as we measure an object moving past us, we measure its length to

8-8-11Chapter 8 - Conservation LawsChapter 8 - Conservation Laws

By these things we know that there is a God in heaven, who is infinite and eternal, from everlasting to

everlasting the same unchangeable God, the framer of heaven and earth, and all things which are in them.

~ D & C 20:17


As we measure an object moving As we measure an object moving past us, we measure its length topast us, we measure its length to

increase. decrease.

I’m going to say incr... I’m going to say decr...

7%0%0%

93%1.1. increase.increase.

2.2. decrease.decrease.

3.3. I’m going to say I’m going to say increase but I think increase but I think this is baloney.this is baloney.

4.4. I’m going to say I’m going to say decrease but only so decrease but only so I don’t flunk the I don’t flunk the exam.exam.


ConservationConservation

A few fundamental quantities are A few fundamental quantities are conserved. conserved. That is, they neither That is, they neither increaseincrease nor nor decreasedecrease with time. with time. They are:They are: MassMass Total chargeTotal charge Linear momentumLinear momentum Angular momentum Angular momentum Total energyTotal energy


Non-conservationNon-conservation

To contrast, these quantities are To contrast, these quantities are notnot conserved.conserved. TemperatureTemperature Kinetic energyKinetic energy Potential energyPotential energy Internal energyInternal energy OrderOrder


Conservation of MassConservation of Mass

Mass is neither created nor destroyed. Mass is neither created nor destroyed. But it can change form or state.But it can change form or state.


Conservation of MassConservation of Mass

The Carbon Cycle is an example of conservation The Carbon Cycle is an example of conservation of mass. Trees do not create wood out of of mass. Trees do not create wood out of nothing, but convert COnothing, but convert CO22 and water into new and water into new

wood.wood.


Conservation of Fundamental Conservation of Fundamental ParticlesParticles

Conservation of mass Conservation of mass equates to conservation equates to conservation of the building blocks of of the building blocks of mass.mass. The number of The number of

fundamental particles fundamental particles stays the same in any stays the same in any interaction.interaction.

Leptons (electrons)Leptons (electrons)

Quarks (protons, Quarks (protons, neutrons)neutrons)


Conservation of ChargesConservation of Charges

Rubbing a balloon on Rubbing a balloon on your head your head does not does not create chargecreate charge, but , but simply redistributes simply redistributes existing charge. existing charge. Negative charge goes Negative charge goes

to the balloon leaving to the balloon leaving your hair positively your hair positively chargedcharged

We assume the total We assume the total charge of the universe charge of the universe is is zerozero..


Conservation of Linear MomentumConservation of Linear Momentum

The mass times velocity The mass times velocity beforebefore a a collision must equal the mass times collision must equal the mass times velocity velocity afterafter a collision. a collision.


If a cue ball hits a billiard ball, can it If a cue ball hits a billiard ball, can it bounce straight back?bounce straight back?

Assume they both have the same mass and the cue ball isn’t spinning in some weird way.

Yes No

Only if it hits it stra...

33% 33%33%1.1. YesYes

2.2. NoNo

3.3. Only if it hits it Only if it hits it straight on.straight on.


Conservation of Linear MomentumConservation of Linear Momentum

When a firecracker When a firecracker explodes in midair, explodes in midair, the path of the the path of the centercenter of the of the explosion is exactly explosion is exactly the same as the the same as the path the firecracker path the firecracker would have would have followed if it had followed if it had never exploded. never exploded.


Conservation of Linear Momentum at Conservation of Linear Momentum at Very High SpeedsVery High Speeds

From the point of view of the astronauts From the point of view of the astronauts there is no momentum before the there is no momentum before the explosion, and equal and opposite explosion, and equal and opposite momentum after. momentum after.


Conservation of Linear Momentum at Conservation of Linear Momentum at Very High SpeedsVery High Speeds

From the point of view of ground control From the point of view of ground control the explosion stops ½ of the object but the the explosion stops ½ of the object but the other half is NOT moving twice as fast! At other half is NOT moving twice as fast! At face value this face value this violates violates conservation of conservation of momentummomentum


Mass Increase!Mass Increase!

We can reason either of two ways at this We can reason either of two ways at this point: Either point: Either momentummomentum is not conserved is not conserved at high speeds or at high speeds or massmass is not conserved. is not conserved. Conservation of momentum follows from Conservation of momentum follows from

position symmetry (Noether’s theorem). This position symmetry (Noether’s theorem). This makes it very solid.makes it very solid.

Mass, as it turns out, is not conserved by Mass, as it turns out, is not conserved by itself. Instead it is conserved together with itself. Instead it is conserved together with energy by the relation E = mcenergy by the relation E = mc22. More on this . More on this next time. next time.


Conservation of Angular Momentum Conservation of Angular Momentum

Because of the Because of the conservation of conservation of angular momentum, angular momentum, divers can control divers can control their rotational their rotational velocity by tucking velocity by tucking their limbs. their limbs.

as we measure an object moving past us, we measure its length to

Documents