Tag Info

21

The main plot below shows the potential energy of a mass in the Earth-Moon system under the unrealistic assumption that the system is not rotating. i.e. This mirrors (at present) all but one of the 4 answers given, in assuming that this point is defined where the gravitational force on a mass due to the Earth and the Moon are equal and opposite (i.e. at the ...

15

Set the forces on the test particle from the Earth and Moon equal: $$F_E=F_M$$ $$G\frac{M_EM_{\text{ test particle}}}{R_E^2}=G\frac{M_MM_{\text{ test particle}}}{R_M^2}$$ The $G$s and $M_{\text{ test particle}}$s cancel, leaving you with $$\frac{M_E}{R_E^2}=\frac{M_M}{R_M^2}$$ but you know that $R_M$, the distance between the test particle and the Moon, is ...

11

Here is how I interpret what happened: You used Excel to compute the coefficients of the Gaussian that best describe the data: mean $\mu$, standard deviation $\sigma$, and magnitude $A$ for a curve $$Y=Ae^{-(x-\mu)^2/2\sigma^2}$$ Then you evaluated that function at a number of X values. Since the X values are not symmetrical about the calculated mean, you ...

8

Earth is about 100x more massive than the moon, and since $F \propto M / r^2$, the distance from Earth to the astronaut would have to be about $\sqrt{100}$ = 10x further than from the moon to the astronaut. Therefore, the astronaut falls "up" about 90% of the way to the moon. [The earlier answers go a lot more into detail (and are more technically ...

8

At Lagrange point L1. Specifically for Earth-Moon L1, these calculations show 326054 km.

4

Airplane wings typically have discharging brushes attached to them so that they can exchange charge with the environment. Charge that builds up on the wing is exchanged for example with water vapor in the air. In this sense, it's a "loop" because the environment closes the circuit. See for example this patent: http://www.google.com/patents/US3585447

4

Hints: In mathematics, a distribution is usually only defined wrt. smooth testfunctions. However the function ${\bf q}\mapsto({\bf q}\cdot{\bf p})^2/q^2$ is not continuous at the origin ${\bf q}={\bf 0}$. Nevertheless, we can e.g. try to evaluate the triple integral using the following representation of the 3D Dirac delta distribution $$\tag{1} ... 4 Indeed the answer is not zero but -4\pi\delta(r) (Dirac delta function). The formula of divergence can be found in any standard textbook on mathematical physics, for example chapter 2 of Mathematical methods for physicists by Arfken. But since this function is singular at r=0 we must be careful. At any other points is easy to calculate it. It is ... 3 This question is about Guassian wave-packet propagation and the corresponding Green's function in ordinary quantum mechanics. Assuming \hbar=m=1 for simplicity, consider the solution (with its initial condition) to the following Schrodinger equation:$$i\partial_t G=-\partial^2_x G \\ G(t=0,x)=\delta(x) $$Now assume the Fourier ansatz for \psi: ... 3 To calculate this by yourself, you need to know that gravity force exerted on an object (for exapmle You) is equal to F=GMm/r^2, where G is gravity constant, M is the mass of the big object (M_m for moon, M_e for earth), m is the mass of small object. r is the distance from the center of the mass. Now you need to know masses of earth and moon ... 3 Just use an equation derogating from the two forces which pull the objects (universal gravitation)to get the equilibrium point, something like (already simplifyed): M/d^2 = m/(384000000 - d)^2 Where M is the mass of earth, m the mass of moon and d the distance from earth. As d gets bigger than this value, you start falling into the moon I get a value of ... 3 The distance I got was 346 084km. Here are the maths I used: (E_m) Earth mass = 5.9736\times10^{24} kg (M_m) Moon mass = 7.3477\times10^{22} kg (D_{em}) average Earth-Moon distance = 384 467km (G) gravitational constant = 6.67384\times10^{-11} (W) my weight = 85kg (D_{fe}) distance from earth = ? The attraction force between two objects ... 2 You're right that in the context of radioactivity, antineutrinos are pretty much only released when a neutron turns into a proton, {}_0^1n\to {}_1^1p+{}_{-1}^{\;0}e+\bar{\nu}. They can also be consumed when a proton turns into a neutron and a positron, {}_1^1p + \bar{\nu}\to {}_0^1n + {}_{1}^{0}e. There are some other processes that involve ... 2 Why do not you try this? The shell is always an equipotential object. When the charge is inside the shell, the potential of the shell is kq/R with R the radius of the shell, so the energy of the charge is kq^2/R. When the charge moves to infinity, the potential energy of it is zero. Therefore, the work is kq^2/R. 2 Generally speaking, when we graph experimental data it's nice for the graph to be a straight line. This is because it's easy to tell if a graph is a straight line just by putting a ruler on it. If a graph is a curve it's hard to tell whether it's a parabola, ellipse, sine wave or some other curve without diving into some maths. In this experiment you are ... 2 Let's address the linear problem first. Both the approaches you have written here are correct: they are essentially the same approach to slightly different situations. The first is the clearer one because it is thinking in terms of momentum conservation of the whole system, which is unquestionably the clearest way to think about these kinds of problems. In ... 2 Rosetta initially went round the comet in a roughly triangular orbit using its thrusters to change direction. Eventually in September 2014 it entered a true orbit at a distance of about 30km going round once every two weeks. The orbit can hold in such weak gravity because it is very close and slow compared to satellites orbiting Earth. It has now moved ... 2 Every time, when you deal with differential equations, the first step is to put it into dimensionless form. There are more reasons for that. First, "small" and "large" has no meaning in dimensional forms, since you can always change the system of units. Second, nature knows no units. Now, when there is no exact solution (this is often the case), you can ... 2 If the block is resting on a frictionless surface, it won't fall over. It'll set in horizontal motion even with little force. To create torque, one edge of the bottom surface of the block needs to be fixed with high static friction and there must be a non-zero angle between the force and the line connecting force application point and fixed point (you can ... 2$$δ^3(q⃗ )=\frac{δ(q)δ(\theta)}{2\pi q^2\sin(\theta)}$$is wrong. The delta function is spherically symmetric, and thus has no θ dependence. Simply use:$$d^3(q⃗ )=\frac{δ(q)}{2\pi q^2}$$instead. Use the Jacobian when you switch coordinate systems (from Cartesian to spherical) (r^2 \sin(\theta)), and you should get the result. 2 It should be in joules. The term \ln \frac{V_2}{V_1} is unitless, because the units on the top and bottom of the fraction cancel. Pressure, in pascals, is defined as Newtons per square meter. You've already put the volume into cubic meters, and the cancellation goes$$\text{meters}^3 \times \frac{\text{Newtons}}{\text{meters}^2}=\text{Newtons} \times ...

