Calculating Centripetal Force? (car moving in circle on a sloped road)

[plat4m6]

Calculating Centripetal Force?!? (car moving in circle on a sloped road)

Hi guyz,
i have problem with this IB standard level mechanics question:

here is the link 2 the question: (it is from an International Bacalaureatte examination):

http://img494.imageshack.us/img494/4303/motionofcar5xa.png

The markscheme (answer sheet) has this as the way they got their answer:

horizontal component = R sin14;
= 8500 tan14;
= 2119 N --> which approx is 2100 N

my area of concern is how they arrived at the value for R, here is my calculation

x = 14 degrees, R = Reaction force of the road on the car, F = Horizontal component of the force R

http://www.rahmahwear.com/mymethod.doc [in this file]

i understand how they got their answr in the markscheme, but what's the error in my logic?

as for part d, i thought that if velocity increase, F will increase (i.e. for example.. instead of 2100 it become 3000 but it is still towards center of circle), therefore the car should slide DOWN the ramp?, they say that
"friction must supply larger force2ward center, car tends to slide UP he ramp"

sorry i couldn't ask my own physics teacher.. he's a great bludger.

Thanks in advance

 

[daniel_i_l]

d) If the car speeds up than the centrafugal force needed to keep it at equilibrium is bigger. the weight stays the same.

 

[plat4m6]

hi daniel

but if car speeds up, the centripetal force will automatically increase since it means acceleration is increasing (a = v^2/r), hence if anything.. the car should remain where it is in same motion.. i think..

i mean.. when u wirl a steel ball attached to piece of string, the faster u spin it.. the Force will continue to piont to center no matter what..

what do u think?

 

[daniel_i_l]

i mean.. when u wirl a steel ball attached to piece of string, the faster u spin it.. the Force will continue to piont to center no matter what..
what do u think?

Yes but you need more tension to keep the ball in place - If you spin it to fast then the string will rip and the ball will fly away.
Same here, as you move faster more friction is needed to hold you in place. If there isn't enough friction then like the ball you will fly away.

 

[plat4m6]

true that, now i understand that part, any suggestion with regard to why my calculation may be wrong?

thanks heaps daniel! lol it actually make sense to me now..