Question

A rock is dropped from a high building. The acceleration due to gravity is -10m/s. Find the distance travelled by this rock from t = 2s to t = 4s. Help!?

Answer 1

It travels 60 m.

Explanation:

I will take the liberty of making this correction: "The acceleration due to gravity is -10m/s^2". Using the kinematic formula
`v = u + a*t`,
we can find that at
t = 2s, `v = 0 + (-10 m/s^2)*2 s = -20 m/s`
and
t = 4s, `v = 0 + (-10 m/s^2)*4 s = -40 m/s`.

The next (and final) step is to find the distance it traveled in a 2 sec period, starting with an initial speed of -20 m/s and ending with final speed of -40 m/s. Using the kinematic formula
`s = ((u+v)/2) * t`,
we can find that `s = ((-20 m/s + (-40 m/s))/2)*2 s = -60 m`

Since the question is asking about distance, which is not a vector, rather than displacement, we can drop the minus sign. So it travels 60 m.

I hope this helps,
Steve