How to Find the Force of Friction Without Knowing the Coefficient of Friction

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To determine how much force friction exerts on an object on a given surface, you normally multiply the force or momentum of the object by the surface's coefficient of friction. If you don't know the coefficient of friction for two items on a given surface, this method is useless. You can determine the total force that dynamic, or motion, friction exerts by using Newton's second and third laws.

Things You'll Need

  • Force gauge
  • Spring scales
  • Stopwatch
  • Measuring stick
  • Determine the mass of the object to be moved by placing it on the spring scales. For example, assume you have a 5-gram weight.

  • Attach the force gauge to the object and pull it along the surface, noting the measurement on the gauge. Assume a reading of 5 newtons.

  • Time it over a given distance to determine velocity; velocity equals distance divided by time traveled. Assume that you have a velocity of 0.5 meters per second.

  • Multiply the mass of the object by its velocity: 0.5 meters per second * 5 grams = 2.5. This gives you the momentum of the object: 2.5 newtons, per Newton's law of acceleration.

  • Subtract the momentum of the object from the original applied force. Per Newton's third law, the law of reaction, you can assume that total force in the system is equal. Because the object has a forward momentum of 2.5 newtons and you applied 10 newtons of force, the other 7.5 newtons is the frictional force.

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