2. if a rock fell down a cliff and hit the bottom of the ravine at 4 seconds, how fast was the rock
going when it hit the bottom of the cliff?

-39.2 m/s or 39.2 m/s directed downwards

Explanation:

This situation is a good example of Free Fall, where the main condition is that the initial velocity must be zero , and the acceleration is constant (acceleration due gravity).

So, in order to calculate the final velocity of the rock just at the moment it hitsthe bottom of the cliff, we will use the following equation:

Where:

is the acceleration due gravity (directed downwards)

is the time it takes to the rock to fall down the cliff

This is the rock's final velocity and its negative sign indicates it is directed downwards

Explanation:

It is given that,

Initial speed of the rock, u = 0

It hits the bottom of the ravine at 4 seconds. Let v is the speed of the rock when it hits the bottom of the cliff. It will move under the action of gravity. Using equation of kinematics as :

a = g

v = 39.2 m/s

So, the speed of the rock when it hit the bottom of the cliff is 39.2 m/s. Hence, this is the required solution.