MIT Physics Demo -- Speed of a Bullet

A pair of spinning wheels 1.5 meters apart is placed in the path of a bullet. A bullet is fired with the wheels stationary for reference position, and fired again with the wheels spinning at a known speed. The second wheel will rotate more than the first as the bullet crosses the gap between them. When the angles between the reference holes and the second holes are compared, the speed of the bullet can be determined.

The balloon is used to show when the bullet has passed through both disks.

This video was shot with a high speed video camera at approximately 7000 frames per second. The frame data can be seen at the bottom of the screen.

Watch the original video on MIT TechTV - http://techtv.mit.edu/videos/887-speed-of-a-bullet

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[RababaInc]

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

[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.

[calebp9503]

@RababaInc da fuck?

[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

[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? ^^

[AZIlluminati08]

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

[danthemanzizzle]

@TheBlackSky000 that would fall well within the margin of error for measuring the exact center of the penetration, not to mention the fact that the bullet is moving at least ten times faster than the paper th,in easily penetratable disks..

[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