SOLUTION: A particle is projected from ground level so that its height above the ground after t seconds is given by 20t-5t^2 m. After how many seconds is it 15 m above the ground? Can you ex

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Question 465947: A particle is projected from ground level so that its height above the ground after t seconds is given by 20t-5t^2 m. After how many seconds is it 15 m above the ground? Can you explain briefly why there are two possible answers?
Answer by richard1234(7193) About Me  (Show Source):
You can put this solution on YOUR website!
Because the height as a function of time is a quadratic, there can be two answers. The graph of the quadratic and the height h = 15 m intersect at two places, as shown:

graph%28400%2C400%2C-5%2C20%2C-5%2C20%2C20x-5x%5E2%2C+15%29

Note that the height vs. time graph is quadratic because acceleration due to gravity is constant (ignoring air resistance) and from this we can show that the position vs. time graph is quadratic using introductory physics and calculus.