FRICTION FORCES AND APPLIED FORCE

The Applied Force: Fapp

Any force could be considered to be an applied force, but we usally let Fapp stand for a force applied by a person or an action which directly pushs or pulls on a system.


When we humans push or pull on an object we exert a force whose magnitude and direction are determined by us. This means that the magnitude and direction of such forces can never be calculated independently, like the weight of an object can be found from W = mg. Applied forces either have to be given or found using Newton's Laws. For example, if you pull down on a spring-scale and it reads 7.80 N, then you know by Newton's Third Law that the force you applied must be equal to that of the stretched spring.
The applied force is just one of the many forces that could be acting on an object. It is not the big cheese, meaning Fapp is not equal to Fnet. If it is the only force acting on an object, then the applied force will be equal to the net force by default. It is extremely tempting to want to set Fapp equal to ma, but here again Fapp is not equal to ma unless it is the only force. This tendency may arise from the observation that when you personally change the magnitude of the force you are applying, the system's behavior will change. However, this is also true for any other force acting on the system--change it and the subsequent motion will change. An object's acceleration is a consequence of ALL the forces acting on a system, not just the applied force.

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Force of Friction
* When the surface of one object slides across the surface of another object and some other force is pressing the surfaces together, a drag force is created between the two surfaces that is parallel to the surfaces.
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For Objects at Rest: Static Friction

f ≤ μs n fmax = μs n
f = Force of Friction. SI: N

μs = Coefficient of static friction. SI: Dimensionless: Table
n = The normal force pressing the surfaces together. SI: N



The coefficient of static friction depends upon the roughness of the surfaces.
The coefficient of static friction does not depend upon contact surface area, provided the two surfaces are made of dissimilar enough material so that any cohesive force between the two surfaces is small.
The force of static friction (like the normal force) is a reaction force in that its value depends upon the magnitude of some external force trying to push the object along the surface.
The force of friction is always oppsite to direction of the other forces imposed.
The force of static friction increases up to a maximum value, after which the object "breaks loose" and begins to start moving.

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For Objects in Motion: Kinetic Friction

f = μkn

f = Force of Friction. SI: N

μk = Coefficient of kinetic friction. SI: Dimensionless

n = The normal force pressing the surfaces together. SI: N



The coefficient of kinetic friction (like the static coefficient) depends upon the roughness of the surfaces. However it is generally smaller in value than the static coefficient.
μk< μs

For relative velocities in the range of cm/s to m/s (typical ranges we will encounter) the coefficient of kinetic friction is approximately constant.




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Force of Friction on a Block
An horizontal force is applied to a block resting on a rough surface. Displayed are the various forces acting on the block as it move along the surface. The mass of the block, the applied force, and the coefficient of friction can be altered. Force of Friction on a Block QT Movie



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Block on an Incline Plane
A block is pushed up an incline plane by an force parallel to the incline plane. The coefficient of friction between the block and the incline plane, the applied force, the angle of the incline plane, and the mass of the block can be varied. Displayed are the resulting forces acting on the block as it moves up or down the incline plane. Block on Incline Plane QT Movie