Question on friction from EK Physics

[shaggybill]

In 1001 Physics, question 226 is:

A car moving at 20m/s brakes and slides to a stop. If the coeff. of kinetic friction is 0.1, how far does the car slide?

Is this possible to answer without being given mass?

Here is their answer:

C is correct. The net force on the car is the kinetic friction force umgcos(theta). From F=ma we can find acceleration and plug this into vf^2 = v0^2 + 2ax.

Where are they getting mass? It's not given in the question.

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[inaccensa]

In 1001 Physics, question 226 is:

A car moving at 20m/s brakes and slides to a stop. If the coeff. of kinetic friction is 0.1, how far does the car slide?

Is this possible to answer without being given mass?

Here is their answer:



Where are they getting mass? It's not given in the question.

Here is my take

Kinetic friction = coeff * F normal
F normal = mg ----1
Force = ma----2

Force of Kinetic friction = coeff *mg = ma
a = coeff*g = 0.1*10m/s =1m/s^2

using the equation, I get 200m.
is that the correct ans?

 

[Naijaba]

C is correct. The net force on the car is the kinetic friction force umgcos(theta). From F=ma we can find acceleration and plug this into vf^2 = v0^2 + 2ax.

Their explanation is clear. You DON'T need the mass because it cancels out.

Net Force = umgcos(theta) = ma

Cancel the m's.

That means a = ugcos(theta)

 

[shaggybill]

Ya'll are right. There is a similar question in the Lecture book that gives the answer as v^2/2gu.

Thanks for your help.