A great demo
26/06/08 07:39 Filed in: Physics
I like demos. They are kind of like magic tricks, but
you get to explain something also. I recorded this
demo and I was going to use it for my astronomy lectures, but
it just didin’t fit in anywhere. I recorded this
demo, but my kids were making a lot of noise, so
I re-recorded the audio. Didn’t quite turn out
like I wanted. Here is the video:
Inertial demo from Rhett Allain on Vimeo.
Inertial demo from Rhett Allain on Vimeo.
I like this demo because it is simple and clearly,
anyone can do it (my 4 year old daughter did it).
Here is what you do:
So, how does this work? The key here is the following:
Which, is essentially Newton’s
2nd law. I can also write this as:
This shows how the momentum
changes with force and time.
Is there a net force on the glass? Yes, but the time is very small, so you will notice the momentum does not change much. In fact, in the video, my two year old pulled the paper slowly. I caught the glass, but it would have fallen.
Why is it easier when you add water? No one really cares about the momentum of the glass, you care about the velocity. Momentum is:
So, for a full glass, the change in momentum is about the same (if the force and the time are the same - really neither of these are the same). If the mass increases, then the velocity of the glass will not need to increase as much.
- Get an object (like a glass) and a piece of paper or cloth. Make sure the glass is not wet or too sticky on the bottom. If you use a cloth, make sure it does not have a hem on it.
- Place the glass on the paper with about 1/2 of the paper hanging over the edge of the table.
- Here is the scary part. Pull really, really fast. Pull the paper DOWN. When you pull it down, the paper comes out and more importantly, the paper does not go UP. If you hesitate, you will lose.
- If you want to go a step further, say that you will make the trick even harder by adding water to the glass (this actually makes the trick easier). Then repeat the above steps. Make sure you don’t get water on the outside of the glass.
- You can put other stuff on the paper as well. Just remember, the heaver the better. Also, the more breakable, the cooler you will look.
So, how does this work? The key here is the following:
Which, is essentially Newton’s
2nd law. I can also write this as:
This shows how the momentum
changes with force and time.
Is there a net force on the glass? Yes, but the time is very small, so you will notice the momentum does not change much. In fact, in the video, my two year old pulled the paper slowly. I caught the glass, but it would have fallen.
Why is it easier when you add water? No one really cares about the momentum of the glass, you care about the velocity. Momentum is:
So, for a full glass, the change in momentum is about the same (if the force and the time are the same - really neither of these are the same). If the mass increases, then the velocity of the glass will not need to increase as much.
