Cinematography is a natural extension of photography and as such shares many of its core traits. Both are built on the idea of taking reality, that has no intrinsic bias, and collapsing it in to a unique perspective; making the audience see the same world but through the eyes of the artist. Sometimes it means reality is distorted naturally, by the virtue of the artist’s bias; other times it is done intentionally, in a specific way for a specific goal. In this paper we will focus on the latter; specifically the distortion of Conservation of Momentum.
In the animated film Ice Age 3 there is a scene I call “the
impossible soap bubble” in which the main character Scrat and his frenemy Scratte
are drifting in what appears to be soap bubbles.
Scratte is slightly ahead and above Scrat and is making a run for his favorite object, the Acorn, that is drifting in a bubble as well. To catch her Scrat starts jumping off the floor of the bubble in order to impact the top with his head and push it higher, getting him closer to his prize.
Scratte is slightly ahead and above Scrat and is making a run for his favorite object, the Acorn, that is drifting in a bubble as well. To catch her Scrat starts jumping off the floor of the bubble in order to impact the top with his head and push it higher, getting him closer to his prize.
Conservation of momentum is distorted here for comedy and
general air of silly shenanigans which makes these lighthearted films so
entertaining to watch regardless of age group. In the film the Scrat is
successful in his attempts and the bubble indeed moves upward. In reality this would not be possible due to conservation of
momentum.
Once Scrat would push off from the floor of the bubble, the
bubble would push off him. Having a much higher mass then the bubble his
position would remain relatively unchanged while the bubble would move
downwards. Whatever small velocity he would gain in the upward direction would
be lost moments later as his head would impact the roof of the bubble and the
relative momentum would return to its original magnitude of zero.
This analysis neglects air resistance, buoyancy and gravity
but the end result is the same. This is not a realistic means of traversing
space.
In the film Lock Stock and Two Smoking Barrels there is a
scene I call “the impossible shotgun” where Hatchet Harry shoots Dog with a
shotgun, causing him to fly back and out the door.
The momentum of the firearm projectiles is greatly over exaggerated in this case for dramatic effect. Realistically the velocity would be far lower, but only if he was on a frictionless floor. If not, the friction between the floor and his feet would convert the impact of the projectiles in to a torque, causing him to tip backwards. Let’s take a look at the physics behind a real life version of these events in terms of conservation of momentum.
The momentum of the firearm projectiles is greatly over exaggerated in this case for dramatic effect. Realistically the velocity would be far lower, but only if he was on a frictionless floor. If not, the friction between the floor and his feet would convert the impact of the projectiles in to a torque, causing him to tip backwards. Let’s take a look at the physics behind a real life version of these events in terms of conservation of momentum.
Assuming:
-
weight of the victim at roughly 175lb (80kg)
- friction-less floor so force doesn’t get
converted to torque
-
elastic collision so no energy is wasted in
crushing bone and tearing tissue
-
all pellets go in to the body
-
Remington's Magnum 00-Buck Load (on the
heavy side of what exists)
The resulting velocity of the victim filled with projectiles is about 36 cm/s as you can see from the calculations above. Take a finger and run it along the ruler from 0 to 36 cm while counting out one-Mississippi. You will see that it’s a very slow speed, about 0.8mph; average walking speed is about 2-3mph.
Below those calculations you can see an extrapolation of
how much of that same shot it would take to achieve the observed velocity of ~2.5 m/s given the same assumptions; roughly 14 of the same shells fired
simultaneously.
Needless to say this is all assuming a friction-less floor and and no energy loss due to deformation/destruction of bone/tissue/cartilage/shot.
Needless to say this is all assuming a friction-less floor and and no energy loss due to deformation/destruction of bone/tissue/cartilage/shot.
In the film Gravity I don't have a fancy name for a scene because conservation of momentum is constantly violated
for unknown reasons; probably incompetence. On multiple occasions items like
the jet-pack and the fire-extinguisher are used to catch vehicles and stations
in the same orbit. It suffices to say that the distances in question are much
too great relative to the amount of available propellant (among other issues
like vast speed disparities, inability to control tumble with fire-extinguisher, etc).
Generally the way this is done is by switching to a lower
orbit to increase velocity (conservation of angular momentum), catch up, and
then go up in to target orbit. Or go in to a higher orbit to slow down, wait
for the target to come around, and then drop down on it. There is simply not
enough momentum to be had from the given propellants to chase objects in the same orbit over the
given distances. And even if there was, as soon as you change your speed you
will automatically change your orbit whether you want to or not (conservation of angular momentum). In a nutshell
they should fire their physics consultant.
If you are going to violate physics you better have good
reason. In animation it’s easy to get away with it the spirit of fantasy. The
vast majority of animated films either take place in a non-realistic universe
or in one where technology has advanced to the point of being perceived by us
as magic; at which point we are back to option one. In non-animated films things are a little
different.
In Lock Stock and Two Smoking Barrels the shotgun scene could have been done better by shooting Dog with something possessing higher mass or blowing a hole though him as opposed to making him fly; which would be more realistic.
In Gravity they should have used the momentum from propellants to shift orbits instead of unrealistically chasing targets. Although to be fair using them would shift orbits automatically; would have helped if they showed it. And not to nitpick but there is no way one could control the tumble induced by the fire-extinguisher expelling propellant.
In Lock Stock and Two Smoking Barrels the shotgun scene could have been done better by shooting Dog with something possessing higher mass or blowing a hole though him as opposed to making him fly; which would be more realistic.
In Gravity they should have used the momentum from propellants to shift orbits instead of unrealistically chasing targets. Although to be fair using them would shift orbits automatically; would have helped if they showed it. And not to nitpick but there is no way one could control the tumble induced by the fire-extinguisher expelling propellant.
I guess in closing I would say movie producers need to
spend more money on physics consultants and less on catering. Although on the
flip side the general public is not very bright so maybe it would just be a
waste of money. Something to think about.
