# Frictional Forces

1. Nov 12, 2012

### 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

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

2. Nov 12, 2012

### Ace.

Are you given mass of the baseball player?

3. Nov 12, 2012

### Confused too

sorry, yeh you're right with the formula

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

4. Nov 12, 2012

### Ace.

Sorry, do you have mass?

5. Nov 12, 2012

### Confused too

No, no mass is given

6. Nov 12, 2012

### Staff: Mentor

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.

7. Nov 12, 2012

### 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

8. Nov 12, 2012

### tiny-tim

Welcome to PF!

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

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

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

9. Nov 12, 2012

### Confused too

Re: Welcome to PF!

thanks

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

10. Nov 12, 2012

### haruspex

Re: Welcome to PF!

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 kf=μmg.)