If an object has zero acceleration, does that mean it has zero velocity?

Astone dropped into a well is heard to strike the water after 8seconds. find the depth of the well if the velocity of sound is 350m/s

When many atoms are split in a chain reaction, a large explosion occurs. this is an example of what type of energy conservation

At very high pressures, gases become and will eventually a) more dense; become hotter b) more dense; change to a liquid or solid c) less dense; combust d) less dense; turn into a liquid