Why does a plastic food container shrink after microwaving?

After I microwave food in a plastic container and remove the container from the oven, the container begins to shrink. It seems like there is a vacuum inside. The lid and all the sides of the container are squeezed inward. Why does this happen? — U.S.

You’re right about the vacuum; the shrinkage you’re seeing is caused by a decrease in pressure inside the plastic container. When you heated the food, you also heated the air inside the container. Additionally, you converted some of the liquid water in the food into water vapor. Both of these effects, heating the air and adding water molecules to it, increased the gas pressure inside the container. Though you might not have noticed, the pressurized gas then leaked out of the plastic container through tiny gaps between its body and lid.

When you then took the container out of the microwave oven and exposed it to cooler room air, the temperature of the container and its contents decreases. The air cooled and some of the moisture in that air condensed back into liquid water. Both of those changes caused the gas pressure inside the container to decrease. While you might expect air to then leak back into the container, modern plastic containers and their lids tend to act as one-way valves — they leak air out, but not in. That’s because when the pressure inside the container is lower than atmospheric, the surrounding higher air pressure pushes the lid onto the container and improves the seal between those two items. So the container & lid leak air and water vapor during heating, but they then seal during cooling.

With low pressure inside the container and atmospheric pressure outside, there are substantial inward forces on the surfaces of the container and the container shrinks inward. The squeezing effect you observe is real — you have created a partial vacuum inside the container and it’s being squished by the surrounding air.

That same effect occurs whenever you cool the gas inside a sealed plastic container — for example, a plastic bottle that you filled with hot water and then closed tightly. The dropping temperature lowers the pressure of the gas and may even cause some of that gas to condense into liquid. With less pressure inside the container than outside the container, the container experiences inward forces that dent it inward. Ultimately, the container dents inward until the net forces on its surfaces become zero. It reaches that new equilibrium situation (new balance between forces) because compressing the gas inside it increases the pressure of that trapped gas and because the bottle or container itself has some elasticity that fights the denting. Either way, the container ends up looking squished.

How can an object spin at constant angular velocity when its parts are accelerating?

How can a spinning object keep constant velocity with the direction of its parts changing at every instant?

When you consider an object as rotating, you normally stop thinking of its parts as moving in their own independent ways and treat the whole assembly as a single object. While it’s true that the various parts of that object are accelerating in response to internal forces those parts exert on another, the object as a whole is doing a simpler motion: it’s rotating about some axis. This ability to focus on a simple motion in the midst of countless complicated motions is an example of the beautiful simplifications that physics allows in some cases.

When you push on a rotating object, when are you doing work?

When you push on a rotating object, when are you doing work?

That’s an interesting question and requires two answers. First, if you push on a part of the rotating object and that part moves a distance in the direction of the force you exert, then you do work on it. In principle, it is possible to identify all the work that you do on the rotating object via this approach.

However, it is also possible to determine the work you do entirely in terms of physical quantities of rotation. If you exert a torque on the rotating object and it rotates the an angle in the direction of your torque, you again do work on the object. That’s the rotational version of the work formula: whereas force time distance is the translational work formula, torque times angle is the rotational work formula.

An important complication arises, however, in that you must measure the angle in the appropriate units: radians. The radian is the natural unit of angle and is effectively dimensionless (no units after it). When you multiple the torque times the angle in radians, the resulting units are those of work and energy. If you use a non-natural unit of angle, such as the degree, then you’ll have to deal with presence of the angle unit in your result.

Rotating “up” or “down” — is that like clockwise and counter-clockwise?

When an object is rotating, both the “up” and “down” directions point along the vertical axis. Do they correspond to clockwise and counterclockwise?

Yes. Distinguishing between the two opposite directions of rotation using words alone requires that everyone agree on what to call those two directions. It also requires that everyone have an artifact that they can use to identify which direction is which. When something is spinning about a vertical axis, a carousel or merry-go-round, for example, then physicists name the two possible directions of rotation “up” and “down” and use a right-hand rule to identify which is which. Since most people have a right hand and know which hand it is, the necessary artifact is built-in.

In more common language, the two directions might be called “clockwise as viewed from above” and counter-clockwise as viewed from above”. In this case, the artifact is an old-fashioned analog clock and is probably more of a remembered artifact than one that is in the room with you. Nonetheless, that common naming convention is fine; it’s just wordier than the physicist’s version.

As a skater changes direction, is the skater accelerating?

When a skater changes direction, so that the skater’s velocity changes as a vector quantity, is the skater accelerating?

 Yes. Any change in a skater’s velocity involves an acceleration because acceleration is the change in velocity with time. So, if a skater is speeding up or slowing down, then it’s clear that the velocity changes because the speed part, the amount part a velocity, is changing. But when the skater’s velocity changes direction, so the skater is turning, even though the skater is traveling at the same speed, the skater is still undergoing acceleration and that acceleration still involves a net force on the skater, pushing the skater and bending the skater’s path.

