Projectile motion car acceleration problem

1. Sep 16, 2011

olliepower

1. The problem statement, all variables and given/known data

A car is parked near a cliff overlooking the ocean on an incline that makes an angle of 29.3 degrees with the horizontal. The negligent driver leaves the car in neutral, and the emergency brakes are defective. The car rolls from rest down the incline and has a velocity of 9m/s when it reaches the edge of the cliff. The cliff is 15.7m above the ocean. The acceleration of gravity is 9.8m/s^2

2. Relevant equations

$\theta$ = 29.3
Velocity = V = 9 m/s
Initial height = 15.7 m
V_x = V cos $/theta$
V_y = V sin$/theta$
r_y = yo +V_oyt + 1/2gt^2 //equation for y component of the motion
r_x = V_xt //equation of x component of the motion

3. The attempt at a solution
1. Find x component
7.848263 = 9 cos (29.3)

2. Find y component
4.404444 = 9 Sin ( 29.3)

3. Find When Y motion = zero

0 = 15.7 -4.404444t - 1/2(9.8)t^2
t = 1.424256 //The car hits the water at this time

4. See how far the car goes in the X direction during the same time interval
X = 11.178448

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Last edited: Sep 16, 2011
2. Sep 16, 2011

kuruman

The y-component of the velocity and the acceleration of gravity must have the same sign because they are both downward.

3. Sep 16, 2011

Spinnor

I got 11.0 meters. Probably rounding differences. I used,

0 = 15.7 - 4.4*t - 4.9*t^2

and solved for t.

4. Sep 16, 2011

olliepower

I did the actual calculation with -9.8m/s^2. I made a typo in the post. Fixed it now.

5. Sep 16, 2011

olliepower

I took it out to 6 decimal units because that is what my HW system requires. I am still getting the wrong answer every time. Am I missing something?

6. Sep 16, 2011

olliepower

I emailed my professor and he responded with " You are solving a quadratic equation to find the time the car takes to hit the water, which is a recipe for wrong arithmetic. You can find the time using a first-order equation, and this is by far the best way to do it... much less chance of making a mistake."

What equation will let me know when height = 0 when i have the velocity?

7. Sep 16, 2011

kuruman

I solved the quadratic and I got 10.957633 m. It's a ridiculous number of decimal places, but that's what it is. Probably your professor is referring to the following derivation for the time.