- #1

manzoor

- 6

- 0

## Homework Statement

A ball is dropped from the top of a tower. If it takes 5 seconds to hit the ground, find the height of the tower and with what velocity will it strike the ground.

**Given Data:**

Initial Velocity = Vi = 0

Acceleration = a = -g = 9.8 ms^-2

## Homework Equations

The above question is my from my school textbook. I have the solution but I don't understand it. I want to know why the the acceleration (

**g**in our case) is negative (minus).

In my book it says that the acceleration is positive because the direction of initial motion is downward. When an object is speeding up, its acceleration is in the same direction as the velocity. All is fine till here. But if

**'g'**is positive then why is there a

**-**in the data?

To find the height of the tower I'll use the this equation

S = Vi * t + 1/2 * g * t^2

But in my book they have used above equation and solved it like this

S = Vi * t

**-**1/2 * g * t^2

S = 0 * 5

**-**1/2 *

**-9.8*** 5^2

S = 122.5 meters

Why is there a

**-**instead of

**+**? and why is the

**g**(why is 9.8 negative)?

Finding the final velocity of the ball is easily done using this equation

Vf = Vi + gt

But here we use positive

**g**i.e. 9.8 not -9.8

Can you please explain?