What is the magnitude of rock's acceleration immediately after being released?

Question

A rock is thrown (not dropped) straight down from a bridge into the river below. What is the magnitude of rock's acceleration immediately after being released? Disregard air resistance.


a) greater than g


b) less than g


c) equal g

Answer 1

It does not matter how we throw the rock. We could throw it straight down, straight up, throw at any arbitrary angle to the horizontal. As long as we do not interact with the rock, only one force acts on the rock - force of gravity.

Force of gravity directed always down and causes the same acceleration to every body - acceleration of free fall g.

Interesting, that when any body is in free fall it has a weight W = mg, but the apparent weight of the body is zero!

Just a reminder: Apparent weight is the force that the body exerts on the surface, scale or string. This force is electromagnetic in nature and its magnitude could vary. At the same time weight of the body at a given location is the same and gravitational in nature.

Answer 2

The acceleration of any projectile at any velocity is g