# Height Question: Projectile launched from a table

## Homework Statement

A toy car rolls off of a table at an initial angle of 0 degrees and an initial velocity of 2.81 m/sec. The ball hits the floor 1.55 m (horizontally) from the edge of this table. How tall is the table?

## The Attempt at a Solution

So far I have:
Vo=(2.81 m/sec) * cos(0 degrees)=2.81 m/sec
Delta x= vot + one half at^2
1.55m=(2.81 m/sec) times t plus one half times (0) times t^2
t=1.55 m/2.81 m/sec=0.55 sec
Vo=(2.81 m/sec) times sin(0 degrees)=0 m/sec
Delta x=vot plus one half at ^2
Delta x= (0 m/sec) times (0.55) plus one half times (-9.8 m/sec) times (0.55 sec)^2=

The result I ended up getting was -1.48225 when I was done; I feel I know I must have made a mistake at some point, as the answer shouldn't have turned up negative. Someone I asked briefly on another site came up with the answer 1.488m for the problem; I'm just not quite sure what I got off. Could someone help me figure this one out please? I feel I'm close, but off somewhat. Thanks for the help & the patience, guys.
Really do want to learn, and whatever help you can give me in making sure I get this problem right would be very appreciated. :)

haruspex
Homework Helper
Gold Member
2020 Award
The formula you used for the vertical motion gives you the displacement at time t. You took up as positive, making g negative. In falling to the floor, would you expect the displacement to be positive or negative?

Negative? I feel kind of ridiculous right now, but I still don't quite understand.... Do you think you could tell me what lines in my data are off and what exactly in them I need to change? I think that would help me.

Was it the -9.8 in the problem that I got wrong? I just noticed that when I make that positive instead of negative, I get the positive answer.

haruspex