# Feels like something is missing from this problem (a car skidding to a stop)

Arwing
Homework Statement:
An automobile's wheels are locked as it slides to a stop from 34.9 m/s. If the coefficient of kinetic friction is 0.266 and the road is horizontal, how long does it take the car to stop?
Relevant Equations:
F=ma, Fg=mg, Fk=coefficient of friction * Fn
I'm not sure where to start, I feel like I'm missing the mass but it is not listed.

## Answers and Replies

Science Advisor
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Maybe you don't need the mass. What do the kinematic equations that don't depend on mass have to say?

topsquark
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Homework Statement:: An automobile's wheels are locked as it slides to a stop from 34.9 m/s. If the coefficient of kinetic friction is 0.266 and the road is horizontal, how long does it take the car to stop?
Relevant Equations:: F=ma, Fg=mg, Fk=coefficient of friction * Fn

I'm not sure where to start, I feel like I'm missing the mass but it is not listed.
You don't need the mass. Go ahead and solve the equations to find the acceleration.

topsquark
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Homework Statement:: An automobile's wheels are locked as it slides to a stop from 34.9 m/s. If the coefficient of kinetic friction is 0.266 and the road is horizontal, how long does it take the car to stop?
Relevant Equations:: F=ma, Fg=mg, Fk=coefficient of friction * Fn

I'm not sure where to start, I feel like I'm missing the mass but it is not listed.
You could assume a) it's a toy car with a mass of ##1 kg##; and, b) a real car with a mass of ##1000 kg##; and see what difference that makes.

Which car will stop quicker? The toy or the real one?

topsquark
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This thread highlights a common bad habit among many students: trying to compute every single partial result numerically. Most of the time it will be more illuminating to make general symbolic considerations for general input parameters and only insert particular values given in the problem once a general expression for the final result has been reached.

If you think mass is relevant, then introduce a mass parameter m and see if it appears in the final result.

topsquark
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Addendum: It can be argued on dimensional grounds that the mass is necessarily irrelevant in this problem.

topsquark