Who proposed a model with electrons moving in specific layers?

An ideal spring has a spring constant (force constant) of 2500 n/m, is stretched 4.0 cm, how much elastic potential energy does it possess? (a) 3j (b) 0.00j (c) 1j (d) 2j (e) 4j

Which feature of a heating curve indicates a change ofn state

For the car in the picture ,in which direction is the normal force? -into the screen -down -out of the screen -up