# Need help with this question regarding kinematics.

## Homework Statement

a) A ball is thrown in the -y direction off of a cliff with a velocity of 7m/s. If the ball takes 1.45s to reach the ground, how high off of the ground is the cliff? (Answer: -20m)

## Homework Equations

Vf=Vi+a(t)

Displacement=Vi(t)+1/2a(t)^2

Vf^2=Vi^2+2a(Displacement)

Displacement= 1/2(Vf+Vi)(t)[/B]

## The Attempt at a Solution

Known Values: [/B]
• Acceleration(a): -9.8m/s^2
• Time(t):1.45s
• Final Velocity(Vf): -7m/s
• Initial Velocity(Vi): 0m/s
• Displacement:?

I plug in the numbers and I'm not getting -20m as my answer. What am I doing wrong? or what am I not doing?

Orodruin
Staff Emeritus
Homework Helper
Gold Member
Initial Velocity(Vi): 0m/s
This is not correct. The problem statement says that the ball is thrown with an velocity 7 m/s in the negative y-direction. This refers to the initial velocity, not the final velocity.

Homework Helper
Gold Member
2020 Award
The only equation you need is ## s=v_i t +\frac{1}{2}at^2 ##. I don't know where the arithmetic is going wrong. Please show us what you get when you plug it in.

This is not correct. The problem statement says that the ball is thrown with an velocity 7 m/s in the negative y-direction. This refers to the initial velocity, not the final velocity.

Yes, I see that now. I also didn't plug in the numbers correctly in my first attempt. I did end up getting -20 this time. If that really IS the correct answer. Thanks.

CWatters
Homework Helper
Gold Member

## Homework Statement

a) A ball is thrown in the -y direction off of a cliff with a velocity of 7m/s. If the ball takes 1.45s to reach the ground, how high off of the ground is the cliff? (Answer: -20m)

## Homework Equations

Vf=Vi+a(t)

Displacement=Vi(t)+1/2a(t)^2

Vf^2=Vi^2+2a(Displacement)

Displacement= 1/2(Vf+Vi)(t)[/B]

Last line is wrong you lost the squares.

Homework Helper
Gold Member
2020 Award
Last line is wrong you lost the squares.
Displacement is the average velocity multiplied by the time. The last line of the OP is correct.

Orodruin
Staff Emeritus
$$v_f^2 = v_i^2 + 2as \quad \Longrightarrow \quad s = \frac{v_f^2 - v_i^2}{2a} = \frac{v_f+v_i}{2} \underbrace{\frac{v_f - v_i}{a}}_{= t}$$