Mass, Speed, Physics and Bullets in the Movies

I watched Harry Brown on DVD, which the great Michael Caine personally lifts from forgettable to above-average. It does though feature a common movie trope, namely that a bullet can thrown a grown adult across a room (Presuming a powerful enough gun).

Mass and Speed are roughly interchangeble forces in comic books and in many films. If a superhero who weighs 200 pounds wants to stop a hurtling train or bus that weighs many tons, it’s entirely a matter of having his feet planted correctly and a good set of biceps.

In college I read in Soldier of Fortune magazine that if a bullet — which not only has small mass but penetrates upon impact — can throw a human being backwards 3 or 4 feet, then a regular feature of baseball games would be batters landing in the upper deck after being beaned by a pitcher.

My personal favorite guy-flying-improbably-backwards comes at the conclusion (about 1:10 of the clip) of the gripping suspense film “Day of the Jackal” [SPOILER ALERT: this is the climax of the movie).

Comments

  1. Harold Pollack says

    I think “every action has an equal and opposite reaction” has some relevance here….

  2. Anomalous says

    One impossibility that gets me is in “Spiderman” our hero jumps after the falling Mary Jane and overtakes her to catch and rescue her from falling to certain death.

    As Galileo proved in his famous experiment by dropping two ojects of differing weight from the Leaning Tower of Pisa all objects fall at the same rate of speed.

    So what Spiderman should have done was to shoot a web after Mary Jane and then snagged a line onto a building swinging he and his true love to safety.

    Is my inner nurd showing?

  3. MobiusKlein says

    And to get it out of the way, explosions in space you can hear, faster than light travel, convex mirrors that make images larger, ESP, ghosts, and singing orphans with hearts of gold.

  4. doretta says

    My favorite is space operas where a moving spaceship becomes disabled and then sinks to the bottom of the screen and stops.

    Back to “Jackal” and the subject of bullets, however tiny that rifle bullet was, it seems to me also improbable that the first one hitting the paving stones would go unnoticed.

  5. Keith Humphreys says

    And laser rifles not only make a loud sound but also have a kick that unsteadies the hand with each shot…

  6. Brett says

    My personal favorite is how friction seems to be an option, rather than a reality, in movies.

    My favorite is space operas where a moving spaceship becomes disabled and then sinks to the bottom of the screen and stops.

    Or how they seem to fight in 2D. Or how so many space opera battles are rip-offs of World War 2 “dogfights” (thanks Star Wars, although it was pretty cool in those movies).

  7. Cranky Observer says

    > One impossibility that gets me is in “Spiderman” our hero jumps after the falling
    > Mary Jane and overtakes her to catch and rescue her from falling to certain death.
    >
    > As Galileo proved in his famous experiment by dropping two ojects of differing
    > weight from the Leaning Tower of Pisa all objects fall at the same rate of speed.

    You might want to look at some YouTubes of acrobatic skydiving. Untrained people in free-fall tend to throw their arms and legs out, and also to tumble, both of which slow their velocity through the air considerably. A trained free-fall skydiver can easily tuck himself into a more aerodynamic position, “fly” over to the tumbling person, and match velocity. In one recorded instance a skydiving instructor actually managed to do this and exchange his reserve chute with the novice in time to save that person’s life.

    Cranky

  8. calling all toasters says

    People outrunning explosions. Bullet wounds to the chest that leave a 2 inch spot of blood on the shirt. Crew members walking down a spaceship’s corridors. Paranoid psychotics always being right.

  9. ACLS says

    The primary thing I remember from high school physics (as opposed to college physics) was a demonstration where we watched the opening of Superman II, where a very stupid boy falls off Niagara Falls and, based on how long it took Clark Kent to notice, jump into the phone booth and change, and then swoop down to the rescue in the nick of time, we were to determine the height of Niagara Falls. The answer was well in excess of a mile.

    Unmentioned in the class was the fact that the boy falling at that speed into Superman’s very very hard arms would probably be no better than the rocks a couple feet below.

