What is the Speed and Time for a Stone Thrown Downward?

[jojo711]

Homework Statement

A stone is thrown downward with a speed of 18 m/s from a height of 11 m. (acceleration due to gravity: 9.81 m/s2)
Your answers must be accurate to at least 1%. Give your answers to at least three significant figures.
a) What is the speed (in m/s) of the stone just before it hits the ground?
b) How long does it take (in seconds) for the stone to hit the ground?

The Attempt at a Solution

When I attempted the first part, I used the equation V^2=Vo^2+2a(X-Xo), and plugged in the numbers, V^2=(18^2)+2(9.81)(-11) and the answer came out to be 10.401 m/s and it was wrong.
When I tried the second part, I used the equation V=Vo+at and plugged in the numbers 0=18+(9.81)t and the answer was 8.19 s.

 

[Doc Al]

When I attempted the first part, I used the equation V^2=Vo^2+2a(X-Xo), and plugged in the numbers, V^2=(18^2)+2(9.81)(-11) and the answer came out to be 10.401 m/s and it was wrong.

Since you're taking down as negative, the acceleration must be negative.

When I tried the second part, I used the equation V=Vo+at and plugged in the numbers 0=18+(9.81)t and the answer was 8.19 s.

Why are you setting V = 0? (Again, careful with signs.)

 

[jojo711]

I got V=0 because the final velocity would be when the stone hits the mud and wouldn't that be 0?

 

[Doc Al]

I got V=0 because the final velocity would be when the stone hits the mud and wouldn't that be 0?

They want the speed just before it hits the ground. (Once it hits the ground, the acceleration no longer equals that of gravity. So that formula would no longer apply.)

 

[jojo711]

Oh, but how do I know what formula to use then?

 

[HallsofIvy]

The stone's initial height was 11m. 11m above what?

 

[Doc Al]

Oh, but how do I know what formula to use then?

You're using the correct formula. Once you solve the first part, then you'll have the final velocity.

Again, be careful with signs. Just to be consistent, anything that points down becomes negative. (You can also choose to call down positive. Just pick a convention and stick to it.)

 

[Physics697]

For the second part use Vf = Vi + at
just as Doc Al said; be careful with the signs ( Vf and Vi )

You can also use this equation yf = yi + Vi(t) + 0.5at^2