MIT Physics Demo -- Potential Energy to Kinetic Energy

Two identical metal balls are dropped from the same height using an electromagnetic release. One ball is dropped freely, while the other hangs from a string, acting as a simple pendulum. Using two photogates, we see that the vertical velocity of the dropped ball is identical to the horizontal velocity of the pendulum, measured at the same height. Potential energy has been converted to kinetic energy equally in both cases.

Watch the original video on MIT TechTV - http://techtv.mit.edu/videos/1491-potential-energy-to-kinetic-energy

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[Maxuras]

Yes, I was confused about this, too. I thought: Hey, that second thing is supposed to be slower, why does it take 0.091 in both cases ...

... Oh wait, that thing does not measure the dropping time, it measures the speed at ground level. - Yeah, now it makes sense.

[gennaman2bit]

@jbignes5 Absolutely wrong, the speed of the ball is exactly the same, the string changes only the direction of the ball (vertical to horizontal) but the speed have same modulus. in 1st try we have full vertical speed, in the 2nd we have no vertical speed (we are in the lower step of the pendulus, vertical speed is converted to horizontal speed due to the string. As u can see, the time of dark picked up by the photo gate is the same, that means same speed (same modulus)

[ceNorene189]

:)yeppp

[dayspeace]

Woah.. this is cool, I really though the result that could be gotten from the two balls would be different, but the result of the experiment proved otherwise. I thought the it would have a significant difference because the other was hooked with a string and the other not. It is nice and very informative, nice video.

[emeemmanuel]

I think its measuring time because the photo gates are placed in the same position. As mentioned before, think of vertical and horizontal components separately, gravity is the only one affecting the vertical direction thus they should reach the same vertical position at the same time. Its like the example of shooting the cannonball horizontally-- it will reach the ground at the same times if the cannon was simply dropped.

[ajskilton]

@Maxuras think about vertical and horizontal motions separately and I mean separately!!

[chemlink1]

I would like permission to use this video in my science classes. I appreciate your consideration.

[cheerklk89]

So, I typed kenetic energy in the search bar to get a BASIC understanding of the definition for my physical science class. I guess I should have read the part of the video labeled "MIT Physics"....ill be back to this video after I get my PhD, which I am guessing will be in about 10 years.

[yogibret]

@Maxuras Thanks for the clarification. I was confused too.

[polarautumn]

(oops, ran out of room)...

And, since t=d/v, t=(0.04m)/(4.429m/s)=0.0091sec­onds. It works out nicely. ;)

[polarautumn]

I'm almost positive the units of the photogate are in seconds: (0.0091seconds). The photogates are measuring the time for the diameter of the ball to pass by the photogate. The balls diameter is about 0.04 meters (same as 4.0cm) and it passed through the p-gate at 4.429m/s if dropped from 1 meter. Ek = Ep, so 1/2mv^2 = mgh. Mass cancels out, so we have 1/2v^2=gh where g is acceleration of gravity at 9.81m/s^2 and h is height. So, v=sqrt(2gh). v=sqrt[2(9.81m/s^2)(1m)] = 4.429m/s. And, since t=d/