An object moves normally toward a plane mirror at 3 m/s while the mirror moves normally toward the object at 2 m/s. Find the rate at which the object-image separation decreases in m/s.
Physics · Geometrical optics · law of reflection and plane mirror
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Question

An object moves normally toward a plane mirror at 3 m/s while the mirror moves normally toward the object at 2 m/s. Find the rate at which the object-image separation decreases in m/s.

G1 Physics · Accepted Answer

Let the object move right at 3 m/s and mirror left at 2 m/s, so vₒ=+3 and vₘ=−2. Then vᵢ=2(−2)−3=−7 m/s. Object and image approach at 3−(−7)=10 m/s.