You have to take into consideration all the energies that are present in this problem. When the dumbbell is at a certain height, it has potential energy. When the Dumbbell is acceleration towards the ground, it has kinetic energy. There you have your answer. As long as you set your potential...
Yeah, start with when the mass leaves the spring. There is no need to take the spring into consideration. It is just there to confuse you. When it comes to these problems, it's best not to over think them other wise you can get overwhelmed easily if you're just starting out. Spend time reading...
Homework Statement
A motorist travels at a constant speed of 39.0 m/s through a school zone; exceeding the posted speed limit. A policeman, waits 7.0 s before giving chase at an acceleration of 3.5 m/s2. Find the time required to catch the car, from the instant the car passes the policeman...