Already in Motion: Kinetic Friction - dummies

By Steven Holzner

Kinetic friction is usually less than the force of friction you need to overcome static friction. Kinetic friction occurs when an object is already in motion.

Static friction is the force that must be overcome to get something to move, and it’s usually larger than kinetic friction, the force that pushes against a moving object. The equation for kinetic friction is

image0.png

In order to keep an object moving at a constant velocity, a force must be applied to that object which exactly balances the force from kinetic friction.

Sample question

  1. Suppose you have a 5.0 kg block of ice, and it takes 5 N to keep it moving across the floor at a constant velocity. What is the coefficient of kinetic friction,

    image1.png

    The correct answer is 0.1.

    1. The force of kinetic friction is

      image2.png

    2. Solve for

      image3.png

    3. Plug in the numbers:

      image4.png

Practice questions

  1. You’re cross-country skiing and need 20 N of force to maintain your velocity. If you have a mass of 70 kg, what is the coefficient of kinetic friction:

    image5.png

  2. You’re skating and need 17 N to maintain your velocity. If you have a mass of 80 kg, what is the coefficient of kinetic friction:

    image6.png

Following are answers to the practice questions:

  1. 0.03

    1. Solve for

      image7.png

      The force due to friction is

      image8.png

    2. The force due to friction that you have to overcome, Ff, is 20 N, and the normal force, Fn, is 70g = 690 N.

    3. Use the equation

      image9.png

      to get the coefficient of kinetic friction: 20/690 = 0.03.

  2. 0.02

    1. Solve for

      image10.png

      The force due to friction is

      image11.png

    2. The force due to friction that you have to overcome, Ff, is 17 N, and the normal force, Fn, is 80g = 780 N.

    3. Use the equation

      image12.png

      to get the coefficient of kinetic friction: 17/780 = 0.02.