A 2kg block at rest on a frictionless surface is pulled for 2s by a 10N horizontal force (pull to the right). Use the Work Energy Theorem to determine the final velocity from the perspective of an observer moving to the left. Does the WET still work that observer? I did the calculations and got 14 m/s for the first part. However, I am stuck on the second part. Why does the Work Energy Theorem works for an observer who is moving at constant velocity?