Coefficient of friction question

AI Thread Summary
The discussion revolves around calculating the initial speed of a car that skidded 290 meters with locked wheels, given a coefficient of friction (μ) of 0.6. The user correctly identifies the acceleration as a = μg, resulting in a value of 5.88 m/s², but initially neglects to apply the negative sign for deceleration. After clarification, it is established that the acceleration should be -5.88 m/s², leading to a positive initial speed calculation. The final conclusion confirms that the user correctly understood the need for the negative sign in the context of deceleration.
wtfcat
Messages
2
Reaction score
0

Homework Statement



If a car's wheels are locked during emergency braking, the car slides along the road. The record for the longest skid marks on a road was reportedly set in 1960. the marks were 290m long. assuming that \mu = 0.6, how fast was the car going when the wheels became locked?

Givens:
Vf = 0m/s
Vi = ?
d = 290m
a = ?
\mu = 0.6
g = 9.8m/s2

Formulas:
\SigmaF = ma
\muk = Fk/Fn
Vf2 = Vi2 + 2ad


Homework Equations



1. Am i missing anything? Givens, formulas, etc.
2. Are my steps correct? ex. in my steps, am i missing a positive or negative sign, did i get units right, etc
3. Is my answer correct?


The Attempt at a Solution



So in this case, \SigmaF = Fk, correct? And Fk = \mukmg, so ma = \mukmg, and cancelling out the m gives me a = \mukg. so a = (0.6)(9.8) = 5.88m/s2.
i sub "a" into the kinematic equation: Vf2 = Vi2 + 2ad ---> 0 = Vi2 + 2(5.88)(290) ---> -Vi2 = 3410.4 ---> -Vi = 58.3986m/s

If it's negative Vi, then does that mean that it was going backwards or something, or am i missing something here? Thanks in advance for taking the time to help me out! :biggrin:
 
Physics news on Phys.org
No, you did everything right except that your car is decelerating so your a value should really be -5.88 and that would give you a positive vi value.
 
oh, ok, got it =)
thank you!
 
Kindly see the attached pdf. My attempt to solve it, is in it. I'm wondering if my solution is right. My idea is this: At any point of time, the ball may be assumed to be at an incline which is at an angle of θ(kindly see both the pics in the pdf file). The value of θ will continuously change and so will the value of friction. I'm not able to figure out, why my solution is wrong, if it is wrong .
TL;DR Summary: I came across this question from a Sri Lankan A-level textbook. Question - An ice cube with a length of 10 cm is immersed in water at 0 °C. An observer observes the ice cube from the water, and it seems to be 7.75 cm long. If the refractive index of water is 4/3, find the height of the ice cube immersed in the water. I could not understand how the apparent height of the ice cube in the water depends on the height of the ice cube immersed in the water. Does anyone have an...
Thread 'A bead-mass oscillatory system problem'
I can't figure out how to find the velocity of the particle at 37 degrees. Basically the bead moves with velocity towards right let's call it v1. The particle moves with some velocity v2. In frame of the bead, the particle is performing circular motion. So v of particle wrt bead would be perpendicular to the string. But how would I find the velocity of particle in ground frame? I tried using vectors to figure it out and the angle is coming out to be extremely long. One equation is by work...
Back
Top