Question

A student throws a rock horizontally from the edge of a cliff that is 20 m high. The rock has an initial speed of 10 m/s. If air resistance is negligible, the distance from the base of the cliff to where the rock hits the level ground below the cliff is most nearly _________.a. 5 mb. 10 mc. 20 md. 40 me. 200 m

Answer 1

Option C is the correct answer.

Explanation:

Consider the vertical motion of rock,

We have equation of motion s = ut + 0.5 at²

        Initial velocity, u = 0 m/s

        Acceleration, a = 9.81 m/s²  

        Displacement, s = 20 m      

     Substituting

                      s = ut + 0.5 at²

                      20 = 0 x t + 0.5 x 9.81 x t²

                      t = 2 s

      After 2 seconds it reaches ground,

Now consider the horizontal motion of rock,

        We have equation of motion s = ut + 0.5 at²

        Initial velocity, u = 10 m/s

        Acceleration, a = 0 m/s²  

        Time, t = 2 s      

     Substituting

                      s = ut + 0.5 at²

                      s = 10 x 2 + 0.5 x 0 x 2²

                      s = 20 m

      Distance traveled is 20 m

Horizontal distance traveled = 20 m

Option C is the correct answer.