Maximum speed of car round banked bend

[jaderberg]

Homework Statement

Car mass m drives round a bend in a circular arc radius 44m with the road banked at an angle \alpha where tan\alpha=3/4. The coefficient of friction \mu=0.6 of the car with the road. What is the maximum velocity the car can travel at without sliding up the banked road?

Homework Equations

F=ma F=mv^2/r Friction(max)=\mur
3^2 + 4^2= 5^2

The Attempt at a Solution

The normal reaction force R perpendicular to the bank will be mgcos\alpha
R=0.8mg

Component of R towards teh center of horizontal circle = R/sin\alpha = mg4/3

mv^2/r= 0.6x0.8mgxcos\alpha + mg4/3
v^2/44=0.384g +g4/3
V=27.2m/s

I've always struggled with this type of question due to taking the wrong components for reaction forces etc so would greatly appreciate any help to see if this is right.

cheers

 

[Shooting Star]

Take the components of the forces along and normal to the slope. It'll seem easier. Draw the freebody diagram. At the maximum speed, the static frcition force along the slope is max.

(Don't plug in numbers to make it messy -- use symbols.)

 

[jaderberg]

Yeah i know...this was actually in a test i did this morning so i did draw out everything. Basically this was the answer i did in the test and I still stand by after working through it again but was just wanting to see if it was actually right

 

[Shooting Star]

After simplification,

v^2/r
= g[sin(theta) + kcos(theta)]/[cos(theta) - ksin(theta)]
= g[tan(theta) + k]/[1 - ktan(theta)],

which gives me v = 32.5 m/s.

(Sorry...)

 

[jaderberg]

ah thanks alot! shame i didnt get it right tho lol