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External Force and Sliding Kinetic Friction
by Ron Kurtus
When an external force is applied to a sliding object in the direction of motion, the object will accelerate, move at a constant velocity, or slow down according to the sliding kinetic friction and amount of applied force.
Questions you may have include:
- What is Newton's Law for acceleration?
- What is the effect of friction on the law?
- What are the results from different external forces?
This lesson will answer those questions. Useful tool: Units Conversion
Newton's Law
According to Newton's Laws, a force pushing on a freely moving object causes an acceleration according to the equation:
Fe = ma
where
- Fe is the external force on a freely moving object
- m is the mass of the object
- a is the object's acceleration
Effect of friction
Now, if the motion of the object is sliding along a surface or material, it is held back by the resistive force of friction. The equation then becomes:
Fe − Fks = mA
or
A = (Fe − Fks)/m
where
- Fks is the kinetic sliding force of friction
- A is the new acceleration of the object relative to the external force
Result of different external force
The relationship between the external force and the force of friction on the object determines whether it will accelerate, move at a constant velocity, or slow down.
Accelerating
If the external force is greater than the kinetic sliding friction force (Fe > Fks), then A is positive and the object is accelerating.
Since static friction is greater than kinetic sliding friction, the object will accelerate when going from static to kinetic.
Constant velocity
If the external force equals the kinetic sliding friction force (Fe = Fks), then A = 0 and the object is moving at a constant velocity.
This can happen by reducing the external force from the accelerating situation.
Slowing down
If the external force is less than the kinetic sliding friction force (Fks < Ff), or if the external force equals zero (Fe = 0), then the acceleration A is negative and the object is slowing down until it finally stops moving.
A good example is when you stop sliding an object. It will start to slow down and soon stop.
Summary
When an external force is applied to a sliding object in the direction of motion, the object will accelerate, move at a constant velocity, or slow down according to the kinetic sliding friction and amount of applied force.
Always do your best
Resources and references
Websites
Friction Resources - Extensive list
Books
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Top-rated books on Friction Science
Top-rated books on Friction Experiments
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External Force and Sliding Kinetic Friction