A ball is projected vertically upward. Its height above the point of projection is h(t)=20t-5t² metres. At what times is the ball 15 m above the point of projection?
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A ball is projected vertically upward. Its height above the point of projection is h(t)=20t−5t² metres. At what times is the ball 15 m above the point of projection?

G1 Physics · Accepted Answer

Set h=15: 20t−5t²=15. Divide by 5 and rearrange: t²−4t+3=0=(t−1)(t−3). Thus the ball is at 15 m at t=1 s while rising and t=3 s while falling.