Dot Physics

I already mentioned the MythBusters’ crashing two cars episode where they correctly doubled the speed of a pendulum type object. Overall, this was a very visual (although expensive) demo. There was one part that left a sour taste in my mouth – the final explanation from the narrator. First, they showed this.

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And then they had an explanation that went something very similar to to this (after restating what the sign above said)

“Although the two-car crash doubles the speed, the energy the crash is transferred to twice the mass resulting in a crash that looks like just one car hitting a wall at 50 mph.”

Here is the graphic that went with that.

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I had to re-listen to this narration a couple of times because something seemed not quite right. First, let me comment on the last diagram. Really, maybe it would have been better to leave this off. It doesn’t really add any useful explanation other than to point out that the kinetic energy is dependent on the square of the velocity. And what about the narrative? I think what bothered me is that they said the two-car crash doubles the speed, but what they probably should have said is “the two car crash doubles the kinetic energy and this energy is spread out over 2 cars.” Using their statement, you would say “oh, double speed means 4 times as much energy.”

Here is the real question: what concept are you trying to get across? Newton’s third law? The idea of kinetic energy? Conservation of momentum? I would just pick one and stick with it. Otherwise, you are kinda implying that “action and reaction” have something to do with kinetic energy. Oh, have I ever mentioned how much I hate “action and reaction” explanation of Newton’s 3rd law? Action? Reaction? How about this for Newton’s third law:

Forces are an interaction between two objects. Forces come in pairs. Or, if you must: For every force there is an equal and opposite force.

Oh sure – you can make the action reaction thing work, but it can also cause problems. Either way, I would suggest sticking with the energy explanation. Here, I want to help. I am going to give an energy explanation that the MythBusters could use and a force explanation for the two-car collision.

Two-car crash, energy explanation

Why are two cars crashing into each other not the same as one car going into a wall at twice the speed?

Explanation: In terms of energy, the energy of motion is called kinetic energy. Kinetic energy depends on the square of the velocity. This means a car moving at twice the speed with have 4 times the kinetic energy.

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So 10 + 10 is not the same as 40.

Two-car crash, force explanation

This is a little more complex, but I will try to make it simple. First, two key points:

  • Forces are an interaction between two objects. Object 1 pushes on object 2 the same as object 2 pushes on object 1 (same interaction).
  • A force on an object changes the object’s momentum where momentum is mass times velocity.

Suppose a car crashes into a wall with a velocity v. While it is interacting with the wall, the wall exerts a force (F) on the car and the car exert a force F on the wall.

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Where the force the wall exerts on the car and the car on the wall have the same magnitude. Now, what if I replace the wall with another identical car traveling at the same speed?

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Since the initial momentums are the same and the forces are same, the effects are the same on the two cars. So, two cars are the same as one car into a wall. If I now double the speed, I will have different initial momentum, so it will not be the same.

Other stuff

I actually forgot that I have talked about the MythBusters colliding two cars before. I wrote this post the first time they did this myth. That is a little more detail than I have here. Also, a similar thing came up when the MythBusters tried to pull two phone books apart. Here is my discussion of forces in that situation.

One more thing. I would like to emphasize how awesome this demonstration was. You hear people discussing things just like this all the time, but no one actually does it. For many physicists, the actual experiment doesn’t mean much. However, to many people this experiment is important. They just need to fix their final explanation (call me next time and I will be glad to help).

Comments

  1. #1 C. Felix
    May 6, 2010

    I can’t wrap my brain around how Newton’s 3rd law logically proves Jamie’s hypothesis wrong.

    Doubling the mass (action) results in what type of reaction? (Sorry for using those terms.)

  2. #2 Rhett Allain
    May 7, 2010

    @C. Felix,

    I am not sure I proved he was wrong, maybe instead I just showed that two cars colliding is the same as 1 into a wall at the same speed. If that works, then I guess it follows that double the speed would not be the same.

  3. #3 John K.
    May 7, 2010

    I started to think about this in terms of reference frames and this really makes me believe that energy is the wrong way to explain this problem to the average person.

    If you look at the proper reference frame of the orange car, because the event is happening to it. The person in the car would see a yellow car with E=40 hitting them or a wall with E=40 hitting them.

    Am I thinking about the reference frame incorrectly?

  4. #4 Matt Owen
    May 7, 2010

    Got this idea from Al Guenther: to teach newton’s third, cut a plastic pear in half and put a force vector in each half. HA! Forces come in pears. Thought you might like that if you hadn’t seen it before.

