Solving a Frictional Problem with No Given Masses

[python023]

In this section, I am trying to determine frictional forces between surfaces and masses. However, in this type of problem, no masses are given. Here is a type of question:

A crate is carried in a pickup truck traveling horizontally at 15.0 m/s. The truck applies the brakes for a distance of 28.7 m while stopping with uniform acceleration. What is the coefficient of static friction between the crate and the truck bed if the crate does not slide?

I have determined the truck brakes at -3.92 m/s^2. The formula I have for coefficient of static friction is Frictional Force = Coeff. of Friction * Normal Force. However, no forces or masses are given. How can I solve this type of problem?

Thank you

 

[civil_dude]

You need to draw your force diagram. You will find that the masses cancel and that the acceleration from friction will be some percentage of acceleration of gravity, and that will be your coefficient.

 

[python023]

I tried to set the mass of the crate to 100kg. After drawing a force diagram, I'm not sure I am doing it right. Force Normal and the Force of Gravity should equal one another. If Net Force = Mass * Acceleration, and the mass is 100kg*-3.92 m/s, you get a negative net force of -392 N. 392 N/981N(Normal Force) = .400N. Is this the coefficient of friction?

 

[lightgrav]

Almost ... the Units cancel (392 N / 981 N) = 0.40 , unitless.
The co-efficient of friction is an efficiency , so it is dimensionless.

By the way, Normal Force = gravity Force here, but it often isn't ... watch out!