According to Newton's second law of motion, if a rigid body of unchanging mass is observed accelerating, what must be happening?

A force is being applied to the mass.

A force was once applied but is now not being applied.

No force was or is being applied to the mass.

The mass must be changing.

Scientific classification

How are period and frequency related to each other? dperiod is the reciprocal of frequency.

This  kingdom  is  made  up of species that  do  not really easily fit into other kingdoms such as the plant, animal,and  fungi  kingdom. some protists, such as the paramecium  and  amoeba, feed  on  other  organisms  while others such as algae are able to  make their own food  through the process of photosynthesis.