Name ______Physics

Date ______

Where the Rubber Meets the Road Lab

Determining the Coefficient of Friction, µ (mu)

Overview: Friction is a term that refers to the resistance to motion that occurs whenever two materials come into contact with each other. Sometimes friction is good. Without it people could not walk along the floor and cars could not move over the road. In other situations friction is bad. Friction in machine parts causes wear and increases energy consumption. In order to simplify problems in mechanics objects are often postulated to move without friction. This makes it easier to understand Newton’s laws. However, in order to consider more realistic situations the effects of friction must be considered. Kinetic friction occurs when the surfaces of two objects slide across each other. The force on an object associated with kinetic friction occurs in the direction opposite to the motion of the object and has a magnitude equal to:

Fk=kFN

where (FN) is the normal force and k is the coefficient of kinetic friction.

Objective: At the completion of this lab, the student should be able to:

Determine µ for an object on a 3 different surfaces.

Relate Force of Normal to Force of friction

Materials:Friction blocks, 3 different surfaces (wood, metal, and rubber), balance, and Newtonspring scales

Methods and Analysis

Part A.

  1. Sketch the setup for the lab using a free-body diagram of the block being pulled at constant velocity by the Newton scale on the study surface. Be sure to include all forces acting on the block.
  1. Determine the weight of the Block.
  1. Place a 200g weight in the block and put the block on the wooden surface. Hook the scale to the object, then using a horizontal force, pull on the scale with a gradual increase in force until it starts moving along the surface at constant velocity. Record the approximate force on the scale to start the object.
  1. Calculate the µs value for this surface.
  1. Repeat for two other surfaces, Record all data.

Part B:

  1. Using the same set up, you will now compare force of normal to force of friction.
  1. Place the friction block on a surface. Hook the scale to the object, then using a horizontal force, pull on the scale with a gradual increase in force until it starts moving along the surface at constant velocity. Record the approximate force on the scale to start the object. Record this data.
  1. Repeat for step 2 adding additional weights of 100g, 200g, 300g, and 400g, Always use the same surface to surface. Record all data.

Part C:

Using the wooden ramp, you will now look at the impact the changing angle has on friction. In this part of the lab, you do the same investigation as before, except your independent variable will be the angle of the incline.

  1. First set the incline of the wooden ramp to the level mark. Place the friction block and 200 g on the surface. Hook the scale to the object, then using a horizontal force, pull on the scale with a gradual increase in force until it starts moving along the surface at constant velocity. Record the approximate force on the scale to move the object at a constant rate.
  1. Repeat step 4 changing the angle of the ramp to 10, 20, 30, and 40o.

Record all data on the answer document.

Where the Rubber Meets the RoadSketches:

Answer Document

Weight of friction Block = ______Newtons

Data Table I: Same Mass, Different Surfaces:

Surface / TotalWeight (N) / FN / FA / FF / Coefficient of Friction (
Wooden
Metal
Rubber

Data Table II: Same Surface, Different Masses:

Mass added / Total Weight (N) / FN / FA / FF / Coefficient of Friction (
0 g
100 g
200 g
300 g
400 g

Data Table II: Same Surface, Same Mass, Different Angle:

Angle
 / Weight / F / FN / FA / F / FF / Coefficient of
Friction (
Level
10o
20o
30o
40o

Conclusions

1. What were the differences in the coefficient of frictions for the different surfaces and what were possible causes of these differences?

2. Were there differences in the coefficient of friction for the different added weights and what were possible causes of these differences?

  1. Does the force of friction depend on the weight of the block? Explain.

4. Does the coefficient of friction depend on the weight of the block? Explain.