Air resistance and Levels of Detail in Physics

Somewhat similarly to friction, air resistance also acts as a retarding force.

The real world is not the same as idealized beginning physics situations; for example, projectiles will not follow the path of a parabola, unless they are in a vacuum.

Also, this air resistance is not a constant value, but instead it is proportional to velocity.

In situations with air resistance the acceleration is not constant either, to deal with that you can express acceleration as dv/dt (the instantaneous acceleration).

Since derivatives are involved, the use of integration, and methods such as u-substitution, can be used to solve these more complicated motion problems.

Because integrals can be complicated, computers were invented for greater artillery accuracy in wartime.

Rocket Explosions, Probability, and Compound Interest

If there are 1000 parts for a space shuttle and each of them has a 99.9% chance of working, then the chances for no malfunction at all is only about 37%.

By multiplying .999 by itself 1000 times without rounding, the true result is found, but if each time it had been rounded up a crew might have piloted the spacecraft and met their doom in an explosion.

When manipulating equations, the numbers should also not be plugged in until the end.

Momentum and Momentum’s SI Units

There is something in physics that does not have a special name like a Newton or a Joule.

It is momentum and can be expressed as mass multiplied by velocity (kg*m/s).

Like energy and matter, momentum is conserved.

If you shoot a gun, there will be a recoil; the bullet goes forward and there is also a force backward.

The greater the force forward is, the greater the opposite force will be, so shooting a shotgun will have more recoil than James Bond’s ppk since the a shotgun shell has greater mass and about the same speed.

Also if you’re ever on a frictionless frozen lake you could throw your boot in the opposite direction of the direction you want to go, and you would end up moving to the edge of the lake (if you disregard friction).

In the real world though there is friction, and losing the toasty interior of the boot might not be the best idea.

Path Independence and Thermodyanamics

Earlier in the year we said that the work necessary for hiking up the slope of Rampina Mountain vs. climbing straight up the cliff route would be the same, independent of the path taken.

This statement is true if you disregard friction and other factors, but one path has a greater length and more friction will be experienced (probably).

May approaches in physics are approximations, but they can be fairly accurate up to a certain point.

If you climb a rope, the friction between your hands and the rope or your gloves and the rope would be unique.  The friction between your shoes and the ground could vary on shoe type, etc.

Or the ground could be icy……

Thus the work is not really path independent.

Even more so, thermodynamics is not path independent-

While throwing a marble up and watching it come down is symmetrical in motion, the rewinding of a camera of a thermodynamic reaction would definitely not be the same as playing it from the beginning (at least in all likeliness).

Heat Flow

Heat naturally flows from warmer to cooler.  Things tend towards equilibrium.

Therefore heat vents are most efficiently placed on the floors of rooms so the heat might rise up throughout the room.

If the vents were on the ceiling, then all the heat would stay towards the top of the room.

And fans are then placed on the ceiling to circulate the air around more efficiently.

All this works according to the formula H=K*L/A(TH-TC) where K is the coefficient of thermal cooling for a substance.

Latent Heat of Fusion | Latent Heat of Evaporation

If you add heat to some substance it will not necessarily change temperature.

Energy also can work towards producing a phase change, such as from solid to liquid or liquid to gas.

Water might heat up and change temperature or some portion of it might turn instantly to steam, leaving the rest of the water at the same temperature.

The “latent heat of fusion” and “latent heat of evaporation” will vary for different materials and can be used to calculate the energy necessary for these changes.

What is Heat? And Heat Transfer

The term heat refers to the flow of energy.

If you drank a cool drink, it would only cool you down the difference in temperature for that amount you drink.

The liquid is brought into your system and therefore will warm up to your body temperature, while your body correspondingly cools down a little bit.

Different materials have different specific heat capacities so something like lead will heat up much faster than water.

The lead will also cool down faster while the water retains heat for much longer, this is partially related to hydrogen bonding in water.

How a Thermostat Works, Thermal Expansion Coefficients

A good thermostat will accurately measure the environment while simultaneously not greatly changing its surroundings.

The basic construct that that makes a non-digital thermostat work is that a bimetallic strip has two layers of different metals.

The two coefficients of thermal expansion will be different, so because the two layers are connected, the thermostat will bend in a certain way according to temperature.

For instance aluminum will expand faster than brass.

Another element, quartz has an especially low coefficient of thermal expansion and is thus used in clocks to keep time since it contracts and expands with regularity only a small amount.

Speed of Light and Relative Speed

If Spaceman Spiff attaches a flashlight to the front of his spacecraft the light is still going to travel at the speed of light c.

Likewise if he attaches a loudspeaker to his ship in order to yell at aliens, those waves will travel at the speed of sound.

While a military plane going mach five (five times the speed of sound) could hit its own bullets, both sound and light travel at speeds independent of the source.

The plane that attains a speed of mach five will also undergo the most stress as it passes mach one since it experiences the constructive interference of the entire cone of sound at that point.

Decibels, Loudness


From woodleywonderworks on flickr

When someone returns from a heavy metal concert they don’t usually comment loudly about how many Watts per meter squared they experienced.

Decibels might make sense.  People tend to know that 120 Decibels is a lot.

This system has been devised and units of decibels measure perceived loudness.

It increases on a logarithmic scale, so something that seems twice as loud really has 10 times the intensity.

Position also can vary the perception of sound such as when a noise emitter comes nearer to you the frequency seems greater and the pitch higher (Doppler Effect).