Off the cliff with velocity and gravity

[rupz28]

Homework Statement

You slam on the brakes and begin decelerating at 20,000 km/h²! The curve sign said you have to be down to 80 km/h before you hit the curve or you'll go over the cliff. You have 10 seconds until you reach the curve. Is your final velocity going to be down to 80 km/h by the time you reach the curve?

The current speed is 180.7 km/hr

Homework Equations

If a = fv-sv/t then solving for final velocity is fv = a(t) + sv

The Attempt at a Solution

divide the number of seconds by 3600 to get the number of hours = 0.002hr

fv = -20,000 km/h²(0.002h) + 180.7 km/h

= 140.7km/h or is it 140.7 km/h²

Homework Statement

So, you go over the edge. This is it, you are dead! The cliff you just fell off is 3000m high. At gravity's 9.8 m/s² acceleration, how many seconds do you have until you meet your doom?

Homework Equations

a= d/t²

The Attempt at a Solution

d = 3000m
a = 9.8 m/s²

I guess need to isolate t² first, to get t² = d/a, so t² = 3000m/9.8m/s²

3000m/9.8m/s² = 306 seconds? this doesn't seem correct, seems like it would take too long to hit the bottom

Thanks for all help

 

[HallsofIvy]

3000 meters, 3 kilometers, is almost 2 miles high! You think it wouldn't take about 3 minutes to fall 2 miles?

 

[cjl]

No, it wouldn't.

First, you're missing a factor of two in your equation to find fall time, and second, you're forgetting a square root. You had the proportionality correct (as t² is proportional to d), but you are completely forgetting about the fact that t is squared in your final answer.

 

[rupz28]

Ok, so I did the problem correct, but I'm not "seeing" the end result as a logical answer is what you are saying??

 

[cjl]

No, you did the problem incorrectly.

First, your equation is wrong. Where did you get the equation that a = d/t²?

(Specifically, it's missing a factor of 2)

Second, near the end, you state that t² is equal to 3000 m / (9.8 m/s²). However, you then solve it and say t = 3000/9.8. You dropped the exponent.