How far will a pullback car go? Spring and friction coefficients.

1. Apr 1, 2013

Totally

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

A pullback toy car is pulled back. How far will it go before it stops?

x(0)=0
v(0)=0
a(x<x0)=-k(x0-x)
a(x>x0)=μg
v(tf)=0
tf=D
2. Relevant equations

Fs=-kx
F=mg
Ff=μF

3. The attempt at a solution

d2x/dt2=1/m(k(x0-x)-μmg)

Later in the run spring constant is irrelevant, so
d2/dt2=1/m(-μmg)

I think I can figure out the μ by pulling the car back on the ramp and lowering the angle between the ramp and the surface until car starts rolling. Then the tangent of that angle tanθ=μ

Would this be correct? Also, what experiment or method would you propose to figure out the coefficient of spring? Disassembling the toy is not an option.

I apologize I didn't put formulas the proper way, however I couldn't get Latex to work even though I followed FAQ instructions.

Last edited: Apr 1, 2013
2. Apr 1, 2013

Sunil Simha

This is assuming you are letting the car go down the ramp and are slowly increasing the angle of inclination.

I believe that the use of a ramp is correct in finding the coefficient of friction but it should be done without puling back the car. If it were pulled back, the spring force may easily overcome the frictional force at a smaller angle and the reading would be incorrect.

3. Apr 1, 2013

mukundpa

Is the car sliding on the horizontal surface? If it has rolling wheels then the coefficient of static or kinetic friction becomes irrelevant and rolling friction and air resistance will be the retarding forces. If it is sliding and air resistance is negligible, we can consider kinetic friction and work energy rule will give the results easily.

4. Apr 1, 2013

Totally

I guess I wasn't clear enough, I was thinking about pulling the car back at the lower end of the plane and lowering the angle until it can climb the ramp.

Interesting approach, thanks for the idea! And yes, the toy is being pulled back on the horizontal surface. Do you have any ideas how to figure out the spring coefficient?