  5. #5 Rhett Allain
    May 7, 2010

    @Matt,

    Great idea. How about this modification. Make a plastic arrow for a vector. Take a real pear and embed this plastic arrow inside. Then when you cut it open in class you can say vectors come in pears.

  6. #6 Evan
    May 7, 2010

    Ummm… Isn’t the velocity in the kinetic energy equation suppose to the relative velocity between two objects, not relative to a fixed point in space or the point of the collision. Isn’t the relative velocity between two cars traveling at 50 mph the same as one car travelling toward a wall at 100 mph.

    Me thinks a few mistakes were made.

  7. #7 Rhett Allain
    May 7, 2010

    @Evan,

    Kinetic energy is not independent of reference frame. If you move to the reference frame of one of the cars, you would see a different total KE before the collision. What you would agree on is the changes in energy during the collision.

  8. #8 Evan
    May 7, 2010

    My statement was that the v in kinetic energy equation is based on relative velocity. Relative velocity is just that, Relative. Using the earth as a universal reference is a very common mistake. Each object’s kinetic energy relative to the earth is irrelevant. We only care about the kinetic energy relative to the objects in the experiment. We’re forced to choose one of the objects as a frame of reference. With the wall the choice is obvious. We have to use the wall as a frame of reference. With two cars travelling towards each other we have to pick one of the cars as our frame of reference and consider that car stationary. Thus, the relative velocity is the same between the two experiments. If the mass of each car is the same and the relative velocity is the same, then the kinetic energy is the same. It’s like saying 1/2m(100)^2 = 1/2m(100)^2.

  9. #9 Evan
    May 7, 2010

    Basically, I’m saying your explanation using kinetic energy is wrong because you calculated kinetic energy relative to the earth and not relative the objects in the experiment.

    I’m also saying that a car hitting the wall at 100 mph hit that wall with the same kinetic energy as two cars hitting head on at 50 mph. Maybe not relative to the earth, but relative to one of the cars.

    Because kinetic energy is dependent on frame of reference, it’s absolutely critical that you choose a valid frame of reference.

  10. #10 Pteryxx
    May 7, 2010

    But nobody has addressed deformation, which Adam and Jamie used *as their measurement* of impact (along with G-forces i.e. deceleration). Of course the clay between the weights on two moving hammers squished only as much as the clay in one hammer did… there was twice as much total clay to absorb the impact. Likewise, when one moving car hits the wall, the wall shows no deformation – only the car is squished. When two cars hit with the same relative velocity as in car vs wall, *both* cars get deformed – so each individual car deforms less.

  11. #11 Evan
    May 7, 2010

    @Pteryxx

    I agree 100%. The difference in damage had more to do with elasticity then it did with force. Elasticity was not constant as the wall was far more elastic then the cars. The amount of force due to relative kinetic energy was the same for the 2 cars traveling at 50 and the one car traveling at 100. The difference was the one car traveling at 100 was the only object in the collision that could absorb energy through deformation. Thus it was damaged far more. The energy had to go somewhere.

  12. #12 Cleon Teunissen
    May 7, 2010

    I agree with Rhett that the action-is-minus-reaction is a particularly awkward way to formulate that law of motion.

    Whatever Newton’s reasons were to formulate the third law in that form, a far superior form is present in the Principia. In the Principia, the three laws are followed by corrollaries, and the fourth corrolary states the third law in dynamical form:

    The common center of mass of two or more objects does not alter its state of motion or rest by the actions of the bodies among themselves.

    In retrospect we would have been in a far better situation if Newton would have asserted the third law in that dynamical form. (In fact many textbooks do present that dynamical form as the third law.)

    I can’t resist fantasizing what things would be like if that dynamical form would be the canonical form. Then that whacky “action-is-minus-reaction” thing would not be there to confuse the guys who write the Mythbuster narrator’s script.

    Going back once more to the dynamical form:
    The common center of mass of two or more objects does not alter its state of motion or rest by the actions of the bodies among themselves.

    This form emphasizes the mutualness, and it is uncommitted as to what force is. It’s a statement about motion, not a statement about force. For better education that “action-is-minus-reaction” thing ought to be phased out, and replaced with the dynamical form.

  13. #13 Carol Braun
    May 9, 2010

    You write: “The two-car crash doubles the kinetic energy and this energy is spread out over 2 cars.” I think this is the clearest explanation for why 2 cars hitting each other @ 50 mph might do the same amount of damage (to Car #1) as would Car #1 hitting a stationary wall @ 50 mph.

