- #1
monkeymind
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Homework Statement
"Consider a square metal picture frame of side length s, mass M, and total electrical resistance R. It is dropped from rest from a height H above a region of uniform magnetic field pointing into the page. The frame accelerates downward under the influence of gravity until reaching the magnetic field. It is observed that, while entering the magnetic field, the frame moves with constant velocity. What is the frame's speed when it begins to enter the magnetic field? What is the strength of the magnetic field? (Solve for B in terms of the given physical quantities (s, M, R, and H) and any constants you need.) After the frame has completely entered the magnetic field, what is the frame's acceleration? Justify your answer in two sentences or less."
Homework Equations
[tex] {V_{f}}^2 = {V_{i}}^2 + 2 a x [/tex]
[tex] F_{mag} = \frac{v l^2 B^2}{R} [/tex]
[tex]F = m a [/tex]
The Attempt at a Solution
First I found the velocity after the metal frame dropped a height H:
[tex]V_{f}^2 = V_{i}^2 + 2 a x [/tex]
[tex]V_{f} = \sqrt{ 2 a x } = \sqrt{ 2 (9.8 \frac{m}{s^2 }) H} = 4.4 H \frac{m}{s}[/tex]
Then I reasoned that the only portion of the square metal picture frame that would factor into the magnetic force was the bottom horizontal portion. I figured that the two sides were moving in the same direction as velocity, and therefore the cross-product would go to zero. I also reasoned that the top horizontal portion would not matter, because as soon as this portion reaches the magnetic field, the flux is no longer changing and there will be no induced magnetic field, and therefore no magnetic force.
I approached the next portion as a dynamic equilibrium problem. Since the velocity is constant, I summed the forces in the positive and negative y-direction. Downward I have [itex]F_{gravity} = m g[/itex], and upward [itex]F_{B} = \frac{v s^2 B^2}{R}[/itex]. I then set them equal and solved for B, obtaining:
[tex]B = \sqrt{ \frac{M g R}{4.4 H s} }[/tex]
After it completely enters the field, I assumed that its acceleration would return to [itex]9.8 \frac{m}{s^2}[/itex] because the flux is no longer changing, so no fields are being induced.
Is this at all on the right track?