# Finding speed of the car, kinematics

1. Sep 24, 2011

### jehan4141

This is an even numbered problem from Cutnell and Johnson 8 edition....Can somebody tell me if the answer is correct? I spent a good 30 minutes dissecting the problem, so here's to hoping I am right :)

A child operating a radio-controlled model car on a dock accidentally steers it off the edge. The car's displacement 1.1 s after leaving the dock has a magnitude of 7.0 m. What is the car's speed at the instant it drives off the edge of the dock?

Given
t = 1.1 s
Voy = 0 m/s
The displacement after 1.1 seconds is 7 meters. This means, from the point where it fell off the edge of the dock, to its position after 1.1 seconds, the distance between the two points is 7 meters.

We are trying to find Vox which is the same thing as Vx because there's no acceleration in the x-direction.

My Work
Voy = 0 m/s

Find Vy when t = 1.1 seconds passes

Vy = Voy + at = 0 - (-9.8*1.1)
Vy = -10.78 m/s

Find the distance it has fallen in the y-direction during the 1.1 seconds.
y = Voyt + 0.5at2 = 0.5(-9.8)(1.12)
y = -5.929 meters

Using A2 + B2 = C2
5.9292 + B2 = 72
B = 3.72114 meters <----this is the distance in the x direction that it travels during 1.1 seconds

So Vx = B/t = 3.72114/1.1
Vx = 3.383 m/s

PRETTY PLEASE LET THIS BE CORRECT *fingers crossed*

2. Sep 24, 2011

### ehild

Hi, Jehan,

your work is correct and nice, but why have you calculated vy? It was not needed.

ehild

3. Sep 24, 2011

### danielakkerma

Hi!
It seems like the reasoning and execution is flawless, good job!
One point though, just for future reference: there's alot to be said for rounding off, on occasion.
You have your v_y there, at 5.92.... It's not unreasonable to bring to ~6...
Daniel

4. Sep 24, 2011

### jehan4141

thank you both! it feels so stressful not knowing whether it is right or wrong. having both your approval puts me at ease! i can finally get some sleep!!! thank you!!!!!