Suppose you had a large empty box. In the bottom of the box you placed a firecracker with an electronic ignition device. On top of the firecracker you pour a pound of flour. You then hermetically seal the box and place it on a scale. What happens to the weight of the sealed box after you ignite the firecracker?
Thanks.
Nothing. If all the reaction products are contained, the mass remains the same. A tiny, tiny, tiny bit is converted to energy, per E=mc[sup]2[/sup], but that’s negligible. since the firecracker is on the bottom, the explosion would cause the bottom of the box to bulge out, making the box jump, so the scale would deflect briefly, but once everything settled down, the mass reading would be the same as before.
Thanks, Q.E.D..
But what about the flour that is no longer resting at the bottom of the box? It’s now floating and therefore weightless, isn’t it??
The weight of the box would go down as the firecracker explodes, jerking the box upwards, as well as throwing the flour into the air. Then the weight would go up as the box settles back down on the scales, and the weight would keep increasing as the flour comes down to rest on the bottom of the box. In the end, it would display the same weight as it did before you exploded the firecracker.
Nope; this is a reworking of the truck-with-flying-birds puzzle. Put a glass of water on the scales, now add an ice cube… will the weight of the ice cube register?.. but it’s floating.
Another way of looking at the question is, does a box with ten hummingbirds sitting on the bottom weigh as much as one with ten humming birds hovering inside it?
That isn’t correct; the system as a whole (which is what you are measuring) consists of a box, some flour, some air and a firecracker; at no point do the components of the system disappear anywhere.
The scales might briefly register some inertial effects as mass is moved about inside it, but across time, these too will add up to zero (otherwise you just invented a reactionless drive method).
I suppose until the flour dust suspended in the air inside the box settled out, it’s weight would not register on the scale. Eventually it would settle and the weight reading would return to what it was at the start.
I can’t help thinking, though, that, even though the dust is suspended in the air, it’s still pressing down, by transmission through the air, but I’ve not got enough of a handle on the physics to be definitive.
Replace the air with another fluid that we don’t intuitively think of as intangible - say water of better still, syrup - flour suspended in syrup adds to the weight of the syrup, now add some melted butter and the weight (actually mass) of the system increases again, now add a couple of eggs, lightly beaten, ladle into a hot pan adn fry for two minutes either side… mmmm waffles.
The flour is supported by the air inside the box, and both are exerting the same pressure on the floor of the box that they were before the explosion.
I’m not understanding how the flour particles can ‘exert pressure’ on the floor of the box.
If I threw a baseball horizontally over a super-sensitive scale, would the pressure from the mass of the baseball show up on the scale?
No, the point being made is that even though the flour particles are suspended in the air, the air is supporting their weight, since they are actually heavier than air. The reason very small, but heavier-than-air particles “float” in air is because their surface-to-mass ratio is very high, and so they have a very low terminal velocity. Or something.
The particles are resting on the air, the air is resting on the floor of the box. No different than if the flour was resting on table and table was resting on the floor.
No (except that the wake left by the ball in the air might disturb it). The reason is that the ball is essentially in free fall, while the flour particles are floating. To illustrate - you can throw that ball in the same manner in an airless vacuum, but you can’t float the flour in a vacuum.
OK, so my answer was completely wrong in every way! Sorry if i mislead anyone…
So what would happen if you had a box with flour falling down in the middle, but with a vacuum in the box, instead of air? Gravity would pull the flour downwards pretty quickly, but until it hit the bottom it would effectively be weightless. Is this correct?
Shouldn’t a similar effect happen in air? Gravity would pull the flour down, but this would be met by an opposing upward force from the air. So the flour would float down, rather than fall like a stone. It wouldn’t be weightless (as there would be a force opposing its fall), but it would have its weight reduced… Hence the scales would show a small reduction in weight… Is this correct?
And to think i’ve got an A in A level physics. I should know better really…
Surreal, the baseball you reference is in freefall, the flour is not. The flour is suspended in the air, resting (if you will) on the air below it.
Gravity is pulling on the flour, which (since it isn’t falling) is being acted upon by a force counter to gravity, that force is supplied by the air, which in turn is pressing against the bottom of the box.
Not all of it- the force would mostly go down through your feet as you threw it. In the closed box the force would all be inside.
Vibration can be transmtted out of the box, especially when you are setting off fireworks…
So if an airplane flew over a super-sensitive scale, would that register on the scale?
Correct.
Not quite, in the vacuum example, the particles are accelerating to the bottom with the full acceleration of gravity (no opposing force). In the air example, once they reach terminal velocity (quite quickly) there is no more acceleration downward, therefore, there is no net force downward (F=MA) the force of gravity is exactly cancelled out by the upward force from the air. Its weight would only be reduced while it accelerated to its terminal velocity.
Surreal, I believe you are correct there, though the scale would register only a very small portion of the weight, as it will spread over a wide area.
Okay, I think I understand now.
Thanks, Cheesesteak.
It’s important to make the distinction between mass and weight. They are often used interchangeably, though they are two different properties. The mass of something never changes, wheter it’s on a scale, floating in water (where its weight is affected by bouyancy), or in orbit (weightless).