Which of the following describes a reference point? A. The location of an object before its motion begins
B. The center point of a grid
C. The point at which the direction of motion changes
D. The position of an object after its motion has stopped

Which of the following best describes internal energy? a. the difference between the kinetic and potential energies of the particles in a system b. the sum of the kinetic and potential energies of the particles in a system c. the sum of the kinetic and thermal energies of the particles in a system d. the difference between the kinetic and thermal energies of the particles in a system