Calculating Distance Slid on Ground with Friction | Baseball Player Example

[Confused too]

Homework Statement

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

Homework Equations

vf^2=vi^2 + 2ad
kf=μm

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]

By kf=μm do you mean F_k = μ_k x F_N?

Note that F_N is the normal force which in this case is equal to force of gravity.

This equation may also be of use to you : a = F_UN / m

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]

I tried subsituting but I need acceleration and distance.

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]

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.

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]

Welcome to PF!

Hi Confused too! Welcome to PF!

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

I don't really understand frictional forces at all

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]

Hi Confused too! Welcome to PF!

thanks

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)

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

 

[haruspex]

But we're not given m

So create an unknown for it (m?) and with luck it will cancel out of the equations later.

and what's the work energy theorem?

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.)