Understanding Friction: Forces and Coefficients Explained

In summary, the graphs of force of friction vs force normal and force of kinetic friction vs weight of the object both pass through the origin. The coefficient of kinetic friction does not depend on the weight of the object, but instead is a property of the surfaces experiencing friction. The surface area of the block affects the force of friction by changing the coefficient of friction, which is a function of the contact area between the two surfaces.
  • #1
kangta
1. Does a graph of force of friction vs force normal pass through the origin? Explain.

2. Does the force of kinetic friction depend on the weight of the object? Explain.

3. Does the coefficient of kinetic friction depend on the weight of the object? explain


This is not on an icline plane.
 
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  • #2
1. yes.

F = μR

μ does not = 0

where F = friction, R = normal reaction force and μ = coefficient of friction.

if F = 0, then R must also equal 0 to satisfy the above equation. therefore a graph of F against R would pass through the origin.

2. yes.

for an object moving on a horizontal plane, R = mg (where mg = weight). therefore, for an object moving on a horizontal plane:

F = μmg

3. no.

F is proportional to R and hence F is proportional to mg. μ is just the constant of proportionality. it is a property of the surface(s) experiencing friction. it is dependant upon the shape and material of the surfaces.
 
  • #3
ummm i don;t get what those things mean...

what is R?

Can you explain it in a simpler way?
 
  • #4
The equations given to me are

ì = Ff/Fn

Fnet = mg
 
  • #5
in the equations given to you, what are i, Ff and Fn? you need to state what the symbols are representing.

in the equation i used, F is the frictional force acting on an object moving on a surface, R is the normal reaction force on the object and μ is the coefficient of friction between the object and the surface.

the frictional force F on the object is proportional to the normal reaction force R acting on the object:

F is proportional to R.

to turn this into an equation, a constant of proportionality is required. this is μ, the coefficient of friction:

F = μR

μ cannot = 0. if μ = 0 there would be no friction.

in a graph of F = μR, F would be the vertical axis and R would be the horizontal axis. μ would be the gradient of the graph. if the graph passes through the origin, then F must = 0 when R = 0. put R = 0 into the equation:

F = μ0
F = 0

so F does = 0 when R = 0 therefore the graph passes through the origin.
 
  • #6
oop i meant u = Ff/Fn u = coefficent of friction, Fn = Force Normal, Ff = Force of friction.
 
  • #7
Ok how about this question?

How does the surface area of the block affect the force of friction or the coefficient of friction?
 
  • #8
Well the surface area will affect the force of friction because quite obviously, the more there is, the more it has to fight to get from A to B
 
  • #9
kangta, the equation you are using is exactly the same as mine only the symbols are different.

you have used Ff for frictional force, and i have used F. you used Fn for normal reaction force, i used R. you used u for coefficient of friction, i used μ.

u = Ff/Fn can be rearanged to Ff = uFn, which is in the same form as i used.

the area in of the block in contact with the surface it is moving on affects the coefficient of friction u. u is a function of the area of the block in contact with the surface, and the material of the block.
 
  • #10
Originally posted by kangta
Ok how about this question?

How does the surface area of the block affect the force of friction or the coefficient of friction?
Coeficient of friction includes surface area and surface area relates pressure and force. So if you increase surface area, the pressure decreases (for a given force) and the 'amount of surface to grab' increases by the same amount. So the friction force stays the same.

I'm not sure if that made sense, so let me try a mathematical explanation: if you break out a surface area term from the coefficient of friction, you also have to divide force by surface area (getting pressure). The two surface area terms cancel out.
 
  • #11
this has nothing to do with pressure.

the coefficient of friction μ is a function of the contact area between the two surfaces amongst other things.

F = μR

μ = f(A) where f(A) is a function of the contact area. thus by changing A you change μ. by changing μ you change the frictional force for a given normal reaction force.

you can test this by experiment. two objects of identical mass but with different contact areas moving on a horizontal plane will experience different frictional forces.

this is why, for example, wide tyres give more grip than narrow tyres.
 
Last edited:

1. What is friction?

Friction is a force that resists the relative motion between two surfaces in contact. It is caused by the irregularities in the surfaces and the intermolecular forces between them.

2. What factors affect the amount of friction?

The amount of friction depends on the type of surfaces, the force pressing the surfaces together, and the roughness of the surfaces. It also depends on the angle at which the surfaces are in contact and the speed at which they are moving.

3. What is the coefficient of friction?

The coefficient of friction is a dimensionless quantity that represents the amount of friction between two surfaces. It is a ratio of the force required to move an object over a surface to the force pressing the surfaces together.

4. How is the coefficient of friction determined?

The coefficient of friction is determined by conducting experiments where an object is pulled or pushed across a surface at different speeds and angles. The ratio of the applied force to the normal force is calculated to determine the coefficient of friction.

5. How does friction affect everyday life?

Friction plays a crucial role in our daily lives. It allows us to walk, drive, and hold objects without slipping. However, it also causes wear and tear on surfaces and objects, which can be both beneficial and detrimental. Friction also helps us to control the speed and motion of objects, making it essential in many industries such as transportation and manufacturing.

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