- #1

EinsteinApple

- 10

- 0

## Homework Statement

Laura propels a stone straight up with her slingshot. The stone strikes a tree branch 4.0m above her. It strikes the branch with a velocity of 5.0m/s. How long does this take?

a= -9.81 m/s2

v1 = 5.0m/s

d= 4.0 m[/B]

## Homework Equations

d=vit+1/2at2[/B]

**d=v1+v2/2 x t**

v2=v1+at

v2=v1+at

## The Attempt at a Solution

ATTEMPT 1: DOESN'T WORK

**I believe I should use d=vit+1/2at2**

therefore:

4m = (5.0)(t)+1/2(-9.81)(t)2

-4.91t2 + 5.0t -4 =0

However I can't do this because using quadratic I get a negative. Therefore it's not an option.

ATTEMPT 2: CAN I ASSUME v2=0?

4=(5)+ (0)/2 x t

t = 1.6s

* But I guess I can't assume the V2=0

Attempt 3: CAN I ASSUME v2=0?

0=5 +(-9.81)t

t = 0.51 s.

I know the answer is 0.53s, however how do I know whether I should use attempt 2 or attempt 3?

I know to use attempt 3 because it's the right answer, but in my mind 1.6 s makes sense too.

Please help!

therefore:

4m = (5.0)(t)+1/2(-9.81)(t)2

-4.91t2 + 5.0t -4 =0

However I can't do this because using quadratic I get a negative. Therefore it's not an option.

ATTEMPT 2: CAN I ASSUME v2=0?

**d=v1+v2/2 x t**4=(5)+ (0)/2 x t

t = 1.6s

* But I guess I can't assume the V2=0

Attempt 3: CAN I ASSUME v2=0?

**v2=v1+at**0=5 +(-9.81)t

t = 0.51 s.

I know the answer is 0.53s, however how do I know whether I should use attempt 2 or attempt 3?

I know to use attempt 3 because it's the right answer, but in my mind 1.6 s makes sense too.

Please help!