# Implications of movie physics

## Recommended Posts

Originally I was going to make this a "relative motion" thread, but I think the notion of analyzing movie physics is more interesting overall.

In "Rat Race," a mechanic startled by the passing supersonic landspeeder fires a bullet parallel to its path. (At about a minute and a half into the clip.) To the drivers, however, the bullet appears to be suspended in mid-air, as it is moving at approximately the same velocity as the landspeeder.

1. Would the bullet's path be kept horizontal for any non-negligible amount of time by air resistance, or would the vertical component of its motion immediately assume downward acceleration like everything else?

2. How quickly would the horizontal component of its motion be slowed by air resistance?

3. Either way, would it be safe for the drivers of this landspeeder to reach out of the window and grab the bullet, provided they maintained the same velocity as the bullet while it was in contact with their hands?

##### Share on other sites

5 minutes ago, ScienceNostalgia101 said:

1. Would the bullet's path be kept horizontal for any non-negligible amount of time by air resistance, or would the vertical component of its motion immediately assume downward acceleration like everything else?

If we ignore aerodynamic effects that might give it lift (I have no idea if that applies to bullets o not) it would fall down at the same rate as if it had been dropped.

6 minutes ago, ScienceNostalgia101 said:

2. How quickly would the horizontal component of its motion be slowed by air resistance?

Don't know. It is probably hard to calculate, but I would bet there are guidelines for different types of bullets.

7 minutes ago, ScienceNostalgia101 said:

3. Either way, would it be safe for the drivers of this landspeeder to reach out of the window and grab the bullet, provided they maintained the same velocity as the bullet while it was in contact with their hands?

They might want to wear gloves, it will probably be hot from air-resistance.

##### Share on other sites

Gah, I forgot all about the thermal effects of all that air resistance.

Anyone know more about air resistance's effects on bullet acceleration?

##### Share on other sites

23 hours ago, ScienceNostalgia101 said:

2. How quickly would the horizontal component of its motion be slowed by air resistance?

Drag force varies by the square of the velocity. It also depends of the drag coefficient for the bullet, which, at high speeds, will also vary with speed ( in a very non-linear way for example, with the bullet I'm considering the Cd  stays constant from 0- mach 0.4, falls between M 0.4 and M 0.9, rises steeply to a maximum from M 0.9 to M 1.1, and then slowly declines past M 1.1)

Using the numbers for a 9mm, 19.44g, very low drag bullet, traveling at mach 1.1, I get a deceleration of ~10 m/sec.   It would be decelerating at the same rate at which it was falling.

##### Share on other sites

• 10 months later...

At 6 minutes and about 20 seconds in, the fish claim they're going to "roll" the bags by swimming in water that's inside the bags. I guess the idea is that they're going to swim in a line that does not cross the center of mass, but that still leaves behind a question. If in swimming forward they push water backwards, is there any way for the torque they generate by this action force to not be cancelled out by the water-pushing reaction-force?

##### Share on other sites

• 5 months later...

Technically a TV scene and not a movie scene, but in this Simpsons scene, an avalanche buries a cabin. Homer finds out by opening the door; only for the snow that formed around the cabin to fall inward.

1. Would the snow, once it has buried the cabin, maintain its shape even if an open window or door gives it a new path downward? Or would gravity force the compressed snow to expand again into the cabin?

2. Wouldn't the snow outside look dark from the inside, because of the snow's scattering of sunlight? Is there any formula for light intensity as a function of snow depth?

##### Share on other sites

1 hour ago, ScienceNostalgia101 said:

1. Would the snow, once it has buried the cabin, maintain its shape even if an open window or door gives it a new path downward? Or would gravity force the compressed snow to expand again into the cabin?

I suspect this depends on how wet or dry the snow is. Wet snow tends to retain its shape better than dry powdery snow and could maintain shape around the cabin... freeze into that shape and show a flat wall when the door/window gets opened.

1 hour ago, ScienceNostalgia101 said:

2. Wouldn't the snow outside look dark from the inside, because of the snow's scattering of sunlight? Is there any formula for light intensity as a function of snow depth?

As you rightly highlight, depth of the snow matters, but I'm not personally aware of any formulas to describe this (though they may very well exist).

Also, I imagine the angle of incidence (which direction the light is coming from and how that compares to where the windows in the cabin are) plays a fairly significant role, too.

##### Share on other sites

1 hour ago, ScienceNostalgia101 said:

2. Wouldn't the snow outside look dark from the inside, because of the snow's scattering of sunlight?

It’s lit from inside the cabin.

##### Share on other sites

6 hours ago, ScienceNostalgia101 said:

1. Would the snow, once it has buried the cabin, maintain its shape even if an open window or door gives it a new path downward? Or would gravity force the compressed snow to expand again into the cabin?

Snow after an avalanche is quite solid. People don't dig themselves out of an avalanche. The snow will maintain its shape even if a door or window is opened.

##### Share on other sites

• 6 months later...

Got it, thanks!

Speaking of The Simpsons...

DISCLAIMER: I would not try this at home. NOR at any convenience store, for that matter.

However, it reminds me of my curiosity about the issue of cryonics. I'm not sure who to believe on this issue.

There are those who claim there are ways to survive being cryogenically frozen, if society would invest in them. Others dismiss it as hopeless. Are the former just wishfully thinking? Are the latter just trying to stop cryonics from cutting in on religion's afterlife action? If I found a professional service to freeze my body, would it be safer to do it immediately after death, immediately before death, or significantly before death in the context of some terminal illness or whatever? (Obviously I'm not going to cut drastically short whatever life I have now just on the offchance of being revived later.)

##### Share on other sites

• 1 year later...

The increasing magnetic field used to destroy the evidence is strong enough to throw metal objects at the wall and leave lights hanging from the ceiling at an angle. However, wouldn't this also make it strong enough to either brighten or dim the lights, due to the Lorentz forces on the electrons in the current carrying wires and whether they are parallel or perpendicular to the electrons' path? Or are there other factors? If not, is there any formula that can be used to estimate the magnetic field in Teslas from the apparent magnetic force on the objects flung at the wall or hanging from the ceiling (if one were to go by its acceleration or angle) and deduce how significant an effect this should have on the electrons in the wires powering the lights? Or does magnetic force not give one enough to go on in estimating magnetic field?

## Create an account

Register a new account