13. A rod with resistance R lies across frictionless conducting rails in a uniform magnetic field B, as shown. Assume the rails have negligible resistance. The force that must be applied by a person to pull the rod to the right at constant speed v is:________.

5. Two blocks of masses m and 2m are held against a massless compressed spring within a box of mass 3m and length 4L whose center is at x=0. All surfaces are frictionless. After the blocks are released, they are each at a distance L from the ends of the box when they lose contact with the spring. Both blocks keeps moving until they collide with the box and stick to it. Find the distance of the center of the box moves from x=0 after both blocks stick to the box.

13. A rod with resistance R lies across frictionless conducting rails in a uniform magnetic field B, as shown. Assume the rails have negligible resistance. The force that must be applied by a person to pull the rod to the right at constant speed v is:________.

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