Q: How can I calculate the impact force of a car crashing into a tree or wall? I want to give a talk to a group of high school students and be able to show them how dangerous collisions really are. Is there a good way to show this without making them glaze over with disinterest? — Megan in Columbus, Ohio
A: The simplest equation for showing impact force is F = ½ m x Vsquared, where F is the force, “m” is the mass or weight of the vehicle and V is the velocity, usually expressed in feet-per-second. A good example to show on a computer screen or blackboard would be a 3000-pound car traveling at 60 mph (90 ft-per-second). One-half the mass is 1500 lbs., multiplied by 90 squared (a big number, at 8100) equals 12,150,000 lb-ft-seconds of force, or roughly 6100 tons/ft/seconds!
All that is simply numbers, though, because most people can’t relate the terms to everyday life. I would suggest you use a person’s weight to demonstrate what their effective force would be hitting against the dash if they weren’t belted in. In the example above, a 150-lb. teenager would hit with a force of 300 tons/ft/second, which would be certain death.
Of course, this is simplistic because the crash duration might be longer than one second and it doesn’t take into account the energy absorption of the vehicle itself, nor of the total frontal area over which the force is distributed. It does demonstrate the incredible forces of physics that show the energy of motion. Good luck in your presentation and be sure to stress wearing seat belts.