Mass = measure of the amount of matter within an object Density = mass per unit of volume Example: Think of one pound of gold and one pound of aluminum, at sea level on earth. They contain the same mass, but the mass is more dense in the gold, because its volume is much smaller.
To assert is defined as  to make a statement with full confidence of its truth and factuality. An example of a person who may assert something is a scientist who is very confident in his supporting data. The definition of assert is to make a claim or defense. An example of someone who may assert is a defense attorney who believes strongly in his client's innocence. To assert yourself means to make your beliefs or desires known in a strong and forceful way.
Synonym Discussion of aggressive. aggressive, militant, assertive, self-assertive mean obtrusively energetic especially in pursuing particular goals. aggressive implies a disposition to dominate often in disregard of others' rights or in determined and energetic pursuit of one's ends aggressive in his business dealings.