Kinetic Friction Experiment #13
Joe Solution E01234567 Partner- Jane Answers PHY 221 Lab Instructor- Nathaniel Franklin Wednesday, 11 AM-1 PM Lecture Instructor – Dr. Jacobs
Abstract The purpose of this experiment was to examine kinetic friction and what factors affect it. We pulled a wood block across a surface to determine whether the surface area of the block or the type of surface affects friction. The surface area did not appear to affect friction, since there was a small percent difference of 6.16% between different surface areas. The type of surface did as the difference in values was large, at 72.2%. (75 words)
Introduction The purpose of this experiment is to examine kinetic friction and the factors that affect it. The two factors that are examined within this experiment are the surface area of the object and the type of surfaces in contact with one another. Both of these will be tested and compared to see which affects the value of kinetic friction Friction is a force that always opposes the motion of an object. Friction can be divided into two different types. One is called static, and one is called kinetic. Static friction is a force between two objects that are not moving relative to one another. For example, an object resting on a slope, but not sliding down the slope, is kept in its position by static friction. Static friction must be overcome to cause an object to move across a surface. Once enough force has been applied to an object, it will begin to slide across a surface and kinetic friction will then act on the object. Kinetic friction occurs when two objects are moving relative to one another with one object sliding across the surface of the other and it opposes the motion of the object. Both types of friction are described by different coefficients. These values are known as the coefficients of static and kinetic friction respectively. The coefficient of static friction is usually higher than that of kinetic friction. A small wooden block was used, with one side covered in Teflon tape to examine the coefficient of kinetic friction. The Teflon tape on one side of the block allowed us to see the effect of different surface types on the coefficient of friction. The small block was attached to a string. This string was threaded over a pulley, which was then connected to a mass hanger. Paperclips were used to add mass to the hanger, increasing the weight of the mass hanging on the string, until the block began to slide across the surface of the table. Mass was also added to the top of block to increase the normal force between the block and the table. The apparatus can be seen in the figure below.
Figure 1- A picture of the experimental set up
In order to calculate the coefficient of kinetic friction, we can look at the set up and begin by examining the forces acting on the hanging mass. Using Newton’s Second Law on the hanging mass, we find ∑ (1)
FT is the force of tension in the string, mh is the mass of the hanger and paperclips, g is the acceleration due to gravity, and a is the acceleration of the hanger. If we assume that the hanging mass is not accelerating, we can solve the above equation for FT and find the following. (2) Next, we can look at the forces acting on the block resting on the table. Since the forces act in two different directions, we must sum the forces separately. To begin, we can look at the forces acting in the vertical direction. ∑ ( ) ( ) (3)
In the above equation, FN is the normal force acting on the block, M is the mass of the block, m is the mass added to the block, and ay is the acceleration in the vertical direction. Since the block isn’t accelerating in the vertical direction, we can set ay=0 and solve the equation for FN. ( ) (4)
Now, we need to examine the forces acting in the horizontal direction by taking the sum of the forces. ∑ ( ) (5)
Fk is the force due to kinetic friction in the above expression. If we assume that the block only just...
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