On 10/10/2018 4:47 PM, wrote:

Frank, If you attempt to stop a moving car you HAVE TO convert the potential energy to force to calculate what sort of FORCE it would take to absorb that potential
energy.

No, Tom, the car does not have potential energy (assuming it's on level
ground as the original photo shows). If it's moving, it has _kinetic_
energy.

And no, Tom, you cannot "convert potential energy to force." Energy and
force are fundamentally different quantities. You're making less sense
than an alchemist trying to convert lead to gold.

I find it odd that you don't know that or that when speaking of ft/lbs you would not assume ft/lb/secs but require someone to hold your hand and give you SI units and perhaps even calculate it out for you...

No, Tom, neither force nor energy are measured in units of ft/lbs. And
no, Tom, in actual physics or engineering calculations, ft/lbs would
NEVER be assumed to be the same as ft/lb/sec ... whatever the hell those
units would be purportedly representing.

One of the most powerful teaching techniques in mechanical engineering
is getting the students to always pay attention to the units of
measurement. Consequently, I and my colleagues always emphasized showing
and properly canceling units in calculations. If the units don't work
out correctly, it's a pretty sure sign you're making calculation mistakes.

Anyone using ft/lb to represent force, or pretending ft/lb/sec = ft/lb,
would probably not pass the course, let alone pass a licensing test to
become a professional engineer.

when it is common sense that a BICYCLE DOES NOT HAVE ENOUGH TRACTION TO DAMAGE A MOVING CAR.

I'm not commenting on the realism of the photo. I'm trying to move
beyond "it's common sense, duh" and instead talk about technical
correctness.


--
- Frank Krygowski