Why is it more difficult to do a sit up with your hands behind your head than when your arms are stretched out in front of you?

An object is dropped from a tower, 400 ft above the ground. the object's height above ground t seconds after the fall is s(t)equals400 minus 16 t squared. determine the velocity and acceleration of the object the moment it reaches the ground. the velocity of the object the moment it reaches the ground is nothing ft/s.

What is a good example of instantaneous speed

What is the major limitation to the light microscope?