Static/kinematic friction trouble

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In summary: But, there is a very simplified way to find the coefficient of kinetic friction between two objects. First, you need to find the mass of each object. Then, you need to find the acceleration of each object. From there, you can use the following equation:μk=maIn summary, the man was trying to move a couch with a mass of 200 kg, across a rug covered floor. He must use an exert force of 700 N to get it to barely move. His daughter has estimated that once it moves, then it accelerates at 1.10 m/s^2. To find the coefficient of kinetic friction between the couch and the rug, the man would need to use Newton's second law.
  • #1
afg_91320
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A man is trying to move a couch with a mass of 200 kg, across a rug covered floor.
He must use an exert force 0f 700 N to get it to barely move. His daughter has estimated that once it moves, then it accelerates at 1.10 m/s^2

determine:
a) coefficient of static friction

b) coefficient of kinetic friction between couch and the rug.

I don't even know what units i should use and what formula is generally given. the section for friction gives like 20 diff units to use but doesn't tell which to use.

please help me understand-i don't want to know the answer-i want to know how to do it!

----------------------------------------------------------------------------------------

also another one-how do you find an object's acceleration if you are given only one angle and force (ie-object of 10kg is sliding upward on a vertical wall with force of 60 and angle at 60. what is the normal force and the object's acceleration)

^^ for the normal force exerted i used:
F cos 60 = 60 cos 60 = 30 N <--does this apply to the latter question??

thanks alot!

:D
 
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  • #2
Have you done free-body diagrams in your class?

If not, think of the couch as a point mass and label all of the forces on it.
...^
...| Fn
...|
fs <----o----> Fa
...|
...| W
...v
(sorry, just did this quickly)
These help sort out which forces you're looking at.

Secondly, the coefficients of friction are unitless-- it's only a ration between two forces:
[tex]\mu=\frac{f}{F_N}[/tex]

To find the coeficient of static friction is pretty easy to find, because the couch is not accelerating in either the x nor the y direction, so:
[tex]f_s=F_{applied}[/tex]
and
[tex]F_N=W[/tex], so you can substitute these into the above equation.

The only difference with the kinetic friction is you have to use Newton's second law to find the force of kinetic friction.
 
  • #3
im looking at the graph and it still doesn't make sense.

f = Fapplied <---this would be the 700 N correct?
FN = W <--- the 200 kg??

so coefficiant would be 700/200 = 3.5 <---no units?
 
  • #4
afg_91320 said:
im looking at the graph and it still doesn't make sense.

f = Fapplied <---this would be the 700 N correct?
FN = W <--- the 200 kg??

so coefficiant would be 700/200 = 3.5 <---no units?

Doesn't weight involve gravity as in m*g?

Yes the coefficient of friction is dimensionless. It is a ratio of like quantities. Newtons/Newtons.
 
  • #5
^^ok so i fixed my FN, which is now:
FN = W = 200 kg(9.80m/s^2) = 1960

= f / FN = 700/1960 = 0.357 <---correct answer??
---------------------------------------
now how do i use Newton's second law to find the coefficiant of kinetic friction bw the rug and couch??
 
  • #6
afg_91320 said:
now how do i use Newton's second law to find the coefficiant of kinetic friction bw the rug and couch??

F = m*a

You have m, you have a, so what force is resulting in acceleration?

Given that 700N was needed to overcome friction and now you know hopw much force is freed up to accelerate it, how much is still being taken by the kinetic friction? Then figure μk in the same way as before.
 
  • #7
^is there a formula i have to use?
 
  • #8
afg_91320 said:
^is there a formula i have to use?

Draw a picture and identify the force elements.

If you try to remember a formula without understanding it, you will likely be heading for problems.
 

Related to Static/kinematic friction trouble

1. What is static friction and how does it differ from kinematic friction?

Static friction is a type of force that acts between two surfaces in contact with each other, preventing them from sliding past each other. It is a force that must be overcome in order for an object to start moving. Kinematic friction, on the other hand, is the force that acts to slow down or stop an object that is already in motion. It is also known as sliding or dynamic friction.

2. How is static friction calculated?

The formula for calculating static friction is F= μsN, where F is the force of static friction, μs is the coefficient of static friction, and N is the normal force between the two surfaces. The coefficient of static friction is a dimensionless number that represents the roughness or smoothness of the surfaces in contact.

3. What factors affect the magnitude of static friction?

The magnitude of static friction is affected by the coefficient of static friction, the normal force between the two surfaces, and the roughness or smoothness of the surfaces. It also depends on the materials the surfaces are made of, their temperature, and the presence of any lubricants or contaminants.

4. How can static friction be reduced?

Static friction can be reduced by decreasing the coefficient of static friction, increasing the surface area of contact between the two surfaces, and using lubricants to reduce the roughness of the surfaces. Applying a force in the direction of motion can also help to overcome the force of static friction and initiate movement.

5. What are some real-life examples of static friction?

Some examples of static friction in everyday life include walking on the ground, pushing a heavy object, and writing with a pen on paper. In all of these situations, the force of static friction is acting between two surfaces to prevent sliding or slipping. Static friction is also important in the functioning of brakes in vehicles, as it helps to slow down the movement of the wheels and bring the vehicle to a stop.

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