Physics lab #5-static&kinetic friction PDF

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Ahmad Morad 9/29/20 Lab Report #5- Static and Kinetic Friction PHYS1033-478 Abstract: In this lab, you will make some basic measurements of friction. First you will measure the coefficients of static friction between several combinations of surfaces using a heavy block and a set of hanging masses. Next, you will measure the coefficient of kinetic friction between two of the combinations of surfaces you used in the static friction part of this experiment.

Theory: The friction forces that arise between any two contacting surfaces are divided into two broad categories based on the relative motion of the surfaces. Static friction exists whenever there is no relative motion and kinetic friction whenever one surface moves relative to the other. The magnitude of each friction force depends on the number of the chemical bonds formed between the two surfaces and their average strength. The direction of each friction force is always opposite the horizontal component of the applied force. Suppose you wish to slide a heavy box of mass m along a horizontal surface with a force F. As you increase F, an equal and opposite static friction force fs arises to keep the box at rest until F equals the maximum value of fs: fs

(max) = μs N

Equation 1

where μs is the coefficient of static friction and N is the normal force. When F exceeds f s (max), static friction is no longer present. The box now slides in the direction of F. A kinetic friction force f k now acts to opposite the motion. Unlike its static counterpart, f k does not match the applied force up to a maximum value. Its magnitude is constant and given by the formula f k = μk N Equation 2 where μk is the coefficient of kinetic friction. If you wish to slide the box at uniform velocity, then F must equal f k by Newton’s 1st Law. However, if you accelerate the box, then F > f k by Newton’s 2nd Law. Coefficients μs and μk are depend upon the average bond strength between the two surfaces. The normal force along a flat surface is simply equal to the weight of the object, or N = mg. N is an indirect measure of the number of bonds formed between the two surfaces. In other words, heavier objects require greater forces to get them moving because they press their supporting surface tighter thus forming more bonds.

Ahmad Morad 9/29/20 Lab Report #5- Static and Kinetic Friction PHYS1033-478 Discussion: 1. You will apply more force to get the block started into motion as in compared to maintaining it in motion because value of static friction is always greater than kinetic friction. 2. Static friction is larger than kinetic friction.

3. Normal force is equal to weight when the object is at rest. Normal force is not equal to weight when the object is in motion. 4. Yes, normal force and kinetic frictions are proportional to each other. There are many sources of error present in the measurement such as error in measuring the weight of the body and instrumental error as well. Conclusion: 1. The coefficient of kinetic friction in table 6.4 is 0.18 and that on table 6.3 is 0.21 so there is a difference of around 14% which is greater than 10%. This is because we do not consider the loss due to heat, amount of moisture, and various types of uncontrolled conditions.

Table 1. Coefficient of Kinetic Friction Total mass (kg) Normal component (N) 1.29 11.3 0.628 6.28 1.63 16.3

Table 2. Static and Kinetic Friction Data Minimum static friction force Maximum static friction force Average kinetic friction force Total mass of block and bars Normal force Coefficient of static friction Coefficient of kinetic friction

Friction component (N) 2.42 1.42 3.42 Mean coefficient of kinetic friction μk

0.0200 0.420 0.180 0.128 11.3 0.372 0.217

μk = 0.215 0.226 0.210 0.217

fk FN

Ahmad Morad 9/29/20 Lab Report #5- Static and Kinetic Friction PHYS1033-478 Percent difference

172%

Table 3. Static Friction Data Using an Incline Angle 1 Angle 2 17 degrees 18 degrees

Angle 3 17 degrees

Coefficient of static friction

0.312

Average 17.3 degrees

Homework for lab 6: 1. A) Frictional force between the sofa and horizontal surface opposes the applied force on the sofa. So, Newton’s first law is not violated. B) Normal force is directly proportional to the coefficient of friction and normal force between the sofa and horizontal surface. There is no effect on normal force due to applied force. So, the force of 80 N is sufficient to move with 2.0 m/s....