# Working out a value for a physics equation based on a graph

#### tatniki

##### New Member
Hey
I need to work out the time of flight for a falling object taking into account air resistance.
Because of the air resistance the acceleration is changing and I can't use any formula to work it out directly
What I have is a graph of displacement vs. time, and I was wondering if there was any way to work out the exact location through which the curve passes the x-axis.
The values are changing, so the location through which it passes will change, and I need some way for this to come up in a cell.
The formula is;
y=(mg/k)(t+(m/k)(e^(-kt/m)-1))
m, g, k and e are constants.
I need the value of 't' for which y=0,
In the data table I have a column for time increments, and a column for 'y' (displacement)
I was wondering if there was any function which would let me say 'when a value in column B=0(eg.B6), this cell=value in column A in the same row(A6)'
A B
1 t y
2 1 10
3 2 9
4 3 7
5 4 4
6 5 0

Does that make sense?

Any suggestions?

In the end I don't really care what way i do it, as long as the end result is a cell which gives me "time of flight"

Thanks

### Excel Facts

Will the fill handle fill 1, 2, 3?
Yes! Type 1 in a cell. Hold down Ctrl while you drag the fill handle.

#### dafan

##### Well-known Member
Can't you use the Goal seeker for Y=0? (Tools -> Goal seek), or the Solver add-in?

#### tatniki

##### New Member
I need to be able to change the constants, and automatically come up with an answer in a cell
Will that work with goal seek?

#### tatniki

##### New Member
*bump*

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