Question

A person is standing in front of a plane mirror.If the mirror recedes with velocity v , the relative speed of person and his image per second is-
1)0
2)v
3)2v
4)v/2

Answer 1

Detailed explanation is here: Distance of Image formed in the mirror is same as the distance of the object form the surface of the mirror.

Thus is distance of object = u

Then distance of image = -u (as the image is formed at other side of the mirror)

Thus distance between image and object = 2u ...(1)

Now if mirroe is receding you/object with speed -v, then distance travelled by mirror in unit time = v units

Thus distance between mirror and object in unit time = u - v

Thus disatnce between object and its image = 2(u-v)....(2)

From equations (1) and (2) distance between image and object is reduced by 2v units in unit time(1 second)

Thus speed of image w.r.t object = 2v