Moonwalking on the moon

rallain — Mon, 29 Jun 2009 22:05:27 +0000

Slate's Explainer has an answer and question post about moonwalking. Here is one of the very good questions answered there.

Would it be easier to moonwalk on the moon?

The Explainer says "absolutely not" and attributes this to the awkwardness of walking on the moon. The article gives an example of Earthly legs being too powerful as the "astronaut's hop". The explainer also says it is awkward because of the pressurized space suits.

I think the problem is almost entirely the pressurized suits. I believe that the astronauts do their moon-hop because it is difficult to bend their legs in the pressurized suits rather than because the gravitational field is smaller.

Suppose you had a space base on the moon so that you could be on the moon and NOT wear a space suit. In this case, I think moonwalking would be essentially the same as on earth. As I showed on my previous post on moonwalk, the move has a lot to do with center of mass and the ratio of the normal forces on the two feet.

On the moon, the ratio of the two forces would still be the same as on Earth. I could be wrong. The only way to know for sure is to build a moon base so that we can test this out. Well, I guess someone could go up in the vomit comet while they are simulating moon gravity.

rallain Mon, 06/29/2009 - 18:05

An Avian Moonwalk

awild — Sun, 28 Jun 2009 11:03:13 +0000

An Avian Moonwalk

Well.Â I'm off to host a photography workshop.Â But in the meantime I can't resist passing along this video. In honor of Michael Jackson, of course.

ht: nyt

awild Sun, 06/28/2009 - 07:03

The physics of Michael Jackson's moonwalk

rallain — Fri, 26 Jun 2009 08:28:51 +0000

The physics of Michael Jackson's moonwalk

Was the moonwalk fake? No, not the Apollo landings. I am talking about Michael Jackson's moonwalk. You got to admit, he had a big impact on a lot of stuff and this is my way to give him respect - physics.

I am sure you know about the moonwalk. Maybe you can even do the dance move yourself, but how does it work? First, here is a clip of MJ doing his stuff.

As a side note, I can't remember where I saw it but there was a great discussion of the history of the moonwalk. If I recall correctly, some were saying Michael didn't create this move. One thing is for sure, he made it popular. Now for the physics.

The key concept here is friction. Friction is actually uber-complicated, but a simple model works for many cases. Static friction is a force exerted on an object when it is in contact with some surface but those two surfaces do not move relative to each other. Kinetic friction is a force exerted on an object when the two surfaces are moving. Suppose I have a block at rest on a table and I pull it with a slowly increasing force. This is what it would look like:

Two key things from this graph. As you pull on the stationary block, the block doesn't move. If I pull with 1 Newton, and it doesn't move then the frictional force is 1 Newton. If I then pull with 2 Newtons and it still doesn't move, the frictional force is 2 Newtons. The static frictional force does what it can to make the thing not move - but not more than it can. This leads to the static friction model of:

In this model, the force is less than or equal to the product of some coefficient (that depends on the two types of surfaces) and the normal force (how hard the two surfaces are pushed together). The direction of this frictional force is parallel to the surface in the direction that prevents the object from sliding.

The other key feature in the graph is the small jump down when the thing starts to slide. This is because the coefficient of kinetic friction is typically smaller than that for static friction. Also, if the object is sliding, the frictional force is constant.

Back to Michael and the moonwalk. The key here is: how do you make one foot slide and the other not slide? If both feet are stationary, then this is dealing with static friction. I could make the frictional forces on these two feet different by changing my center of mass. Here is a free body diagram:

Since he is not accelerating up and down, the following must be true:

These are the forces in the y-direction. They must all add up to zero so that:

There is another condition that must be satisfied. Since he is not rotating, the total torque about any point must also add up to zero. If you want more info on torque, check out this post. But for this post I will just say that torque is like the 'rotational force'. It depends on the point about which you want to rotate and is essentially the force applied times the perpendicular distance to the point of rotation. For the free body diagram of Michael, I have chosen one of his feet to be the point about which he is not rotating (I could chose any point). This makes 3 of the forces have zero torque (N₂, F₂ and F₁ have zero torque because the perpendicular distance to point O is zero). Here I labeled the other important distances:

The only two forces that exert torque about O are the weight and the N₁ force. They have opposite directions of torque because they would cause rotation in different directions. This along with the previous equation gives:

Eliminating mg, and solving for N₁, I get: (I know the indices for the forces and distances don't match)

If his center of mass is in the middle, then r₂ - r₁ = r₁ and the two normal forces would be equal (as you would expect). If the center of mass is more towards the foot on the right, then r₂ - r₁ is less than r₁ and N₁ will be larger than N₂. This will make the frictional force on the foot to the right greater and the other foot slide.

Well, what if r₁ is greater than r₂? One of two things would happen. Either he would fall over, or there would have to be a force pulling the foot on the left down. This is similar to Michael Jackson's trick in "Smooth Criminal".

Here he used special shoes that connect to the floor so that he could do this. More details on this page.

Ok. So that is how Michael gets one foot moving. How does he keep one foot sliding and the other not sliding? It is really the same thing as above except that he can increase the force on the moving foot a little bit more since it is sliding. Sounds easy, but Michael could really make it look cool.

Finally, I just want to show another demo that is essentially the same idea.

Meterstick friction demo from Rhett Allain on Vimeo.

You can find more details on the meterstick demo in this blog post.

rallain Fri, 06/26/2009 - 04:28