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.
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.
Observe friction in action and determine which surfaces and conditions create the least frictional forces.
- Lightweight cardboard box (like a shoebox)
- 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
- Pencil or pen
- Have an adult help you cut a small hole in one end of the cardboard box.
- Insert the balloon through the hole so that the head of the balloon is inside the box and the opening is outside.
- Blow up the balloon and twist the end closed, pinching it between your fingertips. Do not tie it off.
- While still pinching the balloon closed, set the balloon car on a flat countertop, table, or floor and mark the starting point.
- Let go of the balloon and measure the distance the box traveled.
- 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.
- Go back to the first flat surface you tested, and lay out a row of parallel straws about 3 feet long.
- Inflate the balloon, mark the starting point, and set the balloon car on top of the straw runway.
- Release the balloon. Measure and record the distance travelled.
- Repeat Step 8 on the different surfaces you tried in Step 6.
- 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?
- Inflate the balloon, mark the starting point, and release the balloon car across the surfaces you tested before.
- Inflate the balloon, mark the starting point, and release the balloon car across the straw runway.
- Analyze your results! Which setup had the most friction? Which setup had the least friction?
Smooth, flat surfaces like wood floors or kitchen countertops will have small coefficients of friction when compared to rougher, more uneven surfaces like concrete, grass, and carpet.
Placing down smooth straws with the ability to roll along the ground will create much less friction and allow the balloon car to travel farther.
Adding “sleigh rails” to the bottom of the box will allow the balloon car to travel farther also.
It is important to measure to the same spot on the balloon car every time so you can reliably compare distance across different parts of the experiment. Comparing measurements from the starting point to the back of the box in one trial and to the front of the box in another trial would not be a meaningful comparison.
Smooth surfaces create the least amount of friction and are the easiest type of surfaces to travel on. Coefficients of friction, which are properties of a material, are lowest here. Putting the “sleigh rails” on the bottom of the box reduces the frictional force even more because there is a much smaller area of contact, which is means there is a smaller area upon which the frictional force is acting.
When you used the straw runway, you essentially created a simple machine with wheels. When you are trying to move the shoebox while it is on a flat surface, you are working against sliding friction. Wheels and round objects bring in a different kind of friction called rolling friction. The coefficient of rolling friction is much smaller and much easier to overcome than the coefficient of sliding friction.