EK 1001 types of work 365

[GRod18]

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A car moving at 35m/s on dry pavement, skids to a stop in 7 seconds. What is the coefficient of friction between the car's tires and the pavement?

EK says to use equation X = 1/2 at^2 and subsitute in f/m for a where f = umg (u is coefficient). Ek also said you could use the impulse momentum theorem. Can someone show me how they would solve it with the first equation, and explain to me why I can use the impulse momentum theorem in this situation?

Thanks in advance!

 

[Rabolisk]

Impulse is change in momentum, or dp. Since p = mv, impulse, J = mdv. Force is mass times acceleration. F = ma = mdv/dt. That gives J = Fdt. We are given initial velocity, and final velocity, so we know the impulse. Divide that by 7 seconds, and you get the force. Mass cancels out, so you can get a numerical value for the answer.

As far as the first equation goes, I would use V = Vinitial + at. Then acceleration is equal ug. You'll see that the math you do in both cases is the same.

 

[GRod18]

what I would give for a physical science brain like yours!

thanks again!