Here is a video of Danny Macaskill, a highly talented cyclist. The things he can do with his bike almost seem magical. Almost. Physics is involved, of course. Take a look at this video.
How can you lift his bike without pushing on something else. It almost seems like he is lifting himself by his own bootstraps. I thought this would be a good topic to look at.
You might be tempted to say it had to do with the springiness of the tires or the wheels. While that helps, it’s not the main factor at play. If you don’t believe me, go sit on a chair, lift your feet up off of the floor and try scooting around.
A useful visual aid for working with forces is called the free-body diagram. The free-body diagram (FBD) represents each force with an arrow indicating how strong and which way each force is. The most important thing to remember is that a FBD only contains forces that are exerted on one object, and nothing else. Adding up all these forces yields the total force or net force exerted on an object which allows you to figure out the objects acceleration.
Lets take a look at the FBD of the bike and rider. There are two steps to lifting the bike up. The first is when the rider launches himself upward. The bike is still on the ground so it doesn’t accelerate as he pushes down on it. In that instant, the FBD for the bike and rider look like this:
All the forces on the bike cancel so the net force is zero and the bike doesn’t accelerate. There is a net upward force on the rider, so he accelerates up.
Once he has launched himself upward he pulls up on the handlebars and the bike starts to rise as well. As he is pulling up on the bike the free-body diagrams look like this:
The bike is pulled up and the rider is pulled down. However, because the rider was moving up fairly fast, he doesn’t start moving downward right away but his upward speed does start decreasing.
In a nutshell, what he is doing is launching himself up using the bike to push off of, then using his inertia, he pulls the bike along with him. This would be a good place to talk about momentum, but maybe in a later post.
Here is a slowed down video of some of his moves and you can see much more clearly how he launches himself and then pulls the bike with him.
Danny Macaskill looks like a magician, but he is really a master of physics. There is a lot more going on in the video and I’ll address this in future posts.