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  1. J

    Vector addition etc

    Have you done dot or cross products yet? If you have the easiest way to compute the angle is through the association that \mathbf{A}\cdot\mathbf{B}=|\mathbf{A}||\mathbf{B}|\cos{\theta} or with cross products |\mathbf{A}\times\mathbf{B}|=|\mathbf{A}||\mathbf{B}|\sin{\theta} where...
  2. J

    Angular momentum and torque of a particle

    First, you know that a force in the opposite direction and half the magnitude is just 3\hat{k}. So take a look at the torque; \tau=\mathbf{r}\times\mathbf{F}=\begin{vmatrix} \hat{i} & \hat{j} & \hat{k} \\ a & b & 0 \\ 2 & 1 & 0 \end{vmatrix} =[a-2b]\hat{k} So what possiblities could...
  3. J

    Help with gravitation and free-fall acceleration

    I think there is a simpler way to derive the escape velocity (if we're just talking about one surge of velocity at t=0 and not a rocket or anything). From conservation of energy we have K_i + U_i = K_f + U_f Two of these go to zero; the final potential energy (we're going to infinity) and the...
  4. J

    Help with gravitation and free-fall acceleration

    Think about what you're trying to find in part (a)--it's much like what the value g is on earth right? Note that g can be expressed as \mathbf{g}=-\frac{GM}{|\mathbf{r}|^2}\hat{\mathbf{r}}=\frac{\mathbf{F}}{m} For part (b), think of the fact that escape velocity is really something that comes...
  5. J

    What is the time bullet take to get at the animal ?

    I can't tell from the question but is the particle fired from an angle or simply a height above the target in the straight x direction? Nevertheless, any time in projectile motion you are trying to find the maximum height of something the solution is to usually just take the projectile equation...
  6. J

    Speed of a person jumping off a building?

    oh whoops hah sorry. yea i agree, im pretty sure they'd reach it.
  7. J

    Speed of a person jumping off a building?

    Yea, I estimated about 4 1/2 seconds for terminal velocity.
  8. J

    Speed of a person jumping off a building?

    The formula for terminal velocity would be, V_t=\sqrt{\frac{2mg}{\rho A C_d}} where m is the mass of your jumper in kilograms g=9.81 meters per second squared, and there are more specific things involved in the formula like the density of air (\rho) the drag coefficient (C_d) and the area...
  9. J

    Equation of motion problem

    How do you think you would get velocity from that expression?
  10. J

    Inclined Planes; Finding Force of Friction

    Hmm, I'm confused as to what you're asking then.
  11. J

    Inclined Planes; Finding Force of Friction

    The force of friction is defined to be \mu \mathbf{N}\text{ where } \mu \text{ is the frictional constant and } \mathbf{N} is the normal force. But there is of course no friction involved in this problem.
  12. J

    Measurements of the Speed of Light

    You've got it--I think you meant subtract the radii of the earth and the moon though, not the sun. Think about it--velocity is distance over time--so you get... c= 2\left(\frac{3.84 \times 10^8 - (E_R+m_R)}{2.51}\right)