Stopping a Box of Unknown Mass on Frictionless Floor

[TG3]

Homework Statement

A box of unknown mass slides across a frictionless floor with an initial speed of 4.7 m/s. It encounters a rough region where the coefficient of friction is µk = 0.3
Part 1:
What is the shortest length of rough floor which will stop the box?
Part 2:
What is the shortest length of rough floor which will stop the box?

Homework Equations

vf^2 = vi^2 + 2a delta x.
Frictional force = mew times the normal force

The Attempt at a Solution

Final velocity = 0, so
0 = 4.7 ^2 + 2A delta x.
A = the frictional force.
How can I determine the normal force (and after that, the frictional force, and after that, the change in distance) if I don't know the weight of the box? Is there another way to do this problem?

 

[LowlyPion]

F = ma = μ*m*g

a = μ*g

Does that help?

 

[TG3]

Yes- it certainly does. With that I was able to solve both parts of the problem. I'm writing that formula down for future reference...

 

[LowlyPion]

Yes- it certainly does. With that I was able to solve both parts of the problem. I'm writing that formula down for future reference...

Better to understand it and remember it forever.