# Homework Help: Speed distance and not a lot of hair left

1. Apr 25, 2012

### steveandy2002

1. The problem statement, all variables and given/known data
a balls inital velocity is 72 km/ph and falls 100 meters with gravity @ 10 meters per second and i need to work out the time it takes to fall to ground. i was given s=ut+0.5*gt^2

2. Relevant equations
s=ut+0.5gt^2
v^2=u^2+2as
t=(v-u)/a

3. The attempt at a solution

v2=u2+2as
v=squr u^2+2as
v=squr 20^2+2*10*100
v=48.98979486

t=(v-u)/a
t=(48.98979486-20)/10
t=2.898979486 seconds

100=20*2.898979486+0.5*10*2.898979486^2

is this correct??
and is there a way to do this with just the s=ut+0.5at^2 formula ?

Last edited: Apr 25, 2012
2. Apr 25, 2012

### tiny-tim

hi steveandy2002!

(try using the X2 button just above the Reply box )
s = 100, u = 72, a = 10 …

what's the difficulty?

3. Apr 25, 2012

### steveandy2002

not sure what you mean, i need to get time not s

4. Apr 25, 2012

### tal444

All you need to do is plug in your known values into s=ut+$\frac{1}{2}$gt2. Then you move everything to one side and solve the quadratic equation. What are you having issues with?

5. Apr 25, 2012

### Staff: Mentor

Don't forget to change the units on u from km/hr to m/s.