A student of 50 kg steps on a spring scale and compresses the spring by 10 cm. What is the spring constant (in Newton / meter)?

A ball of 5 kg is shot vertically upward with 19.6 m/s. What is the maximum height it can reach (in meter)?

A spring gun with spring constant 4 N/m is compressed 2 meters from its natural length and then shoots a ball of 0.01 kg. What is the speed of the ball out of the gun (in m/s)?