Which of the following statements is not one of newton's laws of motion?
a. the rate of change of momentum of an object is equal to the net force applied to the object.
b. in the absence of a net force acting on it, an object moves with constant velocity.
c. for any force, there always is an equal and opposite reaction force.
d. what goes up must come down.

The answer to your question: d.

Explanation:

a. The rate of change of momentum of an object is equal to the net force applied to the object.  

This is the second a law of motion, so this answer is incorrect.

b. In the absence of a net force acting on it, an object moves with constant velocity.

This is the first Newton law of motion, so this option is not correct.

c. For any force, there always is an equal and opposite reaction force.

This is the third law of motion, so this is not the right option.

d. What goes up must come down.

Newton said this sentence, but is not part of the law of motion.

find the sum of the two horizontal vectors and the sum of the two vertical vectors

isoceles right angled triangle

3. dark matter

empty space could also be a simple vacuum