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.
assertion An assertion is a declaration that's made emphatically, especially as part of an argument or as if it's to be understood as a statement of fact. To assert is to state with force. So if someone makes an assertion, theyâre not just trying out an idea â they really mean it. An assertion can also be an act that seems to make a statement without words. When your dog urinates in multiple places up and down your block, he's making an assertion of his dominance (also called marking) over his territory.