Unit: 1D Kinematics

Prerequisites

Later Topics

None

Multi-Step Problem Preview

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Part 1

A stone is dropped into a well of unknown depth hh meters.

We hear the sound of the splash after ttotal=2.69st_{total}=2.69\,\text{s}.

Our goal is to calculate how deep the well is. The speed of sound is vsv_{s}.

Solve for the time t1t_{1} that it takes the stone to hit the bottom of the well. Write your answer in terms of hh and gg.

Correct!

Solution:

To find the time t1t_{1} it takes for the stone to fall to the bottom of the well, we use the kinematic equation for an object in free fall under constant acceleration. Let's define the downward direction as positive. The displacement of the stone is the depth of the well, hh. The stone is dropped, so its initial velocity is v0=0v_0 = 0. The acceleration is the acceleration due to gravity, a=ga=g.

The kinematic equation for displacement is:

h=v0t1+12at12h = v_{0}t_{1} + \frac{1}{2}at_{1}^2

Substituting the known values for the stone's fall:

h=(0)t1+12gt12h = (0) \cdot t_{1} + \frac{1}{2}g t_{1}^2

h=12gt12h = \frac{1}{2}gt_{1}^2

Now, we can solve this equation for the time, t1t_{1}. First, multiply both sides by 2 and divide by gg:

t12=2hgt_{1}^2 = \frac{2 \cdot h}{g}

Finally, take the square root of both sides to get the expression for t1t_{1}:

t1=2hgt_{1} = \sqrt{\frac{2 \cdot h}{g}}

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