Kinetic Friction along Ramps - dummies

By Steven Holzner

Two kinds of friction — static and kinetic — mean that you have to handle ramp problems where kinetic friction is involved as well as problems where static friction is involved. Kinetic friction is involved any time an object is moving up or down a ramp.. You can solve problems with kinetic friction as easily as those that involve static friction.

Here’s the equation for the force needed to get an object moving and thus overcome static friction:

image0.png

To convert this equation to using kinetic friction, all you have to do is to change from using the coefficient of static friction to using the coefficient of kinetic friction:

image1.png

That’s all there is to it.

Sample question

  1. Suppose that you’re pushing a 40.0-kg plastic crate up a 19.0-degree ramp.

    The coefficient of kinetic friction is 0.100, and you need to apply a force to keep the crate moving. What is the force you will need to apply?

    The correct answer is 165 N.

    1. The equation here is

      image2.png

    2. Putting in the numbers gives you F =128 + 37 = 165 N.

Practice questions

  1. You’re dragging your little sister up the 25-degree wheelchair ramp at the doctor’s office.

    If she has a mass of 30.0 kg and the coefficient of kinetic friction is 0.10, how much force will you need to apply to keep her going?

  2. You’re pushing a box of books with a mass of 25 kg up a 40-degree ramp.

    If the coefficient of kinetic friction is 0.27, how much force will you need to apply to keep the box moving up the ramp?

  3. You’re pushing a chest of drawers up an 18-degree ramp.

    If it has a mass of 50.0 kg and the coefficient of kinetic friction is 0.20, how much force will you need to keep it moving?

  4. You want to keep a 120-kg refrigerator moving up a 23-degree ramp.

    If the coefficient of kinetic friction is 0.20, how much force will you need to keep it moving?

Following are answers to the practice questions:

  1. 150 N

    1. Determine the forces you need to overcome: The force due to gravity is

      image3.png

      and the force due to kinetic friction is

      image4.png

      You need to find the normal force.

    2. The equation for normal force is

      image5.png

      Use the normal force to calculate the force due to kinetic friction:

      image6.png

    3. The force you have to overcome is

      image7.png

    4. Plug in the numbers:

      image8.png

  2. 210 N

    1. Calculate the forces you need to overcome: The force due to gravity is

      image9.png

      and the force due to kinetic friction is

      image10.png

      You need to find the normal force.

    2. The equation for normal force is

      image11.png

      Use the normal force to calculate the force due to kinetic friction:

      image12.png

    3. The force you have to overcome to keep the box moving is

      image13.png

    4. Plug in the numbers:

      image14.png

  3. 240 N

    1. Calculate the forces you need to overcome: The force due to gravity is

      image15.png

      and the force due to kinetic friction is

      image16.png

      You need to find the normal force.

    2. The equation for normal force is

      image17.png

      Use the normal force to calculate the force due to kinetic friction:

      image18.png

    3. The force you have to overcome to keep the chest of drawers moving is

      image19.png

    4. Plug in the numbers:

      image20.png

  4. 680 N

    1. Calculate the forces you need to overcome: The force due to gravity is

      image21.png

      and the force due to kinetic friction is

      image22.png

      You need to find the normal force.

    2. The equation for normal force is

      image23.png

      Use the normal force to calculate the force due to kinetic friction:

      image24.png

    3. The force you have to overcome to keep the refrigerator moving is

      image25.png

    4. Plug in the numbers:

      image26.png