Task: Text Categorization
Input (English): The period of a spring is given by two pi times the square root of this quantity over the spring constant. Heat transfer is equal to the specific heat times the change in temperature times this quantity. A "rest" type is multiplied by the square of the speed of light to give energy. For 10 points, name this quantity that is the "m" in "E equals m c squared."
Output (English): Science