Free Fall: Questions & Solution

[katia11]

Homework Statement

45. A freely falling object requires 1.50 s to travel the last 30.0 m before it hits the ground. From what height above the ground did it fall?

Homework Equations

x = xi + Vit + (1/2)at²
Vf² = Vi² + 2ad

The Attempt at a Solution

First found the initial velocity from 30.0 m left
30m = 0m + Vi (1.5 sec) + (1/2) (-9.81 m/s2) (1.5 sec)²
Vi= 27.36 m/s at 30 m away from ground

Distance from original height to 30 m left
(27.36 m/s)² = (0 m/s)² + 2 (-9.8a m/s2) d
-38.15 m = d

total distance = 68.15 m


I hope this is right! Thank you!

 

[LowlyPion]

Homework Statement

45. A freely falling object requires 1.50 s to travel the last 30.0 m before it hits the ground. From what height above the ground did it fall?

Homework Equations

x = xi + Vit + (1/2)at²
Vf² = Vi² + 2ad

The Attempt at a Solution

First found the initial velocity from 30.0 m left
30m = 0m + Vi (1.5 sec) + (1/2) (-9.81 m/s2) (1.5 sec)²
Vi= 27.36 m/s at 30 m away from ground

Distance from original height to 30 m left
(27.36 m/s)² = (0 m/s)² + 2 (-9.8a m/s2) d
-38.15 m = d

total distance = 68.15 m

I hope this is right! Thank you!

You have the wrong sign for the Vi in the highlighted equation, and the distances should be swapped, but you got the right result.

Otherwise, looks good to me anyway.

 

[katia11]

Yes, I posted this because I was unsure about the signs of the two distances. That makes much more sense.

Thank you very much!

 

[alphysicist]

Homework Statement

45. A freely falling object requires 1.50 s to travel the last 30.0 m before it hits the ground. From what height above the ground did it fall?

Homework Equations

x = xi + Vit + (1/2)at²
Vf² = Vi² + 2ad

The Attempt at a Solution

First found the initial velocity from 30.0 m left
30m = 0m + Vi (1.5 sec) + (1/2) (-9.81 m/s2) (1.5 sec)²

I don't believe this is right.The left hand side should be -30m since the displacement is downward.