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Conservation of Energy – Is This a Violation of Energy Conservation?

This experiment shows us which ball is moving the fastest at the bottom? The answer is behind energy conservation.


Hi, I’m Dr. Bruce DiNardo, here in the physics department of the Naval Postgraduate School at Monterey, California. And we have here a commercial apparatus that has four ramps. You can see the blue one in front, a straight yellow ramp, and the green one here and the red one.

And what we can do with this is release steel balls at rest at the top of these ramps. And there are several questions that we can ask him. One question is, which ball is moving the fastest at the bottom? Energy conservation answers. Because all balls are released from rest and descend through the same height, they lose the same gravitational potential energy, so they will all gain the same kinetic energy.

So all of the final speeds must be the same. One way we can check this is to release the balls from rest at the top of the ramp. Then there’ll be projected horizontally at the bottom and travel downward, and strikes the floor. We can measure the distance that they travel and confirm that the speeds are the same. I have taped a piece of paper to the floor to record where each ball strikes the floor because an indentation is made.

First, the blue ramp. The ball is released from rest, departs from the ramp, and strikes the floor. I’m putting the ball in the indentation. Next is the yellow and then the green. Finally, you can see that the blue, green, and red ramps yield nearly the same range and, thus, the same initial speed. But what about the yellow ramp? You can see that it yielded less range. Does this mean that energy conservation is violated? No. Energy conservation is probably the most single important factor in physics, and it has a substantial theoretical and experimental basis.

What we should look for are some effects that could cause the discrepancy. And I’m guessing some of you may have already noticed this; if you look at the yellow ramp, you’ll see that it does not go flat.

In the end, the ball is not projected horizontally by the yellow ramp, so it doesn’t have as much range. It should fall short of the other ones, and that’s precisely what we observe.

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