MIT Physics Demo -- Speed of a Bullet

[LordAsrielAuB]

Is this supposed to be a "cheap way" of measuring a bullet's speed? If you didn't have this expensive camera, how would you know if the second wheel didnt turn 10 times before having the *observable* angular displacement? Then again, you could change the speed, do it again and compare the two displacements to find out....duuhhh. But then, you wouldnt need the camera^^

Oh, and btw, nice gun? ^^

[AuKNiFe]

@LordAsrielAuB actually it does not need high speed camera because the discs need not rotate fast. we just need markings indicating the angles. It works like this:

1. align the 2 discs such that they start at the same marking

2. confirm that they rotate at same speed by any DC. 1/500s exposure is good enough

3. since they rotate at same speed and start at same marking, there is no complications on how many revolutions the 2nd disc turned before

[RababaInc]

Sometimes I just want to fucking SLAP my dick as hard as possible on my desk!

[calebp9503]

@RababaInc da fuck?

[AZIlluminati08]

change direction? i'm sure they're not shooting at concrete, its MIT, i think they know what they're doing.

[TheBlackSky000]

Are you sure 100% that the bullet did not change its direction at the first penetration?!! Since the first disk is rotating, At the first bullet penetration the first disk will change the bullet direction. Thus, you may have some difference in the distance more than 1.5 meters.

[HoneycombAgent]

Where's the bullet?

[ArtStone]

Much simpler to shoot the bullet into a block of wood suspended on a string and measure the angular displacement, but you can't justify the expense of a high speed camera system in your grant proposal

[s651098]

@ArtStone Not quite, that's assuming a perfect transfer of momentum... The high speed camera isn't the main measuring device either, it's just there so people can see the bullet. Knowing the distance between the two discs and knowing the angular velocity of said discs we can work out the speed.

[unchained1978]

@gettavid You could just solve Maxwell's equations and get the exact value. No need for experiments.

[LLit11]

thats clever!

[taofledermaus]

Very cool video. Interesting way to determine bullet velocity.

[humanidt]

everytime the bullet penetrates a disk, it slows down, but its a minute change in speed.

[looksintolasers]

@gettavid Not quite 200 years ago, but in the 1840's a scientist named Fizeau measured the speed of light by passing it through a rotating disk. The disk had teeth on it, like a gear, and light passed through one gap out to a reflector and returned through the next gap on the disk. By fine tuning the speed of the disk until he could see the returning beam, he got the first experimental value for the speed of light!

[nickharvey7]

Interesting experiment!!

[atomic1234567890]

only problem, hiting the disks decelerate the bullet

[JuryDutySummons]

I would think it would be a tiny amount.

In any case, that amount could be predicted and accounted for.... right?

[chaz706]

Yes it can.

[atomic1234567890]

@JuryDutySummons true that decelerate but also true that a thin piece of paper is almost inefective against a lead ball

[JuryDutySummons]

That's kind of my point. 3 months ago you were saying that the test has a problem because the disks decelerate the bullet.

[KileSimons]

The average speed of a bullet is half the muzzle velocity...

[zdrastvutye]

d=distance of discs

w= angle between shotholes.

omega=angular speed of rotating disks

speed v=omega*d/w.

the smaller the angle the higher the resulting speed.

[jorgearroyop]

Does anyone knows what software can be used to add rectangles and text to videos (like in this clip) ?

[muchadoaboutnothingg]

.50 BMG

647 gr (41.9 g) Speer 3,044 ft/s (928 m/s) 13,144 ft·lbf (17,821 J)

655 gr (42.4 g) ADI 3,029 ft/s (923 m/s) 13,350 ft·lbf (18,100 J)

700 gr (45 g) Barnes 2,978 ft/s (908 m/s) 13,971 ft·lbf (18,942 J)

750 gr (49 g) Lapua 2,618 ft/s (798 m/s) 11,419 ft·lbf (15,482 J)

800 gr (52 g) Barnes 2,895 ft/s (882 m/s) 14,895 ft·lbf (20,195 J)

[PressStudio]

Somebody might be able to tell me... What is the estimated average speed of a flying bullet?

[theodorbrinch]

oh no it explained it in the description!!! sorry!

[pashaahsj]

This is soooooo totally awesome method to find the speed of something really fast!!!!!!!!!!!!!!!!!!!!!!!

its totally mechanical so the result will be sooo tatally incredibly accurate!!!

[theodorbrinch]

i don't get it! can you explain it to me?

[pashaahsj]

ok.. when they fire the bullet, and the two discs are turning at a very high RPM(Rounds Per Minute), the bullet goes through the first disc at a point. but by the time it reaches the second disc, the second disc has turned a bit, so the bullet hits it at a different point, so you can see the difference in the two points and tell the exact speed.

Pasha

[theodorbrinch]

ok, but what if the first spinning disc knocks the bullet a fraction sideways, it wont be totally acurate will it, or would that not even make a difference because of the speed of the bullet?

[pashaahsj]

yes. in the sense that this is a REALLY slowed down footage, so in reality, the bullet will be soooo fast, which is made of solid metal, hence having extremely high momentum when compared to the card/paper type material.

and also, the length of the bullet is very short, as its a small caliber bullet, and so there is not going to be almost any direction change either.

pasha.

[theodorbrinch]

um... i have a physics exam coming up pretty soon, i wonder if maybe you could help me out abit!!!

[pashaahsj]

SURE!!! of course.. i can...

I have an awesomely good physics teacher in college and i understand all stuff very well.

:)

tke. cre.

Pasha.