Acheetah accelerates from 15 m/s to 30 m/s in 3.0s what is the cheetahs acceleration

Consider a system with two masses that are moving away from each other. why will the kinetic energy differ if the frame of reference is a stationary observer or one of the masses?

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

I’ve been stuck on this for a while