PhysicsLAB AP Free Response Question
2010 B6

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The plastic cart shown in the figure above has mass 2.5 kg and moves with negligible friction on a horizontal surface. Attached to the cart is a rigid rectangular loop of wire that is 0.10 m by 0.20 m, has resistance 4.0 ohms, and has a mass that is negligible compared to the mass of the cart. The plane of the rectangular loop is parallel to the plane of the page. A uniform magnetic field of 2.0 T, perpendicular to and directed into the plane of the page, starts at x = 0, as shown above.
(a) On the figure below, indicate the direction of the induced current in the loop when its front edge is at x = 0.12 m. Justify your answer.

(b) When the front edge of the rectangular loop is at x = 0.12 m, its speed is 3.0 m/sec. Calculate the following for that instant.
 i. The magnitude of the induced current in the rectangular loop of wire.

 ii. The magnitude of the net force on the loop.

 (c) At a later time, the cart and loop are completely inside the magnetic field. Determine the magnitude of the net force on the loop at that time. Justify your answer.

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