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How to Reduce Friction

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Author: Erin Bjornsson

Newton’s First Law of Motion states that when an object is set in motion, it will remain in motion until acted on by an outside force. Theoretically, this could mean you could travel at the same speed forever in one direction, right? The reason we don’t see this happen is because of friction. The force of friction is the resistance to motion that is in the opposite direction of the traveling object. This is why if you roll a ball on the ground it eventually stops. Friction is also the reason you can hold a book, and the reason why you don’t slip when walking! The ground and other solid surfaces can cause friction, called dry friction, but fluids (liquids and gases) like water and air can also cause frictional forces, called fluid friction.

The equation for the force of friction is the following:

Ff = µ * FN

Where Ff is the force of friction, FN is the force normal to the object that is moving, and µ is the coefficient of friction.

The normal force of an object is the force that is perpendicular to the surface of the side of the object that is in contact with the surface that is causing the frictional force. For example, if you have a block moving horizontally on flat ground, the normal force points at a 90° angle out of the ground.

Normal Force Diagram

The unit for force is the Newton (named after Sir Isaac Newton), which is equal to 1 (kg)(m) / s2. The coefficient of friction, symbolized by the Greek letter µ (“mu”), is dimensionless, which means it has no units. Dimensionless numbers help scientists, mathematicians, and engineers compare the same characteristic in different objects. In essence, the coefficient of friction describes how easily one object can move over another. Your swimsuit against a water slide has a low coefficient of friction, and so do ice skate blades on ice. We think of these objects as slippery. The rubber of car wheels on the street has a higher coefficient of friction, which is why when there is no force applied the car eventually stops moving. And even higher coefficient of friction can be seen by a big boulder in nature, you can push and push but the coefficient of friction is so large and the normal force of the rock is so big that you might not be able to move the rock at all.

Problem:

Observe friction in action and determine which surfaces and conditions create the least frictional forces.

Materials

  • Lightweight cardboard box (like a shoebox)
  • Balloon
  • Scissors or a box cutter (ask an adult to help you!)
  • Box of plastic drinking straws
  • Tape measure or ruler
  • Glue or clear scotch tape
  • Notebook
  • Pencil or pen

Procedure

  1. Have an adult help you cut a small hole in one end of the cardboard box.
  2. Insert the balloon through the hole so that the head of the balloon is inside the box and the opening is outside.
  3. Blow up the balloon and twist the end closed, pinching it between your fingertips. Do not tie it off.
  4. While still pinching the balloon closed, set the balloon car on a flat countertop, table, or floor and mark the starting point.

Balloon Car

  1. Let go of the balloon and measure the distance the box traveled.
  2. Repeat inflating the balloon to the same size, letting it go, and measuring the distance travelled on difference surfaces. Good surfaces to test are a carpet or a rug, concrete (like the sidewalk), and on dirt or grass.
  3. Go back to the first flat surface you tested, and lay out a row of parallel straws about 3 feet long.
  4. Inflate the balloon, mark the starting point, and set the balloon car on top of the straw runway.
  5. Release the balloon. Measure and record the distance travelled.

Balloon Car Over Straws

  1. Repeat Step 8 on the different surfaces you tried in Step 6.
  2. Glue or tape two straws along the length of the bottom of the box like a sleigh. Will this create more or less friction? Why?

Shoebox Sleigh

  1. Inflate the balloon, mark the starting point, and release the balloon car across the surfaces you tested before.
  2. Inflate the balloon, mark the starting point, and release the balloon car across the straw runway.
  3. Analyze your results! Which setup had the most friction? Which setup had the least friction?
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