An air-track glider of mass 0.100 kg is attached to the end of a horizontal air track by a spring with force constant 20.0 N m (Fig. 6.22a). Initially the spring is unstretched and the glider is moving at 1.50 m s to the right. It is calculated that with an air air track turned off, a glider travels 8.6 cm before it stops instantaneously. How large would the coefficient of static friction have to be to keep the glider from springing back to the left? (b) If the coefficient of static friction between the glider and the track is 0.60 what is the maximum initial speed that the glider can be given and still remain at rest after it stops instantaneously? With the air track turned off, the coefficient of kinetic friction is 0.47. I need help with part b. I know how to solve this but I'm getting the wrong answer because of the sign of the work done. In my book is shows that to solve part b you use: total work = Wspring + W fric = change in KE. What I don't understand is why my book made both the work of the spring and work of friction negative if their forces are acting in opposite directions. After the glider stops, the block will move the left and the force of the spring will be to the left because it is restoring, and the force of friction would be to the right because the friction opposes the motion, so wouldn't the work done by the friction be negative because it points in the opposite direction of the displacement?