# Mechanics problem confusing!

luysion

## Homework Statement

"A cat leaps horizontally from a flat roof 7.5 m above the ground and lands on another flat roof 5 m above the ground and 3.5 m away horizontally.

How long, in seconds, does the leap take?"

## The Attempt at a Solution

Ok, i imagined the situation as half a projectile. with the cat traveling the hypotenuse of the triangle that can be formed (2.5 and 3.5 base). I know vi = 0 at highest point of a projectile and i know that the distance is 4.3 (pythagoras) i used kinematics ( d = vit + 1/2at^2) and get an answer of 0.81..

CHeers

aftershock
Remember that the horizontal and vertical components are completely seperate. If you focus on just vertical components, you should see that you have the values for acceleration, original velocity, and displacement.

Use kinematics formulas to solve for time.

luysion

I did that, and I didnt get 0.7 as my answer. I calculated the vertical component as 4.3 so that's my d(vertical) and i know vi(vertical component) is zero at the highest point of a projectile and acceleration is 9.8..

what am I doing wrong?

songoku
Hi luysion

with the cat traveling the hypotenuse of the triangle that can be formed (2.5 and 3.5 base).
CHeers

This is your mistake. The trajectory of the cat is not straight line but parabolic so it's nothing to do with triangle. Just use simple kinematics as aftershock suggested. Consider the vertical components to find the time needed

aftershock

I did that, and I didnt get 0.7 as my answer. I calculated the vertical component as 4.3 so that's my d(vertical) and i know vi(vertical component) is zero at the highest point of a projectile and acceleration is 9.8..

what am I doing wrong?

There's an acceleration so you need an initial and final velocity. The problem said the cat leaps horizontally... so there would be no vertical velocity at all to begin with. Displacement would be the difference between the two roofs.

Post what number's you're using for original velocity and displacement... and then what equation you're using if you still can't get .7 :)

Kommandant
This is how you solve it.

There are two building, the height difference between both of them is 2.5 meters.

Y (displacement in the vertical axis) = 2.5

We are in Earth, so the acceleration due to gravity is 9.81 m/s^2

a (acceleration) = 9.81

The Cat is resting and then begins descending; therefore, his initial speed is 0 m/s

Vi = 0 m/s

Solve

Y=ViT+(1/2)at^2

2.5=(1/2)(9.81)t^2

(5/9.81)=T^2

Square Root of (5/9.81) = .7139

luysion
Ah Cheers guys! thanks for all the replies. I was using the wrong component! i forgot in projectile we use the vertical component in kinematic equations. I kept using the hypotenuse which made no sense -_- as a projectile is a parabolic path

thanks so much to everyone!1