A concave mirror of focal length 20 cm moves right at 3 cm/s. A point object is 30 cm to its left and moves right at 7 cm/s. The image speed relative to the mirror has magnitude:
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Question

A concave mirror of focal length 20 cm moves right at 3 cm/s. A point object is 30 cm to its left and moves right at 7 cm/s. The image speed relative to the mirror has magnitude:

G1 Physics · Accepted Answer

Object speed relative to mirror=4 cm/s. At u=30, |m|=2, so image relative speed magnitude=4×4=16 cm/s.