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How Kinetic Friction Opposes Movement

In physics, when two objects are in contact with each other and are also moving, you get what’s called kinetic friction. The force due to kinetic friction has its own coefficient, called the coefficient of kinetic friction,


Say that a gold ingot with a mass of 1,000 kilograms has a coefficient of kinetic friction of 0.18.

The forces acting on a bar of gold.
The forces acting on a bar of gold.

If a thief decides to steal it, how much force does he need to pull the ingot along at a constant speed during his robbery? You have all you need — the magnitude of the kinetic friction is related to the magnitude of the normal force by:


Putting in the numbers gives you


The thief needs approximately 1,800 newtons of force to keep the gold ingot sliding while evading the police. That converts to about 400 pounds of force (4.448 newtons to a pound) — not exactly the kind of force you can keep going while trying to run at top speed, unless you have some friends helping you.

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