Frictional Forces

Confused too

1. The problem statement, all variables and given/known data

A baseball player slides into third base with an initial speed of 7.9 m/s. If the coefficient of kinetic friction the player and the ground id .41, how far does the player slide before coming to rest

2. Relevant equations

vf^2=vi^2 + 2ad
kf=μm

3. The attempt at a solution
I tried subsituting but I need acceleration and distance. I didn't know where to go from there

Ace.

Are you given mass of the baseball player?

Confused too

sorry, yeh you're right with the formula

but why is the normal force equal to gravity and what is F_UN

Ace.

Sorry, do you have mass?

Confused too

No, no mass is given

Doc Al

Use Newton's 2nd law to find the acceleration. The only force acting (at least horizontally) is friction.

Just call the mass 'm'. You won't need an actual value.

Confused too

I don't understand. F=ma so a=F/m but we don't have an F. I don't really understand frictional forces at all

tiny-tim

Hi Confused too! Welcome to PF!
You have the coefficient of friction (µ_k = 0.41) …

so if the mass is m, what is the friction force?

(btw, you can then use the work energy theorem)

Confused too

thanks

But we're not given m and what's the work energy theorem?

haruspex

So create an unknown for it (m?) and with luck it will cancel out of the equations later.
In this case, that the kinetic energy lost by the player is equal to the work done sliding against friction. But you might as well stick with the equations you quoted. (The second one is more accurately written k_f=μmg.)