# Implications of movie physics

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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?

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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.

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

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

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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.

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• 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?

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• 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?

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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.

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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.

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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.

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• 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.)

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• 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?

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• 3 months later...

In "Beavis And Butthead Do America" the boys escape a kidnapper by prying open the trunk and jumping out onto the highway. They claim that it's safe as long as they "run as soon as they hit the ground. Now frankly, this particular claim doesn't make sense to me, it isn't consistent with what subsequently happened, and I presume there isn't even supposed to be even a kernel of truth to it. (Though I'm open to any case to the contrary!) However...

A. If they had some means (large slingshot in the trunk, etc...) to have the horizontal component of their car-relative velocity on exiting the trunk be equal and opposite the car's ground-relative velocity (presuming they could convince the motorists behind them to make way and/or safely come to a stop) would the boys' ground-relative velocity be logically equivalent to freefall?

B. If they had rollerblades or skateboards or some other pedestrian vehicle with them in the trunk, and hit the ground through those, would this be logically equivalent to having sped up to highway-speed levels on their own (presuming they have experienced this going downhill), and therefore enable them to gradually slide right toward the gravel shoulder of the highway the same as they would if using rollerblades or skateboards at highway speeds otherwise? If they hit the gravel shoulder, would they be able to avoid or at least reduce injury by leaning against their deceleration, or would they be doomed to go face-flat into the gravel no matter what they do? Would it be safer to just wait and let friction slow them down, and hope the motorists on the highway both notice them and care enough to slow down with them?

C. If alternatively, if the motorists decided to take the law into their own hands and brake-check the driver, what would be the most survivable position for Beavis and Butthead to be in? Would they be safer with their back, their arms, their leg, etc... lined up evenly against the front of the trunk, to keep to a minimum the distance with which any part of them could be accelerated forward relative to the car? Or would it be better to have their knees and arms bent so that when they are accelerated forward against the front of the trunk, they can at least let their bent arms and knees push against the front of the trunk and absorb some of the impact?

(DISCLAIMER: Obviously, I don't intend to try any such high-speed stunts myself and would not recommend anyone here do so.)

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• 1 month later...

So in this Family Guy scene, Peter Griffin attempts to recreate Scrooge McDuck's dive into a vault of coins and it... doesn't end well for him.

While I presume Family Guy's take on such a dive is relatively more accurate than that of DuckTales, I also think back to a childhood hobby of mine called "snow-diving," in which I could jump from a tree or a back patio onto a large enough pile of freshly fallen snow and it would cushion my fall. Is a pile of freshly fallen ice crystals not also "many pieces of solid matter"? If so, what factors determine whether they cushion your fall or act more lie a floor?

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15 minutes ago, ScienceNostalgia101 said:

what factors determine whether they cushion your fall or act more lie a floor?

Some factors and examples of a cause:

-Thickness of the layer of snow (due to amount of snow fallen or wind)
-Density/water content; dry or wet snow (temperature)
-Iced layer under a (too thin) layer of fresh snow (dry, cold snow falling on wet snow)

Relate but maybe not within scope:
-Flat or sloped surface (due to wind or terrain for instance) (This may not qualify as a "floor")
-Iced surface on the fallen snow (wet snow followed by drop of temperature) (This may not qualify as "freshly fallen")
-Snow compacted by for instance a skier (not qualifying as "freshly fallen")

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Ah, so the wetter and more heavily it's packed down, the more like a floor it acts. Thanks!

Man, it is a good thing I didn't attempt snow diving in spring.

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12 hours ago, ScienceNostalgia101 said:

Ah, so the wetter and more heavily it's packed down, the more like a floor it acts. Thanks!

Man, it is a good thing I didn't attempt snow diving in spring.

Yes, think of the crumple zone in a car.

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Posted (edited)
2 hours ago, exchemist said:

Yes, think of the crumple zone in a car.

Let's think of the brakes, first... 😇

Edited by dimreepr
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Posted (edited)

"Interstellar" is one of the few films that deals with the time dilation effect intelligently, [spoiler alert]-

for example they shuttle down to a planet that's close to a black hole and racked by immense tidal forces where they estimate 1 hour=7 years, and when they return to the orbiting mothership they find the other crew member left up there has aged XX years with grey hairs because they miscalculated.

In this scene they're surprised to find the ocean on the planet is shallow, not realising it's because they're in the trough of an incoming mega-tsunami..

Edited by Dropship
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• 3 weeks later...

In The Simpsons, Homer attempts to grow a donut to a larger size using the reactor core. In the show, not only does this not work, it causes a fire that apparently spreads up the sides of the cooling tower for some reason.

A: Is fibre-reinforced plastic combustible enough that a fire that started in the reactor could travel up the sides of the cooling tower?

B: Can radioactivity react with any standard donut ingredients to start a fire? Would the fire be started by the radioactivity itself, or by the heat associated with such radioactive materials?

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There is no expectation that there should be a valid explanation for any of this. There are no actual chemical or physical processes happening. It's a cartoon, and showing things for satirical and/or comedic effect. Sometimes they depict things that can't actually happen, because it's funny (such as when Lisa builds a perpetual motion device. "In this house we obey the laws of thermodynamics!")

"Can radioactivity react with any standard donut ingredients to start a fire?  doesn't even make sense. Radioactivity is a phenomenon. It can't react with anything.

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Bit of a misnomer. What I meant was more so whether the heat or other effects of the radioactivity can force a chemical reaction to occur that could start a fire that wouldn't otherwise have started.

I'm not accusing these cartoons of nefarious intentions or insinuating an obligation to realism or anything like that, just more out of curiosity whether it was a reference to anything real and/or coincidentally resembles something real. A lot of Simpsons moments are known to have had more truth to them than initially realized when it comes to things like, let's say, stuff about the legal system, but whether it's similar for the sciences is a distinct question.

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!

Moderator Note

Considering that cartoons are not tethered to reality it is rather pointless trying to discuss the the underlying physics of the fake realism. If one wants to discuss specific phenomena that are actually grounded in physical reality, please start a new thread. This one will be locked.

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