MacGyver’s Body-Bag Freefall Might Actually Work. Physics Says So
Everyone knows MacGyver. He’s the guy who gets out of sticky situations by cobbling stuff together. Here he is in the reboot, maybe four floors up, with a sudden need to jump from a window. Solution? Use a fire extinguisher and body bag to create an impromptu cushion. Could that really work?
First, a disclosure. I’m the technical consultant for MacGyver, which means I check if MacGyver’s hacks are legit. I don’t say whether someone should try it, and I definitely recommend against this body bag jump. I only make sure it isn’t impossible.
Jumping Out a Window
Jumping from a four story window (say, 40 feet or 12 meters) wouldn’t kill you, though the landing probably would. Anyway, let’s start with the free fall. When MacGyver leaves the window, a gravitational force pulls him down (I’ll ignore air resistance for now) at a constant acceleration of 9.8 m/s2.
How fast would he be moving just before hitting the ground? Since I know how far MacGyver falls but not how fast he falls, it’s best to use the following kinematic equation. It’s not a difficult equation to derive, but you probably want me to handle it.
A change in height of 12 meters and a starting velocity of 0 m/s yields a final speed (right before crashing) of 15.3 m/s (34 mph). That might not seem very fast, but imagine running twice as fast as the fastest you can run and crashing into a wall. That wouldn’t feel too good.
Crashing Into the Ground
When you hit the ground, you stop. Immediately. Going from some velocity greater than zero to a velocity of zero means there is an acceleration. (You might think, “that sounds like deceleration to me.” But in physics, acceleration refers to any change in velocity.) It is this acceleration that you must be wary of—high acceleration can kill you. Just check out this g-force tolerance data of the maximum acceleration for different time intervals. For very short periods, humans can withstand an acceleration of up to 30 G’s—but you don’t really want to. An acceleration of maybe 15 G’s would be better (oh, 1 G = 9.8 m/s2).
A human can easily go from very high velocities to stopping without injury—just think about a car stopping after getting off the interstate. It all comes down to how fast the change in velocity occurs. For instance, stopping a car is a much greater change in velocity than jumping from a building. But stopping a car might take several seconds. Stopping a fall by hitting the ground takes a fraction of a second. That small time interval produces huge acceleration.
One way to think about the acceleration is not with time, but distance. If you fall to the ground, both the ground and your body compress—not much, and your body much more than the ground. This makes the stopping time very short. If you jump into a giant air bag, the air bag will compress over a larger distance producing a much longer stopping time and a smaller acceleration. If I want to calculate this acceleration based on stopping distance, I could use the same formula that I used above to calculate the velocity of jumping from the window.
How to Jump Out of a Building
So. Back to MacGyver’. He needs to jump from of a window. Now, he could land on his feet and use his legs to increase the stopping distance. That’s what the Winter Soldier does after jumping from a bridge in Captain America: Civil War. But MacGyver isn’t pumped up on serum like the Winter Soldier. He’ll need another way of increasing the stopping distance.
He grabs a body bag. Of course, an empty body bag doesn’t provide much cushion. To fix this, he fires a fire extinguisher into the bag, filling it. Foam would work, but an ordinary carbon dioxide fire extinguisher is fine. The key is to fill the bag enough to make it expand, but not so much that it can’t collapse a bit on impact. You don’t want a super-stiff body bag. That’s no better than hitting the ground.
As you can see, MacGyver waits until just before landing to fill the bag. Why not fill it when he leaps? Well, that added time might allow the gas (or foam) to leak out. Waiting until just before impact ensures the bag remains inflated. Also, it looks way cooler.
The bag compresses upon impact, decreasing the stopping acceleration. It also provides an evenly distributed stopping force so no part of MacGyver’s body experiences a force great enough to injure him.
Physics says this would work. The inflated body bag provides a short stopping distance of just 10 to 20 centimeters, but that’s enough. As a real-life example, take a look at Professor Splash. He jumps from 35 feet into a pool just 12 inches deep. He even survives. This is pretty close to the parameters for MacGyver. This jump seems at least plausible, if not advisable. You might survive, you would not feel great afterward.
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