Atrain travels 63 kilometers in 2 hours, and then 72 kilometers in 4 hours. what is its average speed?

The percent efficiency of a machine can never be 100% (or greater), because in the real world some energy is always converted into a. heat b. work c. input force d. output force

Which of the following is the phase that results when the moon is on the opposite side of the earth from the sun? a. quarter moon b. crescent moon c. new moon d. full moon

Matter is needed to transfer thermal energy bya. conductionb. convectionc. radiation d. both a & b.