Solving Friction Forces: Finding Coefficients for Block & Surface

In summary, 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. The coefficient of static friction between the block and the surface is μ(s)n = ma.
  • #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

 
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  • #2
What is the expression for the coefficient of static friction?
 
  • #3
is the expression for static friction just
-Fs = ma
 
  • #4
physics=world said:
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
rl.bhat said:
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
physics=world said:
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
rl.bhat said:
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
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
rl.bhat said:
Normal reaction is opposite to mg. Now you can find the coefficient of static friction.

THANKS! i found it!:smile:
 

1. How do you calculate friction forces?

To calculate friction forces, you need to know the coefficient of friction between the two surfaces in contact and the normal force acting on the object. The formula for friction force is F = μN, where F is the friction force, μ is the coefficient of friction, and N is the normal force.

2. What is the coefficient of friction?

The coefficient of friction is a dimensionless constant that represents the amount of friction between two surfaces in contact. It is usually denoted by the symbol μ and can have values ranging from 0 (no friction) to 1 (maximum friction).

3. How do you find the coefficient of friction?

The coefficient of friction can be found experimentally by measuring the force required to move an object across a surface at a constant velocity. The ratio of this force to the normal force acting on the object will give the coefficient of friction.

4. What factors affect the coefficient of friction?

The coefficient of friction is affected by several factors, including the roughness of the surfaces in contact, the weight of the object, and the type of material the surfaces are made of. It also varies with temperature and can be influenced by the presence of lubricants or other substances on the surfaces.

5. Why is it important to calculate friction forces?

Friction forces play a crucial role in many physical phenomena, such as the movement of objects, the functioning of machines, and the stability of structures. By accurately calculating friction forces, scientists and engineers can design and predict the behavior of various systems and improve their efficiency and safety.

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