Measurement of The Coefficient of Static and Kinetic Friction

Abstract

This report explores and analyzes frictional force data collected in a frictional simulation. The experiment's first part involves measuring static and kinetic friction and recording the data. Secondly, static and kinetic frictional force coefficients are calculated using data collected from part one of this experimentation. The frictional force was measured using three different masses from the simulation, and the coefficients of each were calculated. The final aim of this experiment is to calculate the mass of a gift box in the simulation.

This was achieved by calculating the collected data set throughout the experimentation.

Introduction

If you slide an object over another, some force will oppose the object's movement. This is the friction force parallel to the surfaces of the two objects. The frictional force is classified into many types, the most common being static and kinetic friction. Static friction occurs on surfaces of objects that are at rest and not moving, while kinetic or dynamic friction exists only between surfaces of moving objects.

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There are different experimental methods of evaluating frictional force. Static force is always equal to applied force until the object is set into motion. When the object begins to move, the frictional force acting against its motion is the kinetic friction. Both types of friction are affected by different factors such as the object's weight, the natural surfaces of the objects, and even the coefficients of the type of friction involved. This experiment focuses on defining the coefficient of static and kinetic friction, which is found by using the ratio between frictional force and the normal force.

Theoretical Background

This experiment involved measuring different objects' forces, masses, and speed/velocity. Units used for these measurements are as follows: Force is given in Newtons (N), Mass in Kilogram (Kg), and speed in meters per second (m/s). Applying the same amount of force to different masses results in a difference in the speed the masses move at. For instance, when a force of 120 N is applied to a 50 Kg wooden box in the simulation, the box moves at a constant speed of 1.1 m/s, while if the same force is applied to a 100 Kg mass, it does not even move. If a force is applied to an object and no motion occurs, the applied force is balanced by static force. The maximum value of static force occurs just before the object starts to move. This force is experimentally proportional to the regular forces between both surfaces. Hence static friction (_Fs) is _{Fs ≤ µs N}, where _µs is the coefficient of static friction and _N is the normal force. Kinetic friction is also proportional to the normal force and can be expressed as _{FK = µk N}, where _µk is the coefficient of kinetic friction, and _N is the normal force.

Equipment Used

• Two wooden boxes weighing 50 kg each.
• 40 kg, 80 kg, 100 kg, and 200 kg masses.
• A gift box with an unknown mass.
• Speed timer.
• A force applying body.

Experimentation Procedure

1. Using the frictional simulation, a 50 kg box was placed on a frictionless surface.
2. A moderate force was applied to the wooden box, and observations were made.
3. The friction was set to less, and a reasonable force was applied. The pressure applied was slowly increased until the box was set into motion. Data was recorded.
4. The same procedure was repeated using moderate friction and lots of friction.
5. After several attempts to measure the friction, calculations were carried out to find the coefficient of both static and kinetic friction.
6. The friction slider was kept at the same point on the bar, and friction was measured using the three different masses, including the gift box.
7. The above procedure (step 6) was repeated for three different amounts of friction.

Results and Analysis

Results when a little friction was used:

Results when medium friction was used:

Results when lots of friction was used:

Calculations

The formula: µs = Fs / N, where Fs is the static force, and N is the normal force, is used to calculate the coefficient of static friction.

For the 50 kg wooden box, µs = 125 / 500 = 0.25 when little friction is applied.

The weight of an object equals the normal force acting on the object; hence the weight of the gift box can be calculated by the formula N = Fs / µs = 35 / 0.25 = 140 N.

Weight can be converted to mass by dividing by the gravitational constant; hence the mass of the gift box is given by 140 / 10 = 14 Kg.

Conclusion

From this experiment, it is clear that the static friction of an object is always higher than its kinetic friction. The coefficient of static friction has always been found to be greater than that of kinetic friction. Using the coefficients of friction and forces applied, the unknown weight and mass of different objects can be calculated.

References

1. Kim, Kyung Heon, et al., University Physics (Cheungmunkak, 2013), chapter 4
2. Halliday, D. et al., Fundamentals of Physics, 10th edition (John Wiley & Sons, Inc., 2013)
3. Korean Physical Society, General Physics Laboratory (Cheungmunkak, 1997).
Updated: Jan 05, 2024