Dot Physics

Physics Tips for Media

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.

Force

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).

Momentum

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.

Energy

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

Collisions

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:

i-a0cf4743a9ccf9bf0fbaf3a49a6adc90-2010-04-19_la_te_xi_t_1.jpg

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.

Comments

  1. #1 Blaise Pascal
    April 19, 2010

    I think the issue of force is a bit more subtle than that. While it’s true that forces come in pairs and are caused by the interaction of two objects, it is perfectly accurate and correct to speak of the force or forces on an object. When you talk about the net force on an object, it’s impossible to refer to the “other” object involved.

    At the level of mechanics as shown in these types of science programs, I think it’s fair to say that a force is something imposed on an object causing it to change direction or speed; it is not a property of an object, but something external to it.

  2. #2 Rafael Ribas
    April 19, 2010

    As a high school teacher, I find it really difficult to define energy – I will now check the post you link to.
    One definition I use is that energy is like the “currency” of the natural world. You need to exchange some for anything to happen, and you can find it in different “denominations”, but it is all equivalent.
    Of course, there is no machine that prints “money” in the physical world…

  3. #3 Keith Harwood
    April 19, 2010

    When I was at school in the middle of the last century energy was defined as the capacity to do work. Yes, vague I know. But we went straight on to work is done when a force moves its point of application a certain distance. Then the energy of a system in one state relative to another state is the work done to change from the initial state to the final state. From this expressions for kinetic and potential energy drop out immediately and it’s easy to show that the potential energy in raising an object to great height is equal to the kinetic energy when it crashes to earth.

    By concentrating all the time on the work done when an energy transfer takes place, the idea of energy as the capacity to do work was no longer vague and when we got to thermodynamics and the concept of energy that wasn’t available to do work (even though it had the capacity) didn’t seem contradictory.

  4. #4 V. infernalis
    April 20, 2010

    How about this pet peeve of mine:
    Using “rate of speed” in place of just “speed” (or worse, in place of “velocity”). I think people use it to sound smart, and instead it just makes them look stupid.

  5. #5 brimn
    April 20, 2010

    I like Feynman’s definition of energy: We don’t know what energy actually is; it’s an abstract thing, but we know that we can calculate this number, thing, or ‘energy’, and it would have the same value no matter what happens (in a closed system).

    I despise the energy definition “capacity to do work”. One then has too look up what “work” means. Much like a dictionary that gives you a definition that forces you to look up to other definitions.

  6. #6 Rhett Allain
    April 20, 2010

    @V. infernals,

    that is a good one – forgot about that. I think ZapperZ had a good rant on that – yes: http://physicsandphysicists.blogspot.com/2010/02/more-rate-of-speed.html

  7. #7 Rhett Allain
    April 20, 2010

    @brimn,

    Yes – that is a good definition of energy. Leave to Feynman to give a good explanation.

  8. #8 Eric
    April 22, 2010

    I always wonder when a moving object is being discussed someone might say, “its momentum carried it forward” Would it be more correct to say its inertia? (or less correct?). Is that possibly a way that momentum is misused in media?

  9. #9 Rhett Allain
    April 22, 2010

    @Eric,

    Surely momentum is misused in the media – but I am not sure about “momentum carried it forward”. Clearly, it is not the best thing to say – but I don’t know if it is exactly wrong. I guess it is saying that there is some force on it. This force changes the momentum. Well, I guess the assumption of a force is wrong.