Friction Forces help

  • #1
physics=world
110
0
1. A 25.0-kg block is initially at rest on a horizontal surface.
A horizontal force of 75.0 N is required to set the block in
motion, after which a horizontal force of 60.0 N is required
to keep the block moving with constant speed. Find (a) the
coefficient of static friction and (b) the coefficient of
kinetic friction between the block and the surface.



2. Homework Equations



3. i need help setting up the problem. i would apply the net force model for the x direction and equilibrium model for the y model. i need to know if this is right.
Fx = 75.0N - F(static) - 60.0N = ma
Fy = n - mg = 0

 

Answers and Replies

  • #2
rl.bhat
Homework Helper
4,433
9
What is the expression for the coefficient of static friction?
 
  • #3
physics=world
110
0
is the expression for static friction just
-Fs = ma
 
  • #4
rl.bhat
Homework Helper
4,433
9
is the expression for static friction just
-Fs = ma
Not the above expression.
The expression for μ in terms of normal force and maximum frictional force.
 
  • #5
physics=world
110
0
Not the above expression.
The expression for μ in terms of normal force and maximum frictional force.



oh okay. so i got

μ(s)n = ma

than i got n = mg for the second equation.

do i plug in "n" to the expression μ(s)n = ma ?

which would give me μ(s)mg = ma
 
  • #6
rl.bhat
Homework Helper
4,433
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oh okay. so i got

μ(s)n = ma

than i got n = mg for the second equation.

do i plug in "n" to the expression μ(s)n = ma ?

which would give me μ(s)mg = ma
Correct. Now in the given problem, what is the normal force and ma?
 
  • #7
physics=world
110
0
Correct. Now in the given problem, what is the normal force and ma?

hmm would mg = -245?
i don't know what the normal force is. i thought that mg and n always canceled to make it equal to zero.

opps. i meant would ma = 75
 
  • #8
rl.bhat
Homework Helper
4,433
9
Normal reaction is equal and opposite to mg. mg acts on the floor and normal reaction acts on the block. Now you can find the coefficient of static friction.
 
  • #9
physics=world
110
0
Normal reaction is opposite to mg. Now you can find the coefficient of static friction.

THANKS! i found it!:smile:
 

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