Coefficient of Friction

Coefficient of Friction- Post Lab

Abstract

The purpose of the experiment was to determine to coefficient of friction on a block sliding across a horizontal plane, and on the same block sliding down an inclined plane. This was done by first testing block, and how much weight on a string was needed to move the block at a constant velocity using a pulley system. The block weighed 0.2385 kilograms, and needed a hanging mass of 0.05 kg to move at a constant velocity. This means the coefficient of friction is 0.37. The second block was tested on an inclined plane, and the angle was found at which the block would move at a constant velocity. The angle found was 230. Using the equation μk=tan θk we found the friction to be 0.42. The friction was different because there was more force required to keep the block sliding down the plane at a constant velocity.

Introduction

Frictional forces are universal, in which they are found between two solid surfaces in parallel contact. If an object moves over a surface, the force exerted on the object by the surface is called the Kinetic friction force. This force is in a direction opposite the direction the object is moving. The friction force is proportional to the normal force exerted by the surface on the body. The force’s relationship can be viewed by using the coefficient of kinetic friction, μk, in f=μkn, where f is the magnitude of the force of the friction, and n is the magnitude of the normal force. If the surface is flat, and horizontal, the normal force is equal to the weight of the object, due to the fact that because there is no acceleration in the normal direction, there is no net component in the opposite direction. If the surface is sloped, the normal force is the component of the weight, which is perpendicular to the surface.

If the object is pulled over the surface at a dynamic equilibrium (constant speed), the net force is equal to zero. This is because the normal force is equal to the weight of the body, and its opposite force, while as the force of friction is equal to the force of pulling the object, allowing for each force to cancel the other out, and giving a net force of zero. This means then that if the weight of the object and the force of pulling the object are known, then the normal force and the force of friction can be determined.

If the object is on an inclined plane, with the angle of the plane gradually increasing, then an angle will ultimately be reached at which the object will slide down the plane. At this angle, repose θs, the acceleration and the sum of all the forces acting on the body is zero. The component of the weight parallel to the incline is equal to the friction, while the component of the weight perpendicular to the incline is equal to the net force. By measuring the angle θs and using the equation μs=tan θs. we can find the coefficient of static friction μs. For a moving block, an angle can be found at value θk, where the block will slide down the plane at a constant velocity. At this angle, the acceleration is equal to zero, we can now claim that f=fk= μkn, where μk is the coefficient of sliding friction. We can then obtain μk=tan θk.

Procedure
1. Adjust the incline plane to be horizontal, determine the mass of the wooden block.
2. Run a string from the block over the pulley on the end of the plane. Adjust the pulley so that the string is parallel to the plane.
3. Hang the masses on the end of the string until there is sufficient force to move the block at a constant velocity after it begins moving.
4. The weight of the block is equal to the normal force and the weight of the masses, hung from the string, will be equal to the friction force. Record these values.
5. Repeat Step #4 but increase the mass of the blocks by placing 1, 2, 3, and 4kg on it.
6. Plot a graph showing the relationship of the normal force and the frictional force. Find the value of the coefficient of kinetic friction. Use the slope of the graph.
7. Find the angle at which the block will move down the slope at a constant velocity. Record angle.
8. Use this angle to find the coefficient of static friction.

Discussion

The purpose of the experiment was to find the coefficient of friction on a block moving along a horizontal surface, and an inclined plane. This was done using a pulley systems and using weights. For the block to move at a constant velocity across the horizontal plane 0.05kg of weight was needed for the string. In the second experiment the angle of the plane was increased to 23 degreers, therefor the block was sliding down at a constant velocity. By using this angle we were able to find the frictional force by using the equation μk=tan θk, giving us a force of 0.42N.