A -kilogram disk slides over level ice toward the east at a velocity of meter per second, as shown above. The disk strikes a post and rebounds toward the north at the same speed. The change in the magnitude of the eastward component of the momentum of the disk is
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The correct answer is . The momentum of an object is given by the product of the mass of the object and its velocity. Before the collision, the disk is moving directly east, so the eastward component of its momentum is . After the collision, the disk is moving directly north so the eastward component of its momentum is zero. Thus the change in the eastward component of momentum is the final value minus the initial value, or , which is equal to .