suvat question help

• October 11th 2009, 10:11 AM
silverfalcon
suvat question help
I don't know how to work out the problems for this question :/ i've tried for a while now...

A rock is thrown vertically down from a roof of a 25.0 m high building with a speed of 5m/s.

a) When does the rock hit the ground?
b) With what speed does it hit the ground? (Take acceleration due to gravity to be 10.0 m/s/s

Thanks
• October 11th 2009, 10:52 AM
skeeter
Quote:

Originally Posted by silverfalcon
I don't know how to work out the problems for this question :/ i've tried for a while now...

A rock is thrown vertically down from a roof of a 25.0 m high building with a speed of 5m/s.

a) When does the rock hit the ground?
b) With what speed does it hit the ground? (Take acceleration due to gravity to be 10.0 m/s/s

Thanks

for part (a) ...

$\Delta y = v_{yo} t - \frac{1}{2}gt^2
$

$-25 = -5t - 5t^2$

solve for $t$

for part (b), use the value of $t$ determined in part (a) ...

$v_{yf} = v_{yo} - gt$