    For me, the tricky part is convincing myself why, in the wall case, we don’t need to worry about some of that initial KE getting transferred to the wall instead of going into damaging Car #1. One model that seems to work is if you think of the wall as a spring with spring constant k >> F_max/L, where F_max is the max force w/ which the car pushes back while being crushed, and where L is a characteristic length scale in the process of crushing the car (F_max*L ~ energy that goes into damaging Car #1 during crash).

    Then, in the two-car collision, where each car has initial speed v, all the initial KE is shared equally between Car #1 and Car #2 during the crash:

    2 * 1/2 m v^2 = 2 * (Energy that goes into damaging Car #1),

    => Energy that goes into damaging Car #1 = 1/2 m v^2.

    But in the Car#1-hitting-wall collision, where Car #1 again has initial speed v, the initial KE is shared unequally between Car #1 and wall during the crash:

    1/2 m v^2 = E_wall + (Energy that goes into damaging Car #1)

    Now, IF the wall acts like an ideal spring w/ k >> F_max/L, and if we can assume the wall/spring ends up compressed by a (tiny) distance x, which is how far it had to compress to push back on the car with the required crushing force F_max, we have

    E_wall = 1/2 k x^2
    = 1/2 k (F_max/k)^2
    = F_max^2 / (2k)
    = F_max*L / (2 kappa)
    = O(1/kappa)

    where kappa = kL/F_max >> 1 is the dimensionless large parameter in our approximation. And recall that, by definition,

    Energy that goes into damaging Car#1 ~ F_max*L
    = O(1);

    that is, in the limit kappa->infinity, E_wall is negligible in comparison with the energy that goes into damaging Car #1. Going back to our energy-transfer eqn for car-hitting-wall, and taking the 1st term on the RHS to zero, we thus have

    Energy that goes into damaging Car #1 ~= 1/2 m v^2,

    which is the same amount of energy that went into damaging Car #1 in the two-car collision when *both* cars had initial speed v.

    But is it accurate to treat the wall like a large-k spring in this manner? I don’t know!

  14. #14 Carol Braun
    May 9, 2010

    Re the comments about reference frames: Remember that the COM reference frames of the cars are NONinertial. (As the cars get crushed, they decelerate.) So the physics can’t be expected to work properly in those frames. From his own COM frame, Car #1 thinks there’s twice as much energy to go around than there actually is (1/2 m*(2v)^2 = 2 mv^2 instead of 2* 1/2 mv^2 = mv^2), and he’ll therefore be confused about why there isn’t more damage to the two vehicles.

  15. #15 Rhett Allain
    May 10, 2010

    @Carol,

    I am not ignoring your comments, I am just taking time to think of a response. Sorry for the delay.

  16. #16 peter
    May 11, 2010

    1. One almost always has to make simplifying assumptions when “modeling”.

    2. One must “model” in order to keep the math reasonable.

    3. Sometimes the real world answer is lost in the over simplifications of the models.

    4. In this case, I think the key is to ignore the crumple factor, focus on the choosing the inertial reference frame for each case and let the kinetic energy tell the tale.

  17. #17 deeboo
    June 8, 2010

    They should have made a wall traveling at 50mph and a car traveling at 50mph hit each other.

  18. #18 902siwfty
    February 10, 2011

    The energy argument is wrong, by the way. The momentum one is correct. The problem with the energy argument is that you have all assumed that there is such a thing as “absolute kinetic energy”. Unfortunatley, that does not exist. Only changes in energy are meaningful – this is 1st year physics. I’ll give you an example. Say you are in a train moving at speed v (not accelerating) and the windows are blacked out. You would say that you have zero kinetic energy. Now, and observer on the ground says that you have 1/2 m v^2 kinetic energy. Who is right? It’s meaningless since the notion of absolute KE is just as meaningless as absolute potential energy (PE). If you considered changes in energy, then things would be more meaningful.

    Finally, this is an in-elastic collision, so what you can apply is conservation of momentum. Mechanical energy is not conserved in this collision.

    Summary: Newton’s third law, and the cons. of momentum explain this result. That is all you need. This whole issue of different frames of reference (cars frame, ground frame) is a red herring.

  19. #19 wyatte roberts
    May 30, 2011

    I have a queston if there were to eople in the wouds how would last longer the skiny one or the fat one ??????