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External Force and Sliding Kinetic Friction

by Ron Kurtus (17 November 2016)

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:

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

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

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.


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Resources and references

Ron Kurtus' Credentials

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Friction Resources - Extensive list

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Top-rated books on Friction Science

Top-rated books on Friction Experiments


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