Advanced MathMediumTime when a projectile reaches the ground
Advanced Math practice question
The height, in metres, of an object t seconds after it is launched is given by h(t) = −5t² + 20t + 60. After how many seconds does the object hit the ground?
Answer: 6
Set h(t) = 0: −5t² + 20t + 60 = 0 → t² − 4t − 12 = 0 → (t − 6)(t + 2) = 0. Since t > 0, t = 6.
Common wrong answers
- If you answered 2
- 2 is when the object is at its maximum height.
- If you answered 12
- 12 is the constant term of t² − 4t − 12.
- If you answered -2
- −2 is the negative root, before the launch.
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