How to use the coefficient of kinetic friction?

[Hannah]

The problem could be made much simpler by taking the final case as just on top of the elevated portion. What is the energy equation now? (You now need to take gravity into consideration)

EDIT: Actually no, what you did is correct. Do you get the value of k now?

0+1/2k(0.02)^2+mg(0)=1/2(0.1)(3.286)^2+1/2(m)(0)^2+(0.1)(-9.8)(0.5)
0.0002k=0.53989-0.49
k=249.45
seems pretty large

 

[Hannah]

0+1/2k(0.02)^2+mg(0)=1/2(0.1)(3.286)^2+1/2(m)(0)^2+(0.1)(-9.8)(0.5)
0.0002k=0.53989-0.49
k=249.45
seems pretty large

if i do it my original way i get k =2699.45 which is even bigger

 

[Mastermind01]

Did you do your calculation correctly and follow your equation in #28 ? Then your answer should be correct

 

[Hannah]

Did you do your calculation correctly and follow your equation in #28 ? Then your answer should be correct

okay thank you very much for all your help ! :)

 

[haruspex]

0+1/2k(0.02)^2+mg(0)=1/2(0.1)(3.286)^2+1/2(m)(0)^2+(0.1)(-9.8)(0.5)

Why are you subtracting mgh at the end there?
I believe what MM was suggesting in post #30 was to write
Initial spring PE = final KE + gained gravitational PE

if i do it my original way i get k =2699.45 which is even bigger

But it does have the advantage of being correct.

 

[Hannah]

so just one thing since i used conservation of energy my system would have to isolated so would i have to include the Earth as my system or just track+spring+car since its isolated until it reaches the top part of the track?

 

[haruspex]

so just one thing since i used conservation of energy my system would have to isolated so would i have to include the Earth as my system or just track+spring+car since its isolated until it reaches the top part of the track?

You need to include the Earth, and you do that by considering the change in gravitational PE.