Static and kinetic friction lab report answers

· Follow the instructions and directions below for this lab. Disregard the outline in the manual for your LabPaq Kit.

· Read this document entirely before starting your work.

· Do not forget to record your measurements and partial results.

· Submit a Laboratory Report through Moodle, as shown in the last section of this outline. Remember that the Laboratory Report should include the answers to the questions below.

GOALS

(1) To examine the concept of friction.

(2) To calculate the coefficient of friction of an object.

INTRODUCTION

Friction is an important force, with positive and negative implications. Whenever two objects slide along each other, friction is involved; whether a skier is sliding down an icy slope, or a crate is dragged across the floor. On the positive side, friction between the road surface and the car tires is what keeps the car moving along a highway. On the negative side friction reduces the efficiency of machines. As more work needs to be done to overcome friction, this extra work is wasted as it is dissipated in the form of heat energy.

When one surface slides over another, a resisting force, friction, is encountered. Friction, and the force needed to overcome it depends on the nature of the materials in contact with each other and on the roughness or smoothness of the contact surfaces. It is also affected by the normal force (FN), but not by the contact area or on the speed of the motion. It has been determined experimentally that the force of friction (Ffr) is directly proportional to the normal force (FN). When an object is resting on a horizontal surface the normal force is just the weight of the object (mg). If an object is on an incline, FN has to be corrected via the equation FN= mg cos θ with θ being the angle of the incline. In this experiment, we will only measure friction on a horizontal surface, so we don’t need to concern ourselves with the issues affecting friction on an incline.

The constant of proportionality is called the coefficient of friction, μ. The force of friction when two contacting surfaces are sliding over each other can be calculated by:

PHY-115_Lab-3_equation1.PNG

where Ffr is the force of friction; FN is the normal force; and μk or μs are the coefficient of friction, which is a proportionality constant. The force of friction is parallel to the contact surfaces and opposite to the direction of motion. The term μk stands for coefficient of kinetic (or sliding) friction, which applies when the surfaces are moving with respect to each other.

When an object is at rest on a surface and we attempt to push it, the frictional force is opposing the pushing force. As long as the pushing force is less than the friction force, the object will not move. There is a threshold value of the pushing force beyond which a larger pushing force will cause the body to start sliding. It is this threshold value which is related to the coefficient of static friction, μs. However, when an object is already in movement, the friction that it experiences is related to its coefficient of kinetic friction, μk. When comparing published μk and μs values for identical materials, we see that μs is slightly larger than μk . This indicates that it takes more force to start moving a material than keeping it moving.

PROCEDURE