Lab 3

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Pennsylvania State University *

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111

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Mechanical Engineering

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Oct 30, 2023

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A MODEL FOR KINETIC FRICTION - PART 1 INTRODUCTORY INSTRUCTIONS This lab will be partially graded using scientific ability rubrics. To help you know what we expect, the rubrics have been included along with the instructions for each question. If no rubric is indicated, the question is graded on completeness. Lab goals: In this lab, you will 1) Learn to use the force sensor on the IOLab. 2) Determine different methods to measure kinetic friction. Observing experiments: Read carefully the accompanying “Observing experiment” file which explains what we expect you to do in observing experiments such as the ones below and what we expect you to write in response to the questions. Equipment available: You will need the IOLab with computer/software and a horizontal surface. You may need the “hook” attachment and a small piece of string to pull the IOLab. Phenomenon to Observe: When two objects in contact move relative to one another, the atoms at the interface interact electrically to create a force that resists the motion. This force is called kinetic friction and we know that it is always in the direction opposite to motion and parallel to the interface. We would like to investigate different ways to measure and quantify kinetic friction. Variables: Independent variable: mass of the IOLab Dependent variable 1: measured kinetic friction Dependent variable 2: measured kinetic friction by an alternative method. Controlled variables. Surface on which IOLab slide, temperature of the room or surfaces, etc.. Both dependent and independent variables are quantitative variable (they could be any numbers). Primary Guiding Questions to think about: 1. How can we measure kinetic friction (in Newton?) 2. Does kinetic friction remain mostly the same during the sliding phase. Does the friction change between a big/small push or for a continuous pull?
A Model for Kinetic Friction - Part 1 Page 2 of 7 3. What overall properties of the object does kinetic friction depend on? Does it depend on shape, surface area of contact, mass? Some of these questions can be answered (at least partially) from our analysis in lab 2 “slowing down”. The dependence on shape or surface area are hard to do quantitatively with the material we have so we will focus on dependence on mass. ACTIVITY 1: DISCOVERING THE FORCE SENSOR AND MEASURING THE MASS OF THE IOLAB 1. A first thing to do is to check the direction of pull versus push. Push on the hook. Is the force positive or negative? Does pulling give you the opposite? Does this make sense with the coordinate system given on the IOLab? Pushing on the hook yields a force in the positive Y-axis. Pulling in the hook yields a force on the negative Y-axis this is indeed opposite. This does make sense with the coordinate system given in the IOLab. 2. Now hold the IOLab by the hook, so that it is hanging in the air. Draw (by hand, take a picture or using software) the FBD for this situation, being careful to draw the coordinate system that corresponds to the one used by the IOLab remote. Write Newton’s Second Law F = m a for this situation for the component(s) you think is/are relevant. For this situation Newton’s Second law applies as F=W where W = mg Thus F T = mg 3. What is the force of gravity (F G ) on the IOLab? Specify the magnitude and the direction. The force of gravity on the IOLab is the force pulling on the IOLab cart while the cart is suspended in the air. The magnitude is F G = mg, where the quantity g—a property of the planet in this case 9.8 m / s . Set-up 1. Turn on the IOLab and select the force sensor. 2. Recalibrate the force sensor and the accelerometer to make sure the calibration is up to date. 3. Attach the hook. 4. Hit record and immediately “rezero” the force sensor. You will need to do this frequently.
A Model for Kinetic Friction - Part 1 Page 3 of 7 4. Take a screenshot of your F(t) graph, highlighting the region used to determine the average force with the statistical tool. The average force is -1.93 N. 5. Calculate the mass of the IOLab and explain how you can get this from the measurement of the force of gravity. If |F|=W and W=mg than 1.93 = m*9.81 M = 0.200 kg 6. Now move the IOLab up or down at constant velocity. Does the tension change? Write Newton’s Second Law when the IOLab moves up or down at constant velocity. The tension does not change, an object at constant velocity has an acceleration of zero. Per Newton’s second law F T = mg
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