2

Consider the following proof by contradiction: Suppose you find a state with $E < -V_0$, then its kinetic energy becomes negative at every point $x$ (classically the velocity is imaginary at every point), which means the whole wavefunction (at all $x$) is an evanescent (exponentially decaying) wave, but such a solution is not a physically stable solution ...

1

Your boundaries are at $r=a$ and $r=b$. Notice that the potentials at these two surfaces are independent of $\theta$ (they are spherically symmetric). Look at a list of the first few Legendre Polynomials $P_{l}(\cos{\theta})$. For what value of $l$ does $P_{l}(\cos{\theta})$ not depend on $\theta$? Further, notice that $V(a) = V(r=a,\theta) = V_{0}$, and ...

1

You need to include the 'zero' versions of $\rho$ and $a$ in order for the units to works out, or, to put it another way, to account for initial conditions. As regards the 32, you made an error while integrating: $\int a \mathrm{d}a = \frac{a^{2}}{2}$; the factor of 2 taken to the other side under the square root provides a 4 to makes the 8 into a 32.

1

A gaussian fit is symmetrical by definition, because it is a gaussian. Your orange fit doesnt look like a gaussian, it is not even smooth. I do no think excel had a gaussian fit function (but I dont use excell so cannot tell for sure. You can use other software such as matlab, or likely free ones on the web. Or, just use that data to calculate the parameters ...

1

This proof from Griffiths book introduction to electrodynamics Consider the vector function $$\vec{a}=\frac{1}{r^2}\hat{r}$$ At every location $\vec{a}$ is directed radially outward ; if ever there was a function that ought to have a large positive divergence, this is it. and yet, when you actually calculate the divergence, you will get ...

1

Start with ${b \over g }=m$ and rearrange this equation to get $b$ in terms of $g$ Then you can substitute your expression for $b$ into $g+b=3m$ and you should get the answer.... does this help?

1

Do some substitution: Re-arrange $$g+b=3m$$ to become $$b=3m-g$$ With substitution, $$\frac{b}{g}=10$$ becomes $$\frac{3m-g}{g}=10$$ Can you go from here?

1

Your answer seems to be good. I solved it in the same way. However there are two points in which i have no clarity. The first thing i do not have clear is the conservation of energy in the problem. We use the energy conservation, but how do we guarantee that the normal force is conservative? And, still, in the conservation equations we do not consider its ...

Only top voted, non community-wiki answers of a minimum length are eligible