1. The problem statement, all variables and given/known data A particle P is moving in a straight line with velocity u=10 m/s along the ground. Its line of motion makes angle θ=60° with the X axis drawn on the ground. A flat mirror oriented perpendicular to the ground as well as the X axis is moving with constant velocity of v=20 m/s. This velocity is directed parallel to X axis from right to left. Calculate the relative speed (in m/s) between the particle and its image. 2. Relevant equations VA with respect to B = VA - VB 3. The attempt at a solution The velocity of particle is 10m/s , and angle is 60°. So the velocity in terms of unit vectors is 5i + 5√3j. But since the image will be travelling in y direction with the same velocity , and we want to find relative velocity , we need not consider the y direction . ( Is that correct?) Now in x direction , I've a few problems. First how to calculate the velocity of image ? My friend told me that it is 2m-o ( where m=velocity of mirror and o = velocity of object ( in this case particle ) ) . Is that right? So velocity of image is = -40i-5i=-45i ( since v. of mirror is -20i) . Now Velocity of image with respect to particle is v= -45i-5i = -50i . So relative speed is 50 m/s. But the answer is wrong. Please help me.