- A rock is dropped off a cliff that’s 80 m high. If it strikes the
ground with an impact velocity of 40 m/s, what acceleration did it
experience during its descent?
Here’s How to Crack It
If something is dropped, then that means it has no initial velocity: v 0 = 0. So, we’re
given v 0 , ∆s, and v, and we’re asked for a. Since t is missing, we use Big Five #5.
Those Pesky Signs
Make sure you are clear
which direction is positive.
If an object ends farther in
the negative direction than
it started, ∆s is negative.v^2 = + 2a∆s ⇒ v^2 = 2a∆s (since v 0 = 0)Notice that since a has the same sign as ∆s, the acceleration vector points in the
same direction as the displacement vector. This makes sense, because the object
moves downward and the acceleration it experiences is due to gravity, which also
points downward.