# Homework Help: Circular motion

1. Sep 1, 2007

### krnhseya

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

car is on a horizontal circular track of 80m radius and car starts from rest to 100km/h in 10 seconds. what's the magnitude a of ttotal acceleration of the car 8 seconds after start?

2. Relevant equations

normal acceleration = v^2/r

3. The attempt at a solution

convert v to m/s and r = 80. then i found normal acc.

i just have no clue on how to do this...class notes and book reading dont help me at all.
what are things i need to consider?
thank you

2. Sep 1, 2007

### rootX

there are two accelerations:
tangential, and normal.

3. Sep 1, 2007

### krnhseya

tangential is what's messing me up...
books says it's equal to v with dot above it (derivative)
or also equal to r*theta(with dot above as well)

i dont get what they actually mean or how to get it...

4. Sep 1, 2007

### rootX

well, I am assuming here that tangential a is uniform.
Just ignore them, I guess you really don't have an equation to differentiate.

just use that simple for: a = vf-vi/t
10 seconds, and assume that it's same for 8 seconds.

I am pretty sure that this would be correct

5. Sep 1, 2007

### krnhseya

Last edited: Sep 1, 2007
6. Sep 1, 2007

### hage567

Yes, you have found the acceleration. But the velocity that you use in the a = v^2/r equation must be the velocity of the car at t = 8 s, NOT at t =10 s, or Vf. You must use the acceleration you just found to find the velocity of the car at t = 8 s.

Also, the total acceleration is the vector sum of the tangential and centripetal accelerations.

7. Sep 1, 2007

### krnhseya

well can i use v = vo+at ? considering that a is constant and vo is zero(?)
how can you use vector to do this problem...i dont know how to set up the vector...
no coords :(

 well i can sum up and get final vectors i think...only problem is how to get normal acc. and tang acc. in vector form...

Last edited: Sep 1, 2007
8. Sep 1, 2007

### krnhseya

alright, here's what i have...

v = vo + at (given that vo = 0, v = 100km/h, t = 10)

i get 2.78m/s^2 as acceleration.

v = vo+2.78t (given that vo=0, t = 8)

i get v = 22.2m/s^2

an = v^2/r (given that v = 22.2 and r = 80)

i get an = 6.17m/s^2 and it should be NEGATIVE.

for the vector sum up part...since they are perpendicular to each other, i just find magnitude of two acceleration which is...

sqrt(((-6.17)^2)+(2.78^2)) = 6.77m/s^2

answer is right but just confirming if this is how i should do it.

thank you so much!

Last edited: Sep 1, 2007
9. Sep 2, 2007

### learningphysics

Yes, that looks right to me.

10. Sep 2, 2007

### krnhseya

ok thank you very much :)