Modern Marvels Race Car Crash Physics Question - Page 1
Forum section: electronics / projects

author: bostonman
author_rank: Super Contributor
timestamp: May 06, 2025, 11:00:36 pm
content: Last night an old episode of Modern Marvels aired that discussed crashes (I forgot the title of the episode),It stated if a race car with a mass of 3600lbs traveling at 180 miles/hr hits a wall straight on, the driver can be launched five-miles into the air (I'm assuming vertical, but maybe it's the horizontal distance). At that point they showed on the corner of the screen the total Kinetic Energy to be: 3,900,000 ft-lbThe height seemed a bit extreme to me and an opportunity to calculate it. I converted all the numbers to SI units, calculated a Ke of 5.3MJ, and converted to ft-lb to confirm the numbers matched. Using the same Ke formula, assuming the mass of a driver to be 80kg, I solved for his speed at the moment of the crash, and got 361m/s (or 807 miles/hr).Finally I used the formula Vf^2 = Vo^2 - 2as and solved for 's' to get the height of the driver which came out to be 6650 meters (or 4.1 miles).Still uncertain whether I did this correctly, ChatGPT told me the driver (based on the same numbers) would only go less than a mile in the air, however, it then pointed out the driver would only be going the same speed as the race car when it crashed (180miles/hr).After reading/using ChatGPT, I confused myself because typically I'd agree the driver would be ejected at 180 miles/hr, but then asked myself, Ke must come into the equation, so why is ChatGPT telling me less than a mile into the air, but at the same time, five-miles seems extreme?Was my method in solving this correct or did Modern Marvels grossly exaggerate that five-miles (if I'm correct, then Modern Marvels just rounded up to five-miles from my 4.1-miles)?This all assumes no loss has been experienced in the crash, the car instantaneously stopped, etc...

author: Benta
author_rank: Super Contributor
timestamp: May 06, 2025, 11:18:47 pm
content: I think you should place this in "Dodgy Technology" or "Chat" instead.It certainly does not belong in "Projects".

author: bostonman
author_rank: Super Contributor
timestamp: May 06, 2025, 11:23:31 pm
content: I apologize.I contemplated which one to post this question in. Now it won't let me delete it nor can I locate how to move it.

author: eugene
author_rank: Frequent Contributor
timestamp: May 07, 2025, 02:06:41 pm
content: We can settle this quickly. I'll convert to SI units: 3600lb = 1633kg, 180mph = 80.5m/s.The kinetic energy of the moving car: KE = mv^2 / 2 = 1633*80.5^2 / 2 = 5300000 Joules.All of that goes into potential energy of the man in the air: PE = mgh. We'll assume the man weighs 80kg. Solving for h:h = PE/mg = 5300000 / (80*9.8 ) = 6750 meters = 4.2 miles.BTW, ft-lb are units of torque, not energy.

author: bostonman
author_rank: Super Contributor
timestamp: May 07, 2025, 02:37:20 pm
content: QuoteThat threw me off too, but then I read and found:The term foot-pound is also used as a unit of torque. In the United States this is often used to specify, for example, the tightness of a fastener (such as screws and nuts) or the output of an engine. Although they are dimensionally equivalent, energy (a scalar) and torque (a Euclidean vector) are distinct physical quantities. Both energy and torque can be expressed as a product of a force vector with a displacement vector (hence pounds and feet); energy is the scalar product of the two, and torque is the vector product.

author: SteveThackery
author_rank: Super Contributor
timestamp: May 07, 2025, 06:39:11 pm
content: Pedant alert!Here in the UK, and I suspect everywhere, in engineering, foot-pounds and pounds-feet are both clearly defined and definitely different.If the force comes first, it's a measure of torque, sopounds-feet (lb-ft) is the one for torque. It's easy to remember because in the metric system torque is measured in newton-metres (Nm). See how newtons (the force) comes first. Nobody ever says "metre-newtons".Thereforefoot-pounds is the measure of energy. It is the energy required to lift a one pound weight through a distance of one foot.The big problem with foot-pounds and pounds-feet is that they are incredibly easy to mix up, and all sorts of people get them wrong - even engineers! Some so-called "engineers" treat them as if they are interchangeable, which is as wrong as can be.Check any engineering text book: there is no ambiguity about their meaning. It's not a matter of opinion.

author: Benta
author_rank: Super Contributor
timestamp: May 07, 2025, 08:03:36 pm
content: ...and furlongs-fortnight is a measure of speed.Those archaic units break me up. We're in 2025, folks.

author: bdunham7
author_rank: Super Contributor
timestamp: May 07, 2025, 08:14:36 pm
content: Quote from: bostonman on May 06, 2025, 11:00:36 pmEnergy is not how you solve this problem because it is an inelastic collision that absorbs a great deal of kinetic energy. Fallacious conservation of energy arguments are classic beginners mistakes in physics. Assuming there is some mechanism--such as a teflon-coated 90 degree curved slide) that can translate the drivers momentum completely efficiently from horizontal to vertical, then he might fly about 1100 feet into the air if you ignore air resistance (which would be a significant error in this case). Anyone that claims the driver can be launched higher will have to explain by what process his momentum is not only translated but increased.

author: bostonman
author_rank: Super Contributor
timestamp: May 07, 2025, 09:56:59 pm
content: I fully agree that many other forces will change the outcome to be far less in height.My first step was to confirm whether or not the show provided a false statement; and to confirm whether I was approaching it correctly exclusively based on a perfect transfer of energy.It seems obvious that the show exaggerated that five-miles because I calculated (and eugene confirmed) it's only 4.2miles.Based on what you're saying with the 1,100 ft (less than 1/4 mile), the show was grossly off and provided an inaccurate detail.