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Old July 11th, 2007, 11:00   #34
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Originally Posted by WhatTheWho View Post
It's not gryoscopic. It's the Magnus effect which explains why hop-up works.

Gravity is constant not matter what mass object; it does not have less of an effect because a mass is greater or lesser than another. You calibrate hop-up for a straight trajectory where in effect, you are adding a force with an acceleration of 9.81m/s^2 to counteract that of gravity's.
WhatTheWho's got it pretty close, IMO.
The magnus effect is where you create a pressure differential between the top and bottom of the bb. The backspin creates higher air velocity (velocity of the air moving over the surface of the bb) on the top of the bb and slower air velocity on the bottom of the bb. This velocity difference results in a low pressure on top and a high pressure on the bottom (like how an airplane wing works). The high pressure wants to "push up" to the low pressure to obtain an equilibrium, thus creating "lift" (a force upwards counteracting the weight force down). This is why if you have too much backspin (hopup) you will actually get the bb's to rise as the "lift" generated is greater than the weight force.

Originally Posted by WhatTheWho View Post
The only reason why a heavier BB "could" retain it's hop-up effect better than a lighter BB, and therefore counteract gravity better, is due to it's mass being greater such that the force of friction from the surrounding air is not able to impart enough force to decelerate its spin when compared to a lighter BB.

I.E. Backspin is reduced faster on a BB with lighter mass when compared to a BB with heavier mass.
This is due to momentum! Momentum is loosely defined as the difficulty to stop an object. So we're both correct in a way, I believe.

Originally Posted by WhatTheWho View Post
This explanation itself holds true to why a heavier BB retains more kinetic energy than a lighter BB when both energy outputs are equal; air resistance has less effect on an object with more mass, therefore it decelerates less than an object with less mass.

Less deceleration means it retains more of it's velocity in a given period of times than a lighter BB, and so it will retain more energy, and that dictates impact power.
This is also true but again, its due to it's greater momentum that it retains it's velocity (or energy) better, for lack of a better term.

It appears to me you haven't studied mechanics/dynamics extensively as you say impact POWER. Power and energy are, again, very different! Units of energy are in joules, units of power are joules/second. Not meaning to cut you down; maybe just a mental lapse...

Originally Posted by WhatTheWho View Post
Another thing to note is that a sphere traveling faster experiences more drag (air resistance) than a slower traveling sphere; that would also be indicative of why a heavier BB will keep its velocity, therefore its kinetic energy, better.
Close...if you fired bb's of different masses at the SAME velocity, the one with the greater mass will have it's velocity decrease at a slower rate.

Originally Posted by WhatTheWho View Post
An equation to relate why velocity and mass dictate air resistance, and how that relates to the retention of energy:

a = (Cd * r * V^2 * A / 2) / m
a = deceleration due to air resistance
Cd = coefficient of drag for a smooth sphere, being 0.5
r = density of air, being 1.229 kg/m^3 at sea level
V = velocity, being variable
A = cross-sectional area of the ball, being 2.826e-5 for a 6mm BB
m = mass of BB, being variable

Simplifying the equation yields:

a = (8.68e-6 * V^2) / m (at sea level)

This shows the relationship of velocity and mass to air resistance, and that an increase in velocity or a decrease in mass will increase air resistance. That causes more deceleration, and more loss of energy.
(This equation is for an instantaneous calc. of deceleration with a given velocity, as the velocity decreases for the next calc. of deceleration since air resistance is continuous and therefore velocity changes continuously)

So even if two different mass BB's were fired with the same Joules and with their corresponding velocities, the heavier BB will retain more energy and have more "impact power".
Can't completely comment on this except you have to be careful. You're finding its acceleration (or deceleration in this case) which deals with FORCES (F=ma) and then talking about energy then mistake the term "impact power" which should be (I think) impract energy as they would be completely different.

I'm not completely sold that impact energy (kinetic energy) is the definitive way of saying what will hurt more. I could be wrong and will look into it as I'm quite curious....

I remembered we did a test one day out at the field to see what hurts more, .20's or .25's. We had mixed results to say the least. I think the test must be done point blank (or close as possible) so that they (in theory) have relatively the same impact energy (ie. little to none is reduced by drag)

Just look at the Mythbuster's "frozen vs thaw" chicken test. They thought that the penetration should be equal as they will have the same kinetic energy when they impact the windshield. This was obviously not the case as they saw the frozen had MUCH greater penetration. True, they do have the same "impact energy", but they transfer this energy completely different! Not quite a good analogy for our BB case but it gets your mind around energy being the "be all, end all"....

A lot of terminology is very confusing and gets thrown around loosely. I find it tough for me to keep it all straight at time. FYI/summary for those interested:

-Mass isn't the same as weight! Mass is essentially "how much stuff is in something". You have the same mass on earth as on the moon, however your WEIGHT will change! (weight=mass*acceleration due to gravity). Weight is also a FORCE. Note that imperial units (pounds,etc) are weights (forces) and metric units (kilograms,etc) are masses. Gets tricky dealing with both!

-Momentum and energy are NOT the same. Have the same parameters but are quite different. Note that work is also energy (ie. you must do X amount of work to obtain X amount of kinetic energy)

This is a light-hearted debate (on my mind) and enjoy getting the brain going even though it is summer time! This is a huge portion of what I study at school so it quite interests me. Time to get something done at work now...
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