# Tag Info

36

That's not true, Newtons's laws do not say that. What's important here is conservation of momentum. Inside the phone, there is an oscillating mass. While the mass inside has a momentum and thus a velocity in one direction, the (friction-free) phone has to have the same momentum in the opposite direction. It "vibrates". Homework: Get on a skateboard (best ...

23

Yes! In fact, this kind of phenomenon is very common. For example, the mass of a proton is much greater than the sum of the masses of the constituent quarks. Much of the extra mass comes from the gluons that bind the quarks together; like photons, gluons are massless, but they contribute to the inertia.

20

From here: Higgs is an atheist, and is displeased that the Higgs particle is nicknamed the "God particle", because the term "might offend people who are religious".Usually this inappropriate nickname for the Higgs boson is attributed to Leon Lederman, the author of the book The God Particle: If the Universe Is the Answer, What Is the Question?, but the ...

15

At least one mobile phone I've heard about uses an unbalanced spinning weight. As the weight moves in one direction, the phone moves in the other, in accordance with Newton's Third Law of Motion.

15

On the whole, static friction is higher than dynamic friction. This means that if you can brake without your wheels skidding, you will come to a halt more quickly. So let's assume that the truck brakes without skidding, and see where that gets us. Let's assume that your truck has weight $W = Mg$ with a haystack with additional weight $w = mg$ on top. ...

14

Not to detract from Floris' answer, but I think this is an instance where it is nice to think in terms of limits. If the hay is tied down, you're stopping an object with mass (truck + hay). If the hay isn't tied down, but on a sufficiently sticky surface such that it doesn't move, it should be the same as stopping it if it were fixed, since the outcome is ...

13

When the cosmonaut sneezed they would start moving, and rotating, in the opposite direction, but when the sneeze hit their faceplate (ugh!) this would stop the motion. The net result is that the velocity of the cosmonaut would not have changed, but their position and angle would have. According to Wikipedia a typical breath is 500cm$^3$ and a sneeze ...

12

It would help if you gave some context. Is there any evidence, or even theoretical work, that suggests neutrinos are not affected by gravity? I suppose you could argue that the similar arrival times of photons and neutrinos from SN 1987A was evidence that neutrinos and photons are following the same path through spacetime and both being "gravitationally ...

11

If the asteroid is in parallel to the orbit of the earth and at rest it will feel the gravitational attraction and will fall with velocity growing as $g\cdot t^2.$ This force will be there whatever the angle and velocity of the asteroid, centrifugal forces may make it miss the earth in a parabolic orbit, or be caught in an elliptical as the path of the ...

9

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The term "God Particle" is used only by journalists. It's a wholly inappropriate term and I'd be very surprised if any physicist used it (outside of the lower end popular science TV programmes). General Relativity tells us that inertial and gravitation mass is the same thing. The Standard Model isn't going to say anything directly about gravitational mass ...

8

The answer depends on the identity of the dark matter. In the most widely believed scenario, dark matter is composed of "weakly interacting massive particles" ("WIMP"). The adjective "weak" really means that the particles interact via the weak nuclear force. This pretty much guarantees that they interact with the Higgs boson, too: the WIMPs carry the ...

8

Similar questions are: "why does electric charge happen?" and "why does gravity happen?" etc. The "art" of physics is in the identification of the fundamental "stuff", stuff for which the question "why" is actually misguided. You see, if there are fundamental "things" then, by the definition of "fundamental", these are the givens that we accept without ...

7

Yes the things/humans inside the vessel will keep going forward due to inertia. Since the things/humans have kinetic energy due to motion, they would keep moving. Whether the cabin is oxygened is not important: If you are comparing between a vessel filled with oxygen (or some other gas) and a vacuum vessel, the difference would be that the things/humans ...

7

going very fast and suddently stops (maybe it is not possible but that is not the point) Well, "stops" isn't actually well defined, but if it is subject to a thrust "backwards" any unsecured contents will all bang up against the "forward" bulkheads. So, the answer is to your first part is yes. Now, if there is an atmosphere present that will also slosh ...

