Joined: Aug. 2007
|Quote (C.J.O'Brien @ Mar. 18 2008,15:44)|
The last formal education in Physics I had was longer ago than I care to contemplate, so I have an (admittedly very basic) question to ask of those wiser than I.
So, we had my son's birthday party at Lawrence Hall of Science in Berkeley over the weekend. The temporary set of exhibits was called "Speed," and they had all kinds of hands-on gizmos illustrating various aspects of "speed": velocity, acceleration, drag, etc.
And one was this pneumatic thingy that shot a penny at great speed into a metal plate and then spit it out so you could see how it deformed on impact. They had some exemplars of what happens, based on composition (zinc vs. copper) and angle of impact (the pennies could "flutter" in the tube). It was pretty cool, and we fed it several pennies. But my issue is with the explanatory sign accompanying the exhibit.
It gave F=ma, talked about the force being applied to the penny. So far so good, but then it got into the "equal and opposite" reaction to the force, and then came the inexplicable talk about (paraphrased) "the plate pushes back" and said (verbatim) "the plate is the big F." (meaning the 'F' in F=ma.)
Is it me, or is this just crazily wrong? As I understand it, F is the initial force applied to the penny to shoot it down the tube, and the "equal and opposite reaction" is the deformation of the penny, i.e. the shedding of kinetic energy into heat and stresses in the material of the penny, causing it to be smashed, cracked or folded.
The way they explained it is analagous to saying that if the force of gravity is pushing you into the ground, then the opposite reaction is somehow the ground "pushing back." And that's all wrong. The ground, or the plate in the exhibit, don't exert any force at all, right? In the gravity example, the equal and opposite reaction is all going on inside your body: it's the various stresses and tensions on your skeletal-muscular system involved in keeping you upright, or, if not upright, then at least not a puddle. Right?
And if so, why does a freaking science museum at freaking UC Berkeley have signs that don't hold up to basic high school physics?
The museum's explanation is accurate.
Remember the three laws:
A robot shall not harm a h-
Wait, not those laws. Newton's:
1. A body will remain at constant velocity unless acted on by a net force.
2. F = ma
3. All forces act in equal and opposite pairs.
These seem to be true, although schools have yet to teach the controversy about the failings of Newtonian gravity at speeds close to that of light.
What this means is that the Earth does indeed push up on you with the same force as you push down on it - it has to be, or it'd be violating the first law (you're not moving up or down, so the net force on you is a zero, balanced force).
Similarly, when it comes to your body, all the internal forces are balanced, by tension (which is also a force). If you go to a planet with super-gravity, then you'll find what happens when your body doesn't have the ability to balance the forces; they get balanced for you when your bones and organs puddle onto the ground.
It's the same with the penny, but horizontal. The penny is travelling at a huge velocity until it hits the plate and decelerates to zero. The first law says that a force must have applied to cause this. The third law says that a force is also applied equal and opposite to that. The second law tells you a little about the forces effect.
Since the plate is large and probably mounted, it'll shrug off the penny's force without much problem. However, when the penny takes the force in the other direction, its tiny lil body doesn't have the internal strength to decelerate and keep its shape, so it bends and crinkles.
Try to envision what happens if the plate is replaced with a piece of paper. This time, the penny wins; the paper can apply a force to slow the penny, but can't possibly take the reaction force without tearing, so the penny makes a hole in it.