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5

Acceleration does not kill us any more than speed. If your head and feet do not move at the same velocity long enough, whatever the cause, you are in trouble. Velocity does not kill us when the whole body has the same velocity. Similarly, I doubt acceleration kills us when all parts of the body accelerate, but without having to transmit forces. It is said ...


4

There does not need to be a force on an object for it to move, only for it to accelerate, as can be seen from Newton's second law: $$F=m \cdot a$$ I think your confusion arises from forgetting to take into account frictional forces. In practice, a moving object will slow down because of friction: the net force is not zero! Therefore you need to apply an ...


3

Wormholes don't just exist by themselves, they have to be created by a form of matter called exotic matter (which almost certainly doesn't exist, but let's gloss over this). To construct a wormhole you need to gather up some exotic matter and arrange it in a particular configuration. So if the exotic matter is moving the wormhole will move along with it. ...


3

The flaw in your reasoning is that you believe that pressure and velocity are said to be inversely proportional, $p=K/v$, by the law. Instead, Bernoulli's law says that these two variables are in inverse relationship (but not "proportionality") which means that one of them is a decreasing function of the other, and you wrote what the function is. $p=K/v$ is ...


3

If the velocity decreases by a little bit, it goes into an elliptical orbit with the apogee the same as the original orbit. The orbit will be stable if no other changes happen. Only when the velocity decreases to the point where the elliptical orbit intersects the Earth's atmosphere will the object crash into the Earth.


3

The theory of relativity tells us what is the answer to this question. See Wikipedia, as John Rennie recommends, http://en.wikipedia.org/wiki/Velocity-addition_formula#Special_case:_parallel_velocities If in the formula $$v_{rel} = \frac{v_1 + v_2}{1 + v_1 v_2/c^2}$$ you set $v_1 = v_2 = 0.9 \, c$, you get $v_{rel} = 1.8/1.81$, i.e. slightly less than ...


3

To answer the question "same wavelength, different frequencies, which arrives first?": naturally, the one with biggest speed, which is proportional to frequency AND wavelength according to the formula: $$v = \lambda f$$ So, for the same wavelength $\lambda$, the one with bigger frequency $f$ will have bigger speed $v$, thus arriving earlier. For the ...


3

Are Lorentz transformations more adequate representations of motion, than the more intuitive velocities? Yes. The non-associativity that bothers you simply arises because there is no group of three dimensional boosts. Confined to one dimension, boosts form a rather lovely one parameter subgroup of the Lorentz group $SO^+(1,3)$. So everything "works", ...


2

babou, this is really an extended comment to your own answer. I think your answer is pretty much spot on, but I would simply the reasoning a bit. What kills you is when the distance between different parts of your body changes. You give the example of the separation between your head and feet changing by more than 5% (something exploited by hangmen over the ...


2

The only thing that the device "knows" when it is hit, is the force with which it gets hit, and the duration of that hit. Transfer of momentum $m\Delta v = F\Delta t$. So what matters is the momentum of the hammer's head - or more specifically, the momentum that you are able to transfer. Ultimately it comes down to giving the most momentum to the head of the ...


2

The electron does not move - it has no well-defined position in the orbital state, and hence no well-defined momentum. Neither does it "teleport" around - as long as it is not interacting with something that forces it to be at a definite position, its state is "smeared" all over the electron as an electron cloud. Yes, this is essentially the Bohr model, ...


2

Different parts of the blades have different speeds, but all parts of the blades have the same instantaneous angular speed, that is all parts travel through the same angular displacement in a given amount of time. This is always true for a rigid body (when the angular speed is measured about the rotation axis). As ACuriousMind pointed out in a comment, you ...


2

If you fly head first into a wall, at the moment of impact the top of your head will accelerate very rapidly while the est of your body continues to travel at a constant speed until they make contact with the wall. Clearly having different parts of your body rapidly accelerate in different direction will lead to some very large forces on various parts of ...


2

It really is the stress that kills you. Velocity, acceleration & jerk are all fine as long as they are spatially uniform. It is a postulate of general relativity that you can not even detect acceleration due to a uniform gravitational field, no matter how intense. However if the there are spatially non uniform forces applied to your body, then there will ...


2

The problem is, there isn't just one way in Newtonian mechanics to kill someone. You can cause as little or as much acceleration as you want. A few things worth analyzing are: Whiplash. If you're under constant acceleration and you reach a steady state (and aren't dead yet), a change in acceleration (jerk) could cause a whip effect. The Earth-Sun system. ...


1

Since gravity is constantly acting on both spheres once released, both objects have constant downward directed acceleration and, thus, have zero velocity only for infinitesimal time. That is to say, the velocity of both objects is not constant at any time between their release and their impact with the ground. Assuming the objects have different upward ...


1

You can cast your problem in terms of what is the velocity of the car at any instant of time after it started. The answer is $v = a_{\rm eff} t$ from the first kinematical equation. In the same time, the car would've traveled a distance $s = (1/2)a_{\rm eff} t^2$ from the second kinematical equation. The other option is - if your inclined plane is of any ...


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If i understood your question correctly, you seem to not know the displacement of the car but you still need to increase the velocity of the car. Velocity of the car has to be expressed in terms of a quantity. I suggest you use velocity as a function of time, $$v_f = v_i + at$$ $a = g \sin \theta $ as you told correctly. So $$v_f = v_i + g \sin \theta ...


1

Try this method for solving the majority of Momentum/Impulse Problems with these two simple equations. Michel Van Biezen on Youtube teaches this method. Sure beats m1v1 + m2v2 (initial) = m1v1 + m2v2 (final). Given: Girl- Mass of 45.5kg; Velocity +1.47m/s Plank- Mass of 140kg Questions: QA Find the VELOCITY of Plank (that is girl + plank) on ice. QB ...


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Hint to the question (v2): For a velocity-dependent force ${\bf F}$ (such as e.g. the Lorentz force), the relationship between force ${\bf F}$ and potential $U$ is $$ {\bf F}~=~\frac{d}{dt} \frac{\partial U}{\partial {\bf v}} - \frac{\partial U}{\partial {\bf r}}. $$ See e.g. Goldstein, Classical Mechanics, Chapter 1. See also e.g. this and this Phys.SE ...


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First, the context is a function of time that is periodic which means that it is repetitive with repetition period $T$. $$g(t) = g(t + T)$$ So, if one sampled the function every $T$ seconds, one would get the same value each time. Now, we have the period of time $T$ which tells how long it takes for the signal to go through one cycle. The inverse ...


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How to understand non-associative composition of velocities in STR? Special relativity introduces a weirdness about how your axes can be related to other observers' axes: if your axes are aligned with observer A's axes and theirs are aligned with observer B then special relativity (i.e. the Lorentz transformations) say that B's axes will be rotated with ...



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