Recent content by Matthaeus_

  1. M

    Electromagnetic Radiation and Health

    That is complete bull****. Microwave radiation is not stored anywhere in cooked food, the waves simply move the water molecules up and down as they follow the sinusoidal changing electric field so that they warm up through friction. The food cannot become a source of microwaves itself. It is...
  2. M

    Electric field/equipotential field questions

    1) Equipotential surfaces are always perpendicular to field lines. 2) \displaystyle \Delta V = E\cdot \Delta x (for a uniform electric field)
  3. M

    Constant performance vs friction and drag

    Then you'll have to solve the differential equation: \displaystyle F_{motor} - \mu m g = m \ddot{x}\qquad \text{(Newton's II Law)} \displaystyle \ddot{x} + \mu g - a_0 = 0 Where a_0 is F/m. In this case, it is clear that the acceleration \ddot{x} of the body will be zero if and only if \mu g...
  4. M

    Constant performance vs friction and drag

    Please use LaTeX. Friction force dissipates a power: \displaystyle P_f = \frac{F\cdot \mathrm{d}s}{\mathrm{d}t} = \mu m g v Drag force: \displaystyle P_d = c_1v^2 + c_2 v^3 For some drag coefficients c_1 and c_2 Therefore the limit speed is the real solution of: \displaystyle...
  5. M

    Doppler effect spectrum of a star

    Sure. That immediately follows from ~E = hf.
  6. M

    Doppler effect spectrum of a star

    Gee! You're using the wrong equations for energy! A photon carries a momentum p = h/\lambda. According to special relativity: \displaystyle E^2 = p^2 c^2 + m_0^2 c^4. But photons have no mass! Therefore, no \frac 1 2 mv^2!
  7. M

    Magnetism, component values, and three dimensions

    \displaystyle \vec{B} = (56\,\mathrm{mT})\hat{\i} + (46\,\mathrm{mT})\hat{\j} \displaystyle \vec{v} = (2.6\,\mathrm{km/s})\hat{k} When you have components, it is often wasier to calculate the cross product by finding the determinant of the matrix: \displaystyle \vec{v}\times\vec{B} =...
  8. M

    Doppler effect spectrum of a star

    The star is approaching the Earth with a speed v \approx 52\ \mathrm{km/s}, which you can calculate by solving: \displaystyle \lambda^{\prime} = \lambda \left( \frac{1+\beta}{1-\beta}\right)^{1/2} \quad \text{ where } \quad \beta = \frac{v}{c} You probably overlooked that 6.561 is smaller...
  9. M

    Projectile Motion Help: Angle 15, Time 3.5s, Unknown Distances & Velocity

    Ah, well, putting y_0 = 0 then you can solve: \displaystyle \left \{ \begin{array}{ll} y &= (v_0 \sin 15^{\circ})t - \frac{1}{2} gt^2\\ v_y &= (v_0 \sin 15^{\circ}) - gt\\ x &= (v_0 \cos 15^{\circ})t \end{array}
  10. M

    Find the maximum induced emf in the large coil solenoid

    I think Ohm's Law and Faraday's Law would help: \displaystyle V = iR \displaystyle \oint\vec{E}\cdot d\vec{l} = \mathcal{E}_{ind} = -\frac{d\Phi_B}{dt}
  11. M

    What Is the Speed of 200Hz Sound in Air?

    Speed of sound is independent of the frequency. Otherwise it would be impossible to listen to an orchestra...
  12. M

    Electric Fields ( i attached a file)

    You could also have noted that the components in the x direction cancel each other out by simmetry, without having to worry about cosines ;)
  13. M

    Calculating Force Exerted by Laser Beam on a Mirror

    Whew... Thank you Doc for checking :)
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