You probably already know how I feel about the “media” and their physics explanations (see attacks). Let me summarize the problem. There are a whole bunch of cool shows on tv that deal with sciency stuff – that is good. These shows then try to teach some science along with their demos and explosions and stuff. This is also good. However, they usually butcher the explanation part. Some of them (ESPN’s Sport Science) must just literally make stuff up that sounds cool.
I understand that in common usage, things like “force” can be used lots of different ways. I am ok with that. Also, sometimes a person on a show will use a term incorrectly (MythBusters do this all the time). This isn’t so bad if it is not an “official” explanation. If there are special graphics and a narrator, the explanation should at least not be wrong.
I want to help. Here are some commonly used “physics” terms and definitions that don’t suck.
Definition: A force is an interaction between two objects – usually describe as a push or a pull.
Other stuff: Forces are measured in units of Newtons or Pounds. If there is only one force on an object, it will change the object’s speed and or direction. Force is a vector -this means direction means something.
Wrong: It seems the most common error is to associate a force with one object. “That ball had a lot of force” or “that ball had a lot of force behind it”. This maybe confusing force with momentum (which is different).
Definition: Momentum is a measure of how much “oomph” a moving object has. This IS a property of the object. Mathematically, momentum is the product of mass and velocity (if you are not moving near the speed of light).
Other stuff: Momentum is typically used when people are talking about collisions – and that is ok. Also, it is common to say that momentum is transferred from one object to another in a collision. I guess this is ok – but really it is that the total momentum vector for a closed system remains constant.
Wrong: I can’t think of a wrong use of momentum off the top of my head. Probably the biggest wrong use is to not use momentum but to use the term “force” instead.
Definition: It might be surprising to you, but I think energy is not so simple to define. Most texts say energy is the ability to do work – I don’t really like that one. For this use – I guess it would be easiest to say that there are two types of energy – particle energy and field energy (I know that is not a definition). Maybe I should just say that using “energy” is a way of looking at interactions.
Other stuff: Energy is not a vector – direction does not matter. I would consider it a property of an object. Here are some examples of different types of energy that come up:
- kinetic energy
- gravitational potential
- electric potential
- thermal energy
- light energy
I don’t have a definition for collisions, but it comes up a lot on shows. What happens? Well, let me say what doesn’t happen. If an object is crashing into a wall, it does not transfer its kinetic energy to the wall. It is common also to talk about how energy is dispersed in a collision. Maybe this isn’t really wrong – but it isn’t the best thing to talk about. There are two important things. First, the momentum principle (more details on the momentum principle). It says for a given object:
Basically, the total force on an object changes its momentum. For a given change in momentum (like an object stopping) you can use a smaller force if the time of impact is larger. If there are two objects (and no external forces) then the total change in momentum of the whole thing is zero vector.
What about damage? Damage to a human is probably best done by looking at the acceleration (but even that is sometimes tricky). The MythBusters use this a lot – and usually correct. The problem is that if you have two different mass people falling from the same height, they will have different momentums and different forces but similar accelerations.