Table of content:
page
Abstract
2
Introduction
2
Objectives
4
Apparatus
4
Procedures
5
Precautions while conducting experiment
8
Data Result
8
Discussion
10
Conclusion
10
ABSTRACT
This experiment was conducted to accomplish the objective of finding the static and sliding forces with respect to the area, weight and the material the object was experimented on. Adding to that we determined the friction coefficient for the rolling case implemented on the wooden block experimented on. The wooden block has a layer of plastic hence the experiment was conducted on both the wooden and the plastic side separately to compare the results. This experiment was accomplished by measuring the static and sliding forces of the wooden block using a spring balance. The results of this experiment proved quantitatively what we as students before could only measure using formulas and equations. The average maximum force values were plotted against the normal force for each mass, and the slope was determined to be the coefficient of static friction. Next, the average mean force was plotted against the normal force, and this slope was determined to be the coefficient of kinetic friction. We found that all of the factors mentioned are directly related to the coefficient friction.
INTRODUCTION
Friction is a resisting force that acts along the tangent to two surfaces in contact when one body slides or attempts to slide across another. Normal force is the force that each bod exerts on the other body, and it acts perpendicular to each surface. The frictional force is directly proportional to the normal force. There are two different kinds of friction. Static friction (F s) occurs when two surfaces are still at rest with respect to each other, but an attempt is being made to cause one of them to slide over the other one. Static friction arises to oppose any force trying to cause motion tangent to the surfaces. The static frictional force F s is given by F s ≤ µsN Where N stands for the normal force between the two surfaces, and µs is a constant called the coefficient of static friction. The meaning of Equation 1 is that the static frictional force varies in response to applied forces from zero up to a maximum value given by the equality in that equation. If the applied force is less than the maximum, then the frictional force that
arises is equal to the applied force, and there is no motion. If the applied force is greater than the maximum, the object will begin to move, and static friction conditions are no longer valid. The other kind of friction occurs when two surface are moving with respect to each other. It is called kinetic friction, and it is characterised by a constant µk called the coefficient of kinetic friction. The kinetic frictional force F k is given by F k = µk N Where N is again the normal force. Equation 2 states that the kinetic frictional force is a constant value any time the object is in motion. In fact, the coefficient of kinetic friction does vary somewhat with speed. It is assumed for this laboratory that at the slow speeds used, µk does not depend upon speed. To a good approximation both coefficients are independent of the apparent area of contact between the two surfaces. According to velocity, friction may be classified into static friction (when v =0) and kinetic friction (when v0). According to type of motion, friction can also be classified into sliding friction (in the case of sliding motion) and rolling friction (in the case of rolling motion).
Then there’s coefficient of friction. This defined as the ratio between the force necessary to move one surface horizontally over another and the pressure between the two surfaces. In
this experiment, there’s two type of coefficient of friction be determined that is coefficient of static friction and coefficient of kinetic friction.
The friction coefficient (μ) between two solid surfaces is defined as the ratio of the tangential force (F) required producing sliding divided by the normal force between the surfaces (N)
μ = F /N
OBJECTIVES
This experiment has three main objectives to accomplish 1) To determine the static and sliding (kinetic) friction forces ( F S and F K) as function of: (a)- Area, (b)-Weight and (c)-Material , and then to compare the friction forces (F S and F K) for different areas, weights, and materials. 2) To compare rolling and kinetic (sliding) friction forces ( F R and F K) as a function of weight, 3) To determine the friction coefficient in rolling case.
APPARATUS One set of seven weights, 0.1 to 2 kg with hook, One set of six stand rods, L=100 mm, D=12mm, One spring balance (dynamometer) 10N, One pair of wooden blocks for friction experiments,
PROCEDURES
For each objective a different set of experiment was conducted hence a total of 3 sets of experiment were conducted. The following shows the procedure for each se of experiment.
Set 1- Static and sliding friction forces as a function of the area, the weight and the material (refer to Fig. 1),
1.
The small block is placed on the workbench surface with the plastic (rubber) side down. The static and sliding friction forces, F S and F K are measured.
2.
The wooden block placed on the workbench surface with the wide wooden side and then the narrow wooden side down. The measurement for F S and F K recorded.
3.
The measurements repeated with the large block for friction experiments.
4.
Results of F S and F K plotted as a function of area, weight, and material.
5.
The corresponding S and K were determined.
Set 2- Static and sliding forces as a function of the force of gravity, (Refer to Fig. 2)
1.
The small block is placed on the workbench surface with plastic side down. After that the static and sliding friction force is measured.
2.