  10. SP says

    As a shout out to nerds, every time they show a target getting thrown backwards they should show the same thing happening to the shooter.

  11. Brett Bellmore says

    In the *comic book*, when Spiderman’s love, Gwen Stacy, was thrown off the bridge by the Green Goblin, and he managed to snag her with his webline, when he pulled her up he discovered that the sudden acceleration had broken her neck. Tragically accurate physics.

    “If a superhero who weighs 200 pounds wants to stop a hurtling train or bus that weighs many tons, it’s entirely a matter of having his feet planted correctly and a good set of biceps.”

    Ironically, in one of the movies Spiderman tries exactly this, and accomplishes nothing but breaking a bunch of rail ties, and getting sore feet. Superman may be almost infinitely strong, but what good is that if the ground he’s bracing against is just dirt?

    My favorite movie firearms bit, where they actually get it right, is in Tremors II, where Burt shoots one of the monsters with a .50 BMG single shot rifle; Not only is the monster dispatched, the bullet continues on through a cement wall, and the engine block of the car they’d meant to escape in. It really would have, too, with the right load.

    Notably, Burt is not thrown backwards by the recoil; This is all accomplished by kinetic energy, not momentum.

  12. MobiusKlein says

    Well Brett, I’ll suggest that the gun scene in Tremors II is way inaccurate, as it happened during the Clinton administration. No way that gun-confiscating commie would have let such ordinance stay in the hands of the civilian rabble.

  13. Brett Bellmore says

    Well, I’ve heard rumors that Obama would like to ban them, in order to keep the BATF from shipping any more to the Mexican cartels… But, no, they weren’t covered by the 94 ‘assault weapon’ ban.

  14. Anderson says

    “thanks Star Wars, although it was pretty cool in those movies”

    Story has it that when Fox was getting cold feet on the SW project, Lucas had to show a rough edit to the suits, and because the dogfight scenes weren’t finished, he spliced in WW2 dogfight footage in those parts.

    This more or less led to the notorious deal where, to keep Fox from pulling the plug, he agreed to take his compensation in a cut of the tie-in royalties. “What tie-ins?” the suits laughed.

  15. docdave says

    Many years ago, American Heritage ran an article examining the question of realism in movie gunplay. One ilustration showed the disruptive effects of a bullet on a bloc of a gelatin material similar in density to the human body. The photo was dramatic–the block might not have been thrown against a nearby wall, but there was not much of it that hadnm’t been churned up pretty thoroughly.

    Effect of impact is usually assumed to have a relation to the size of the projectile. In theory, the bigger the slug, the greater the impact/”push.” I don’t know how this plays out with higher-velocity rounds, but it _sounds_ reasonable. Having been shot only once, and that with a low-powered .22, I will gladly leave this question to those with greater curiosity about such things. I know that when I was plugged I moved dramatically, but this had less to do with physics than with my being hopping mad and in a fair amount of pain.

  16. SamChevre says

    In college I read in Soldier of Fortune magazine that if a bullet — which not only has small mass but penetrates upon impact — can throw a human being backwards 3 or 4 feet, then a regular feature of baseball games would be batters landing in the upper deck after being beaned by a pitcher.

    I’m pretty certain this is wrong. If you figure a bullet at 175 grains, 2000 fps (a reasonable performance for a .30-06) and a baseball at 5.125 oz, 100 mph, and use the standard energy = mass * velocity^2 equation, the bullet has 14 times as much energy.

  17. Keith Humphreys says

    SamChevre: It could well be wrong, considering the magazine source and the failings of my memory about what the article said all those years ago. But are you not leaving out an important part of the equation — a bullet will penetrate but a baseball will bounce off, transmitting all the force it has into knockback.

  18. Brett says

    This more or less led to the notorious deal where, to keep Fox from pulling the plug, he agreed to take his compensation in a cut of the tie-in royalties. “What tie-ins?” the suits laughed.

    And they let him keep the merchandising rights. Definitely one of their worst decisions of the latter 20th century.