Friday, November 16, 2007

Stonehenge: Just add physics

Wally Wallington can move 2-ton cement blocks or barns over large distances by himself. No heavy machinery. No help. Just physics! He thinks he may have figured out how Stonehenge was built, and that it could have been done with very few people. This video shows not only some very basic physics principles in action, but illustrates that physics needs creativity and ingenuity to be put to work.

This video is about 6 minutes long, but it’s worth a look. Full of lots of physics and really awesome feats!

Read on to learn a little more about why Wally can move a barn with a 2x4. Getting it up on the pivot is something you’ll have to figure out from the video. The physics involved: center of mass and inertia; an object in motion tends to stay in motion; basic lever. This might be old hat if you’ve had basic physics classes, but I had to check my book to make sure I got the mechanics right.

Balancing a Barn- Put a pivot under and object's center of gravity, and the object will balance. No matter how big or oddly shaped the object is, that little point means equilibrium and that means it will sit tight. If you threw a tractor in the back left corner, gravity would pull more on that side, and the center of gravity would change. In terms of the barn, that point is relatively close to the center.

Rotating it- Wally uses a lever to push the barn around. Try opening a door by pushing on a spot right next to the hinges. It takes way more effort than to push it out by the handle, but you move your hand further using the handle. If you tried to rotate the barn by pushing directly on it, it’d be much harder than pushing a long 2 by 4 extending off the side of the barn. It works like a crowbar; you can move lots of mass but you have to move yourself a greater distance. Wally has to go all the way around the barn just to move it a few inches. So crowbars, see-saws, and barn movers still move a lot of mass, but they have to move over a larger distance to do it.

Getting it going - Once the barn is balanced on the pivot, gravity only plays a part in the frictional forces between the pivot and the ground. The motion of the barn in the horizontal direction is in the hands of its rotational inertia. Heavier objects are harder to push because of their mass, not their weight: Mass=stuff it’s made of; weight=stuff it’s made of + gravity; gravity only pushes down

The inertia is Newton’s second law. The barn doesn’t want to move while it’s at rest, but once it gets going it doesn’t want to stop! You have to overcome the initial energy needed to get it going, but then as you can see in the video, the barn moves much easier. Frictional forces eventually slow it down again

Moving the Barn Forward -Think of the center of mass as being in the center of a cement block. Now picture two pivot points on either side of the center of mass. If you put a mass on one side of the block you can change the center of mass to one of those pivot points. You can change the point where the block will balance. If you rotated that mass 180 degrees on one of those side pivots, the longer end would be pointing in the other direction. The block has moved in that direction. Change the center of mass to the other side of the block, and the long end is now the short end. Swing it around and the block has moved again.

Ok, think about it like walking with your knees and hips locked in place so your legs are totally straight. You step on one foot and pivot forward, step on another foot and pivot forward. Kind a like that.

In addition, Wally’s lever has its load on the pivot. This anchors the pivot and maximizes the work the lever can do.

Side Note on COM:

Center of mass is easy to imagine in an evenly weighted square block: it’s in the center. But if you really want to see how center of gravity works take a piece of paper cut like a pear or a kidney, and try to pin it to a wall. Keep the paper so it’s not touching the wall, or friction will play a part. Try pivoting it on the pin and it will usually slip back to a place of equilibrium where the heavier end is pulled down. But pin that piece of paper at its center of gravity, and you can pivot it all you want, and it will never slip down. The larger end can point up, down, or sideways. That’s equilibrium! It’s being pulled on equally from all sides.

Check out Wally Wallington's Website:

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