7

Inertial mass describes an object's resistance to change in velocity. The more inertial mass something has, the harder it will be to change its velocity. Gravitational mass describes an object's ability to attract other matter (and under GR, to curve spacetime). The more gravitational mass something has, the more attracted to it other things will be. When ...

7

Well, technically, the answer is no as the other answers and comments also say. The approach speed can not be less than escape velocity. But in order for such a thing to happen, nature has to be really creative and totally in our favor. For example, the asteroid can have a very very lucky combination of these: The asteroid has right kind and amount of ...

6

Inertia does not suddenly "break" in the sense that the axis will remain fixed until some force threshold is reached, and move thereafter (for that matter, an ice skater cannot change direction by any clever combination of heel-toe maneuvering). In reality, any change in the mass distribution of the earth will move the orientation of the axis. Small changes ...

6

The thing you throw in the air is also traveling at the same speed you are, in the same direction. When you throw it up, it doesn't matter that the earth below is moving backwards at speed, nor that the moon is moving past even more quickly, nor that the earth itself is spinning and moving relative to the sun. The ball has a speed and direction and ...

6

The distance from London to Australia is about 17,000km. If you wanted to minimise the acceleration you'd feel during the trip you'd accelerate continuously for the first half of the journey (8,500km) then decelerate at the same rate for the second half. To work out what acceleration is required you use the SUVAT equation: $$s = ut + \frac{1}{2}at^2$$ ...

6

You ask, "How do we describe mass to the aliens, who don't know about our (g)?" This is an example of a class of questions referred to by Martin Gardner as "Ozma problems." The classic Ozma problem is how we describe to aliens the distinction between right and left; the answer is that we do it by describing the weak nuclear force. Your statement of your ...

6

The concept of inertia is indeed useful in two ways. I think your notion of it as a technical promotion of the everyday word "sloth" (without the baggage given it by the Roman Catholic translation of the "deadly sin" Ἀκηδία) as extremely close to the mark. In physics the notion of "inertia" has two, very alike uses: The first is practical, through a weak ...

6

Like Wikipedia says: "Moment is a combination of a physical quantity and a distance." This 'physical quantity' could be various things. To take the examples you mention: Moment of momentum (commonly known as angular momentum) is expressed as $\vec{L}=\vec{r}\times m\vec{v}$, and is a measure for the rotational momentum of an object around some axis. Moment ...

6

Yes, mass and energy are equivalent. A more competent relativist might be able to give you the complete description, but to first order you can say that the mass of an object is simply the total energy in its volume divided by c^2. That mass is equivalent to the inertial mass by the weak equivalence principle, which is a cornerstone of GR. That is to say, ...

5

I have to start by saying I don't know anything about the derivative method shown in this excerpt. I tried some calculations but it doesn't even seem to give the same result as the standard definition, so I'm guessing he is calculating something different from what we call "moments" in modern physics. Anyway, by way of explanation: The word "moment" is ...

5

The author appears to be assuming that $a$ is indeterminate in an empty universe. That assumption fits in nicely with some people's philosophical preconceptions (including Mach's), but of course we don't know it to be true. In particular, in general relativity, one can have an empty universe described by good old special-relativistic Minkowski spacetime. In ...

5

Thank you for your interesting question. The following is what I assumed in the paper. If you accelerate to the right, the Rindler horizon to your left is a boundary beyond which things are in principle unobservable for you. So, as soon as the nearer Rindler horizon forms, the far cosmic horizon behind it becomes unobservable and therefore (following the ...

5

That's exactly the case. If you look at the trajectory of any given spacecraft, you will see that it has a few burns of the rocket engines punctuating very long periods just coasting along in orbit around some other body. For example, the flight path of Apollo 8 has something like eight different rocket burns: launch, translunar and transearth injection (to ...

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