What role does force play in a self-defense blow?

My self-defense instructor encouraged me hit the dummy with more force, so I exerted more force on my arm and it accelerated more rapidly. Yea for physics! But how does my increased force on my own arm cause me to hit the dummy with more force? — RL

Suppose you’re defending yourself against an attacker and you find that you have to hit them, either with your hand or with your fist. Two of the most important features of the impact between your hand and the person are how hard you push on the person and for how long.

The technical term for that push on the other person is a force; you exert a force on the other person. And a force is one of those physical quantities that has a direction to it. You can exert a force on someone toward the right or you can exert a force on someone toward left. Direction matters. Another thing about forces is that they’re always exerted between two thing, for example, you pushing on the other person. Forces don’t just exist by themselves; you can’t carry a force with you. You exert a force on something else.

That leaves this intuitive notion that you carry something with you during the wind up to an impact a little fuzzy. What is it you’re caring if you’re not carrying a force? Well, there is something you’re carrying: it’s known as momentum and momentum is a conserved physical quantity. That means you can’t create momentum or destroy momentum; all you can do is move it from one object to another.

In this respect momentum resembles money. Money is a conserved quantity, too, assuming that you don’t print it up in your basement (you are a law-abiding citizen) or you don’t destroy it (you’re not goofy). Money is conserved and goes from person to person to person. It is the conserved quantity of finance. Correspondingly, momentum is the conserved quantity of motion.

If you want to start moving to the right, you I have to accumulate some rightward momentum. Momentum, like force itself, has a direction to it. There’s momentum to the right; there’s momentum to the left. They are different, so if you want to move to the right, you have got to accumulate rightward momentum. The same is true of your hand or your fist, when you’re going after that attacker. If you want your fist to be really quick and move to the right rapidly, you have to invest a lot of rightward momentum into your fist.

To do that, you have to get that momentum from somewhere because you can’t make it. You can’t just cooking it up from nowhere. It comes from the ground and from the rest of your body. You pour rightward momentum—let’s suppose the bad guys are over to your right—you pour rightward momentum into your own fist. That momentum comes out of the rest of you and you do this how? By exerting a rightward force on your own fist.

You can do this—you can think of yourself as two separate parts: (1) your overall body and maybe your shoulder, and (2) the rest of you, your arm and your hand. So you’re pouring rightward momentum into your fist, at the expense of everything else. You actually can end up going backwards if you’re not careful

So you pour the rightward momentum into your hand and the amount of rightward momentum your hand accumulates is equal to the force you exert on your hand times the time over which you exert that force. The harder you push your hand and the longer you push your hand, the more rightward momentum it accumulates. If you want a fierce impact, You want to put a lot of rightward momentum into your hand. That means you push hard and you push long. You don’t go gently; you get going! You pour the momentum in so that its all accumulated.

This is this is the case not just for for punching somebody. It’s also the case for throwing a baseball. If you really want it to go fast, you need to take a long windup and you pour the rightward momentum into the baseball over as long a distance and with as much force as you can summon. Pack it full of rightward momentum, and off it goes. The same with a hammer. You pour rightward momentum into it, get it going and pack it full of momentum. When it hits the nail, its going to pack a wallop.

Okay, so now on to the impact. You have invested momentum in your hand; now when your hand hits something, it invests momentum in what it hits: the other guy, the bad guy. Your hand, which is chock-full of rightward momentum impacts that other person and transfers much or maybe even all its rightward momentum to that person by way of a force for a time. It’s passing along that momentum and it turns out that it can pass all of its momentum in a variety of different patterns. It can either pass along all its momentum with a gentle force over a long time, by pushing the person as they go away, or it can transfer all its momentum with a giant force for a short time.

If you hit knuckles to jaw, that impact is fierce and involves a big force, but not for very long. All the moment goes over in a jiffy. So a momentum transfer, it turns out, the amount of momentum you the put into something or transfer to something, is just this product of force times time. You can use a little force for a long time, or a big force for a short time; both of them can transfer the same momentum.

Well, if you really want to stun somebody, you want to make the transfer quick—short time, big force—and so that’s the bare-knuckle fight. It hurts. On the other hand, if you put on big fluffy gloves and delay or prolong the impact, it’s a little force for a big time. It more pushes you, but it doesn’t have that peak impact force that hurts.

So there you have it: if you’re trying to defend yourself against an attacker and you punch them, you do it by accumulating as much momentum toward the bad guy as possible. Use a big force for a big time, whatever you can do to get a lot of forward momentum into your knuckles and into hand. At the impact point—the moment when when you touch the other person—you want to transfer all that momentum to the other person, perhaps by way a big force for a short time. That’ll hurt everybody involved, you included, but, in any case, hopefully it will have the desired effect of getting the bad guy to go away and leave you alone.