Dot Physics

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:

i-93de04d1710e8321f0b82ae6990e24e2-friction_graph_.jpg

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:

i-baef1fccae7219b043a39260a2934dbf-la_te_xi_t_1_4.jpg

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.

i-f42ba367d71432307e93c6fcc88bae05-la_te_xi_t_1_5.jpg

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:

i-ad62544aaf830d79fcc2ea9074bb562e-mj_fdb.jpg

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

i-d92bd6a9c8c6cc08fd6d0e3c4b8447a3-la_te_xi_t_1_6.jpg

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

i-38f7816df6eaa1015339be699c1be4bd-la_te_xi_t_1_7.jpg

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:

i-9ae782747c7736a3a5bb6cc30c7d878c-torque_2.jpg

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:

i-515b6140e0837271cf6418ec0f09995d-la_te_xi_t_1_9.jpg

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

i-10c67e78989e112b292ec334fb19c0fa-la_te_xi_t_1_10.jpg

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”.

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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.

Comments

  1. #1 Samantha
    June 27, 2009

    The moonwalk will forever be remembered and so would the LEGEND himself,Michael Jackson!

  2. #2 bodydetoxdietgirl
    June 27, 2009

    It is such a great loss that a man with great talent like Michael Jackson dies. RIP King of POP

  3. #3 myarthritistreatments78
    June 28, 2009

    Michael Jackson is my favorite pop artist ever since i was a child. He is truly the King of Pop and i am saddened by this news.

  4. #4 chris
    July 2, 2009

    michael jackson was, is , and will always be music history. I grew up to his music and loved it, now my 3 yr old is wanting me to play his music. im a real m.j. fan stop breaking his dancing down a real fan doesnt say his moonwalk is fake i believe he did it and its the real deal. R.I.P MJJ.

  5. #5 Stephen
    July 2, 2009

    that demo with the meterstick – wtf ? people cannot be that stupid

  6. #6 Tom Livsey
    July 3, 2009

    Talent and efford are not the same. No matter what you are trying to do. Individuals with god given talents can not explain why they can do certain things, while others with endless amounts of practice will never crest the bell.

    Talent is what most societies call this power. The beautiful thing is when a person can identify or discover his true god given talent like Michael Jackson. The moon walk was one of his birth talents.

    However for you all needing practice…watch this video, click on the link below:

    http://www.ourtown.com/snellvillega/article/2009/6/29/learn-how-to-moonwalk-like-michael-jackson.html

  7. #7 Raika
    July 9, 2009

    Beautiful.

    But I am told my demo is beautiful (even erotic !) and certainly instructive

    http://www.youcanmoonwalk.com

    Peace

    R x

  8. Michael Jackson will be the greatest pop singer in the whole world. Everyone will miss him.

  9. #9 Anxiety Disorder Health
    October 17, 2009

    | Michael Jackson is truly the King of Pop. i am a die hard fan of him and we are going to miss him now that he is gone.

  10. #10 Kayla Smith
    November 1, 2009

    i am already a great fan of Michael Jackson ever since i was just a little kid. i would really miss the King of Pop -

  11. #11 Dawood
    November 9, 2009

    no one ever will do moonwalking as MJ did…..

    all notable tributes and quotes on Michael Jackson from peers: http://www.tributespaid.com/quotes-on/michael-jackson

  12. #12 Jonathan
    December 31, 2009

    Michael Jackson is truly the King of Pop. He made a lot of great songs in the area of Pop Music. His death is a great loss to the music industry.

  13. #13 stacey
    January 4, 2010

    wdf was the meter stick all about like one hand stoppedd nd one hand kept goin we all arent dat stupid nd its just weird nd mj is de best i love him

  14. #14 Hayden
    January 10, 2010

    Michael Jackson is one of the greatest singer in our time. He is really the King of Pop and we would really miss this great person,

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