# 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 ...

19

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 ...

12

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 ...

8

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 ...

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 ...

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 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: $$s = ut + \frac{1}{2}at^2$$ ...

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

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 ...

4

Electromagnetic induction. All roads the hovercraft drive on need to have spatially-varying permanent magnetic fields. The hovercraft has a circuit with high induction sitting on board, but the circuit is usually broken. When you want to brake, close the circuit and power drained by the induced current will slow the hovercraft. This could potentially be ...

4

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 ...

4

This question is impossible to answer comprehensively, but conceptually we might be able to offer some new insight. Certain aspects of trade can affect rotation, but there are many human activities for which we have no expectation of impacting the Earth rotation. I will try to enumerate some here. ships should not affect the Earth's rotation. A ...

4

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 ...

4

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 ...

3

The "effect" is quite real, but I don't believe there's a name for it. It has to do with rotational stability and the intermediate axis theorem. I'm sure you've also observed this effect when throwing a spatula, frying pan, or remote control. In terms of rotational inertia, the "easiest" axis of rotation for the racquet is straight through the handle. The ...

3

You can think of ${\vec F}_{\rm net}$ as consisting of all of the external forces acting on a system of particles, irrespective of where those forces are applied. The only place where the center of mass comes into play is on the other side of Newton's second law: the ${\vec a}$ in $\sum {\vec F}_{i} = m{\vec a}$ is most assuredly the acceleration of the ...

3

The moment of inertia is merely a generalisation/application of the ‘usual’ inertia to rotations. Since translations and rotations are different kinds of motion, it appears sensible (to me) to have different kinds of inertia associated with them. Regarding your second question: Imagine a particle at position $(x,0,0)$ which you would like to rotate with ...

3

Any object has an inertia that is proportional to the mass. Inertia is defined as the tendency of a body to resist acceleration. It's this inertia that determines how fast the object will accelerate under a given force. In fact, in Newton's (and all subsequent) equations, mass and inertia are assumed to be the same thing, i.e. the proportionality factor is ...

3

That's just the way the world is. The fact that the resultant force $F$ on an object is proportional to the object's mass $m$ and its acceleration $a$, i.e. $$F=ma,$$ is a fundamental principle and cannot be derived from anything else (unless you count minimum-action principles and fancier, but equivalent, formulations of classical mechanics, or you see ...

3

Ask yourself the following questions: In what spatial coordinate(s) do you want to integrate? How do you express $dS$ in these spatial coordinates? An alternative strategy would be to make smart use of the perpendicular axis theorem.

3

Yep. You're pushing. In fact, with one jump, you will rocket straight up and probably bash your head agaist the ceiling. By the equivalence principle, the freefalling elevator is equal to an inertial box in space. If you jump in the box, you will push it "downwards" (meaning away from your feet--space has no up), and you will go "upwards", by momentum ...

3

Original Newton idea was that $$\vec{F} = m \cdot \vec{a}$$ meaning in inertial frame of reference (net or total) force equals product of mass of body and its acceleration. Indeed, as you mentioned similar expression appears in non-inertial frame of reference $$\vec{F'} = - m \cdot \vec{a'}$$ meaning in non-inertial frame of reference you have to ...

3

$T$ must be accounted for when you sum the forces. The application of $T$ away from the center of mass is equivalent to moving $T$ to the center of mass and adding an appropriately sized couple.

3

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 ...

3

For both interpretations, the answer is 'yes' since force still acts in an opposite force on anything which has mass. As you accelerate, your velocity increases and therefore mass will increase. The increase in mass will bring about an opposite force. The greater the mass, the greater the inertia.

3

If you ignore air resistance, the answer is "neither of them". There are no forces acting on either of the balls, so they will keep on moving at the same speed the train was moving originally, until they roll (or rather slide, if there is no friction at all) off the surface or hit something. In the frame of the train, they both receive the same acceleration, ...

2

To deal with lift fan failure, you'd need some landing pads anyway. If you design those properly you can use them for braking, too. The biggest problem might be how you'd lose the air cushion rapidly, in case of emergency braking. Maglev trains use a similar solution as they've got the same problem.

2

Do you mean air-supported or magnetically supported? Air-supported hovercraft need to be able to generate adequate horizontal thrust, for accelerating, decelerating, turning, whatever. Just to give you an idea of how much thrust you can get from a propeller, a Cessna 172 weighs about 2000 lb and has propeller thrust of about 400 lb at full power. That's ...

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