A girl is throwing a ball.
If I have an equation $h= -4.9*t^2+20t+1.2$, where $t$ is number of second after the girl has thrown the ball, and $h$ is the height above ground. I want to find out when will the ball hit the ground. My solution is set $-4.9*t^2+20t+1.2=0$, and then I get $t=4.1$ seconds. Is it correct?
An it is wrong to say that we solve $-4.9*t^2+20t+1.2=4.1$ and then solve for $t$. Am I right?
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$\begingroup$So that the question will not remain unanswered, here is an answer.
You calculated the (positive) value of $t$ such that $-4.9t^2+20t+1.2=0$ correctly, and this was the right thing to do.
I get that $t\approx 4.14$. That kind of precision (to the nearest hundredth of a second), is physically unreasonable, since the quadratic model assumes that there is no air resistance.
As to the final question, setting $-4.9t^2+20t+1.2=4.1$ has no connection to solving the problem of finding the time when the ball hits the ground.
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