The weights of the block increased by adding in turn the weights of 0.1 kg, 0.2 kg, 0.5 kg and 0.8kg; the measurements repeated.
3.
The results of F S , F K , S , and K plotted as a function of the force of gravity, i.e. weight, W.
Set 3- Rolling and sliding friction as a function of the force of gravity, (Refer to Fig. 3)
1.
The stand rods are lie down next to each other and large block were placed on the rods with the plastic side down.
2.
The horizontal pulling force measured, which maintains a uniform motion on the rolling rods as the rolling friction force F R.
3.
The weight of the block increased by adding in turn the weights 0.1 kg, 0.2 kg, 0.5 kg and 1.0 kg; measurement repeated each turn of the weight.
4.
After that the block aligned parallel to the rod axes to measure the sliding friction force F K.
5.
Graph of sliding friction force and rolling friction force are plotted as a function of the force of gravity.
PRECATION WHILE CONDUCTING EXPERIMENT Make sure the block and table are smooth and clear Make sure constant velocity was maintained to find accurate F k Make sure while taking readings to keep eye sight perpendicular to readings so as to avoid parallax error Check or adjust spring balance to avoid error DATA RESULTS
a) SET 1:
Material Plastic
Wooden Small Area
Weight
Static Friction (Fs) 1.275 2.747
Large Small
Kinetic Friction (Fk) 0.981 1.57
Large Area
Fs (N)
Fk (N)
Fs (N)
Fk (N)
0.3924 1.079
0.2943 0.7848
0.3924 1.079
0.2943 0.7848
b) SET 2 -by using Plastic side Normal Force (FN) 3.242 4.223 5.204 8.147 13.052
Fs
Fk
(s)
(k)
2.845 4.120 5.10 7.652 >9.81(immeasureable)
1.57 1.962 2.747 4.905 8.044
0.8775 0.976 0.980 0.939 null
0.484 0.4645 0.528 0.602 0.616
-by using wooden side Normal Force (FN) 3.242 4.223 5.204 8.147 13.052
Fs
Fk
(s)
(k)
1.079 1.668 2.060 3.335 4.709
0.7848 0.981 1.177 1.766 2.747
0.333 0.394 0.396 0.409 0.361
0.242 0.232 0.226 0.217 0.210
c) SET 3 Rolling Mass (kg)
FR
3.242 4.223 5.204 8.147 13.052
0 0 0 0 0
(R) 0 0 0 0 0
*Forces are too small to be measured. Sliding Mass (kg) 3.242 4.223 5.204 8.147 13.052
FK 0.981 1.177 1.373 1.962 3.728
(K) 0.3026 0.2787 0.2638 0.2408 0.2856
DISCUSSION
Friction is an opposing force that resists the motion or attempted motion of one surface across another surface. Friction depends on the material of both the object’s surfaces and weight. The heavier the block is, the larger the friction. However friction is independent of the area of the surfaces in contact. This can be proved by observing the graph of Set 1 where the value of friction varies when different material or weight is being experimented on but the value does not change when different area is used. The frictional force is presumed to be proportional to the coefficient of friction. These coefficients of friction are quoted for a given pair of surfaces - a coefficient of static friction and a coefficient of kinetic friction. The amount of force required to move an object starting from rest is greater than the force required to keep it moving at constant velocity once it is started. Therefore, Static friction is greater than kinetic friction. In the Set 3 experiment, the friction caused when using rolling rods is so small that it can be considered to be insignificant. This is because the table is horizontal and there is no component of force tangent to the surface; hence even if the coefficient of static friction were zero there would be no sliding. Therefore the static friction force is zero. The relative velocity between the surface of the ball and the horizontal sur face near the point of contact the velocity is wholly normal to the plane. Therefore the kinetic friction force is zero. Therefore we could say that: Static friction > kinetic friction > rolling friction for the same combinations of surfaces. Due to inconsistent force in this experiment when pulling the block, it is best when one person pulls the blocks 3 trials. Also some of the experiment cannot be measured, therefore a larger and more sensitive dynamometer needed in the future. This makes the data more accurate and less erroneous. CONCLUSION
Due to inconsistent force in this experiment when pulling the block, it is best when one person pulls the blocks 3 trials. Also some of the experiment cannot be measured, therefore a larger and more sensitive dynamometer needed in the future. This makes the data more accurate and less erroneous. In conclusion to my report, static friction’s force will and should always be greater than that of Kinetic friction due to that more force is needed in exerting force to be able to move an object while that of Kinetic Friction, all it has to do is to maintain. Adding to that different material will make the frictional force vary while the best way to reduce the frictional force to move the block would be the rolling method since insignificant frictional force was observed in this method.
