How much energy does this produce?

[frank mccune]

      Hi All:

      I would like to know how much energy in English terms does the following produce.

      A 16 ounce airplane striking an object at the speed of 90 miles per hour.

      I have worked it out but my results may not be valid. 

                                                                 Tia,

                                                                 Frank

[Tim Wescott]

Enough to hurt.  A lot.  Even if the spinner's not pointy.

In proper English units of energy, that's about 4356 pound-force feet (that is what you asked for!).

(For our international audience, that's about 363 Joules, or 1/10th of a Watt-hour).

Calculations:

90 miles/hour = 132 ft/sec = 40.2 m/s
16 ounces = 0.5 slug = 0.45 kg

energy = m * v² / 2, always, if you have consistent units for mass (m) and velocity.

(0.5 slug)(132 ft/sec)² / 2 = 4356 pound-feet (pound-force implied by the use of a slug as a unit of mass)

(0.45 kg)(40.2 m/s)² / 2 = 363 Joules

(363 Joules) * (1 watt-second/Joule) * (1 hour/(3600 seconds)) = 0.1 watt-hour

[Tim Wescott]

90 miles/hour = 132 ft/sec = 40.2 m/s
16 ounces = 0.5 slug = 0.45 kg

energy = m * v² / 2, always, if you have consistent units for mass (m) and velocity.

(0.5 slug)(132 ft/sec)² / 2 = 4356 pound-feet (pound-force implied by the use of a slug as a unit of mass)

A weight of 16 pounds would be 1/2 slug.  16 ounces is 1/32 of a slug.  So I'm off, by a mere factor of 16 (there's a reason engineers generally prefer the metric system).

(0.031 slug)(132 ft/sec)² / 2 = 272 pound-feet.

Still enough to hurt, but not as much as that magnum slug (which doesn't mass a slug, but rather -- oh, never mind).

[David Hoover]

But the force produced on impact is entirely dependant upon how fast it decelerates.  If it smacks into asphalt pavement or a brick wall the deceleration and resulting impact force are very high and the results are quite spectacular and generally somewhat expensive.  If it plops into wet, soft, squishy turf, not so much.

[jim gilmore]

i am really confused here.
I do not think any energy is created in the above simulation....
 

[Brett Buck]

A weight of 16 pounds would be 1/2 slug.  16 ounces is 1/32 of a slug.  So I'm off, by a mere factor of 16 (there's a reason engineers generally prefer the metric system).

    In this case the only advantage to metric would be that instead of the usual English mistake of trying to use lbm where you should use slugs, metric people tend to use kgf where they should use newtons. Ask anyone from a metric area how much they weigh and they will tell you in kg with the "f" implied. It's exactly the same sort of error, it just happens to work out favorably for this particular case. kg just happens to be a fundamental unit of mass which is where this earlier calculation went wrong.

    Do enough real-world engineering in metric and you will soon find mysterious fudge factors of "9.81" sprinkled in, just like you find mysterious fudge factors of "32.174" in English units. Either is perfectly acceptable and neither has any particular advantage, despite what people claim. A better criteria for chosing is which system requires the fewest conversions, end-to-end. That depends on what you are doing.

   Brett

[Mike Keville]

In layman's terms, it would leave a mark.

[Tim Wescott]

Do enough real-world engineering in metric and you will soon find mysterious fudge factors of "9.81" sprinkled in, just like you find mysterious fudge factors of "32.174" in English units.

I don't know what those French revolutionaries were thinking -- they should have redefined the fundamental unit of time so that the acceleration due to gravity was one meter per <whatever> squared.

I do a lot of work with motors and electronics, where going between volts, amps, and whatnot to pound force and feet would be a lot more tiresome than going between volts and amps and Newtons and meters.

[Tim Wescott]

i am really confused here.
I do not think any energy is created in the above simulation....

It takes energy to accelerate a body up to some speed.  That energy is present in the moving body in the form of kinetic energy.

But the force produced on impact is entirely dependant upon how fast it decelerates.  If it smacks into asphalt pavement or a brick wall the deceleration and resulting impact force are very high and the results are quite spectacular and generally somewhat expensive.  If it plops into wet, soft, squishy turf, not so much.

If it rolls to a stop after a nice landing, even less.

Yes, and that makes a difference, and I was going to try to calculate how fast an elephant would have to run to have the kinetic energy of a rifle round or some such.  Then I decided to let someone else do that work.

[Igor Burger]

This will be OT, but I will jump in, because I see here little bit mixed informations :- ))

No, French revolutionaries did not think about better definition of second, they even did not think about coherence of units at all (that is what means that force = mass * acceleration; energy = force * distance; power = energy / time etc without scaling by funny constants). Coherence is only little more then 100 years old concept comming especially with SI metric system. SI is that what we use, there are several metric systems known in history, and most of them are NOT coherent. For example BAR is metric unit for pressure which is NOT coherent, it is aproximately equal to atmosferic pressure. But Si metric system has another COHERENT unit Pascal which is [kg * m^-1 * s^-2].

That funny constant 9.81 for G acceleration is not an exception from coherence. Metric SI system does not know any "G" unit, it has [m * s^-2] what is again coherent (automatically), object of mass 1kg will need force 1N to accelerate 1m/s/s. That constant 9.81 is not unit, it is property of our planet, SI system is the same on Mars or Venus or Moon and they have certainly different gravities :- ))) ... but object of 1kg will need still the same force 1N to accelerate 1m/s/s.

BTW during my life and education in schools university etc, I did not meet unit kgf ... ever :- )))))))))))))))) yes here is non SI unit kp (kilopond) but it is unit which is not used in SI system and it is mentioned only on place of physics books in history part where is explained unit Atm or Bar (pressure) as kp/cm^2 what means aproximately atmospheric pressure, just to give students overview how strong pressure it is :- ))

[Chuck_Smith]

Assuming the plane's velocity goes to zero, the change in kinetic energy will be the same regardless of how fast it decelerates. Since work is the change in energy the amount of work required is again the same.

The force experienced will be a function of the time it takes to stop. We often look at it in terms of "impulse" , hence..."impulse engines" are in fact, real!