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Frictional Forces

by Confused too
Tags: forces, frictional
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Confused too
#1
Nov12-12, 04:24 PM
P: 6
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
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Ace.
#2
Nov12-12, 04:41 PM
P: 52
Are you given mass of the baseball player?
Confused too
#3
Nov12-12, 04:43 PM
P: 6
Quote Quote by Ace. View Post
By kf=μm do you mean Fk = μk x FN?

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

This equation may also be of use to you : a = FUN / m
sorry, yeh you're right with the formula

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

Ace.
#4
Nov12-12, 04:45 PM
P: 52
Frictional Forces

Sorry, do you have mass?
Confused too
#5
Nov12-12, 04:50 PM
P: 6
No, no mass is given
Doc Al
#6
Nov12-12, 04:52 PM
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Quote Quote by Confused too View Post
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
#7
Nov12-12, 05:01 PM
P: 6
Quote Quote by Doc Al View Post
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
#8
Nov12-12, 05:37 PM
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Hi Confused too! Welcome to PF!
Quote Quote by Confused too View Post
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
Quote Quote by Confused too View Post
I don't really understand frictional forces at all
You have the coefficient of friction (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
#9
Nov12-12, 05:56 PM
P: 6
Quote Quote by tiny-tim View Post
Hi Confused too! Welcome to PF!
thanks

You have the coefficient of friction (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
#10
Nov12-12, 10:06 PM
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Quote Quote by Confused too View Post
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 kf=μmg.)


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