In the last episode of MythBusters, they tried to reproduce the following experiment. Suppose you are driving in a car at 60 mph and you shoot a ball backwards at 60 mph (with respect to the car). Will the ball just drop (with respect to the ground)? Actually, it is a cool demo – I saw some Japanese show did this a while ago.
So, what is the problem? The problem is with the MythBusters’ use of their terms to explain this thing. Let me look at a couple of the things they said to explain this (surprisingly, they described it several different ways). This first one is my favorite.
Bad physics 1
“it makes sense that an object subjected to equal and opposite forces would drop like a rock.”
Where to begin? First, I am not sure where they get these equal and opposite forces. I would love for the science diagram to include a free body diagram for the ball after it is shot out of the truck. It would probably look something like this:
Oh, I know. Neither of those are real forces – but that must be what they are thinking. Now for the next problem with that statement. Here is a correct force diagram for an air plane flying with a constant velocity.
I am not going to talk about planes – this was just the first example I came up with. In this case the force pushing forward DOES equal the force pushing back. Does the plane drop like a rock? The idea that a net force of zero (vector) means no motion is what Aristotle would say.
Bad Physics 2
Here is another quote regarding the ball:
“Does the forward momentum and the backward momentum cancel out?”
I think the main error here is that they are using the term “momentum” to either mean velocity or force – not sure which. Also, if they are talking about momentum, the momentum of what? The truck and the ball?
Bad Physics 3
“…see if the energies cancel out”
Can you get energies to cancel? Well, you could get the potential plus kinetic to be zero joules, but would that count as canceling? You can never get two kinetic energies to add up to zero because 1) it isn’t a vector and 2) it is always positive.
I usually don’t attack the MythBusters, but I couldn’t help it in this case. Dear MythBusters, if you want to explain some stuff in the future give me a call. I will gladly look over your stuff for you.