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Static Sliding Friction on an Incline
by Ron Kurtus
When an object is placed on a flat incline or ramp, it will not slide down the surface if the force caused by gravity acting on the object is less than the resistance from friction. Since the object does not slide, the resistance is called static sliding friction.
To determine this resistance, you need to know the friction equation for an incline, the gravity forces pushing on the object, and the static coefficient of sliding friction.
The static coefficient of sliding friction can easily be determined by the incline angle when the gravity force equals the static sliding friction resistance.
Questions you may have include:
- What is the friction equation on an incline?
- What is the gravity force for object on incline?
- How can static coefficient of sliding friction be determined?
This lesson will answer those questions. Useful tool: Units Conversion
Static sliding friction equation on incline
The static sliding friction equation is:
Fss = μssN
where
- Fss is the static force of sliding friction
- μss is the static coefficient of sliding friction for the two surfaces (Greek letter "mu")
- N is the normal force perpendicular to the surface
Normal force on an incline
When the object is on an incline, the normal force is:
N = W*cos(β)
where
- W is the weight of the object
- β is the angle of inclination (Greek letter "beta")
- cos(β) is the cosine of angle β
Equation for static sliding friction
Thus, the equation for static sliding friction is:
Fss = μssW*cos(β)
Static forces on object on inclined surface
Gravity force for object on incline
The force from gravity acting down the incline is the weight times the sine of the angle of inclination:
Fg = W*sin(β)
where
- Fg is the force from gravity pulling the object down the incline
- sin(β) is the sine of angle β
Will not slide
When the resistance from static sliding friction is greater than the sliding force from gravity, the object will not slide:
Fss > Fg
Coefficient of friction
The static coefficient of sliding friction is:
μss = Fss/N
For a given set of materials, the static coefficient of sliding friction is typically greater than the kinetic coefficient of friction.
There are a number of factors that can affect the coefficient of friction, including surface conditions. Values of the coefficient of sliding friction can be a good reference for specific combinations of materials.
(See Coefficient of Sliding Friction and
Friction Experiment: Measure Coefficient of Friction with a Ramp)
Summary
An object is placed on a flat incline or ramp will not slide down the surface if the force caused by gravity is less than the resistance from static sliding friction.
The static coefficient of sliding friction can easily be determined by the incline angle when the gravity force equals the static sliding friction resistance.
Be clever
Resources and references
Websites
Friction Concepts - HyperPhysics
Friction Resources - Extensive list
Books
(Notice: The School for Champions may earn commissions from book purchases)
Top-rated books on Friction Science
Top-rated books on Friction Experiments
Questions and comments
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Static Sliding Friction on an Incline