, settling the question of whetherheat was conserved or not (it was not)

In Germany Rudolf Clausius said Joule and Carnot were both right

Carnot was right that the temperaturemust fall for heat to become mechanical force

Joule was right that heat became mechanical force (heat not conserved)

Because of this not all of the heat becamemechanical force. There was always some that was merely transferred from a warmbody to a colder one.