You should memorize and use the basic equation of linear motion for bodies under constant acceleration:
y(t) = y_0 + v_0 t + (1/2)at^2
You may see this equation with different terms, such as using 'x' or 'd' for displacement, or v_i for initial velocity; whatever symbols you're used to you should use; it really doesn't matter which version you use as long as you are consistent.
Here you want to solve for time 't' given the initial displacement y_0 = 20m, the final displacement y=0, the initial velocity v_0 = -12 m/s, and acceleration a = g = -9.8m/s^2. By the way, you don't say what the direction is for he initial toss of the water balloon - it makes a big difference! If upward then v_0 = +12 m/s; if downward then v_0 = -12 m/s; if horizontal then v_0 = 0.
You can solve for 't' using the quadratic equation. Note that this will give you two answers - one with a positive value for 't' and one with a negative value. The positive value is the correct answer for this problem, but what do you think the negative value of 't' denotes?
Post back with your solution and we'll check it for you.
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Last edited by ChipB; Sep 28th 2018 at 03:04 PM.
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