Lab 4

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Lab – Mechanics, Heat, and Sound (102M) | 1 Lab - Mechanics, Heat, and Sound (102M) NAME: ______________________________________________ Experiment 2: Newton’s Laws We encourage you to complete this lab using whatever equipment is available to you. There is no penalty for inaccurate measurements. Your grade is based on your reasoning about data that is collected and the rubric in the Assessment section of this Lab. If you are unable to set up the experiment and analyze data using available equipment, please contact your HS Instructor. Materials List Qty. Lab Item Alternative Option 1 Block with hooks/rings This is the object that will be sliding (or not) while we study friction. It may be a wooden block, but it could also be any object that is flat on one side and slides with some resistance on a surface, such as a shoe, textbook, notebook, etc. 1 Pulley and string These are used to connect the vertically-hanging masses to the side of the block, so that a horizontal force may be applied. You may consider using a spool cylinder or small wheel of some kind as a pulley, so long as it can be mounted on the edge of a table and rotate freely. 1 Set of hanging masses These masses are used to apply an increasing amount of force to the block. You may also use anything with a known mass. For example: • a penny is 2.5 grams • water has a density of 1 gram per mL, and can be placed in a Styrofoam cup or other light container that will not add much extra weight. You might consider another object with a known mass, either from an internet search (roll of coins, cup of rice, etc.) or an object whose mass is labeled (can of tuna or soda) 1 Electronic scale This is used to measure the mass of the block. If you are using an object with a known mass, then you do not need a scale and can determine the mass by searching the internet or inspecting the label.

Lab – Mechanics, Heat, and Sound (102M) | 2 Lab - Mechanics, Heat, and Sound (102M) NAME: ______________________________________________ Background In kinematics, we focused on motion itself, without attempting to explain the interactions that caused or changed the object’s motion. Now, with the introduction of forces , we are building a broader set of models that incorporate interactions between objects (forces), which can cause or change an object’s motion. In your reading assignment, you encountered Newton’s 1 st and 2 nd laws , which are briefly summarized here. • Newton’s 1 st Law : A body at rest remains at rest, or, if in motion, remains in motion at a constant velocity unless acted on by a non-zero net external force. • Newton’s 2 nd Law : The sum of all external forces acting on an object or system is equal to the mass of that object or system multiplied by the object’s or system’s acceleration. ΣF #⃗ = m a #⃗ (࠵?࠵?. ࠵?) These laws, along with our representation of forces acting on a body called a free-body diagram , give a new set of tools to building models to determine what can happen to an object or system when it experiences external forces. Friction In this experimental inquiry, we will focus our investigation of forces on friction , a particular type of force that resists relative motion between two surfaces. The force of friction between two surfaces changes depending on whether two surfaces are moving or not moving relative to each other. Static friction applies to the case where two surfaces in contact do not move relative to each other (static derives from the Latin word that means “not moving”). Objects that are static can be subject to a range of forces before they begin to move, giving a model for the force of static friction of the form f ! ≤ µ ! N (࠵?࠵?. ࠵?) where f ! denotes the magnitude of the force of static friction, µ ! is a dimensionless quantity representing the material properties between the object and the surface, and N represents the normal force on the object from the surface.

Lab – Mechanics, Heat, and Sound (102M) | 3 Lab - Mechanics, Heat, and Sound (102M) NAME: ______________________________________________ Kinetic friction applies to the case where two surfaces in contact do move relative to one another (kinetic derives from the Latin word that means “in motion”). Unlike static friction, the model for the force of kinetic friction is an equality, giving a single value, defined by f " = µ " N (࠵?࠵?. ࠵?) where f " represents the magnitude of the force of kinetic friction, µ " is a dimensionless quantity representing the material properties between the object and the surface, and N represents the normal force on the object from the surface. Both models of friction assume that the magnitude (either maximum or constant) of the force of friction: (1) is proportional to the normal force that is exerted on an object from the surface on which it is placed, (2) depends on the material properties of that object and the surface, and (3) is independent of the surface area of contact between the object and its surface. However, friction can frequently exhibit deviations from these models due to the specific ways that surfaces interact while sliding. It is good to bear this fact in mind when considering your conclusions or comparing your results with those of other students. Experimental Question Before posing our research question for this experimental inquiry, let’s consider why we know there must be friction in the world around us. Imagine a box in the middle of your physics classroom. The box has some amount of mass. You give it a gentle push, applying a force that accelerates it to some speed. After you stop pushing, the box slides across the floor. If there were no friction, the box would continue to slide at that same speed until it encountered an obstacle. If we perform this action, this is not what we observe. Instead, the box will either come to a stop or slow down before hitting some obstacle (you are encouraged to try this yourself to make sure OnRamps Physics isn’t making this up). From Newton’s 1 st law, this means that there must be some external force changing the box’s speed and slowing it down. This is a justification for our idea of friction, that there is some force which acts in opposition to an object’s motion.

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Lab – Mechanics, Heat, and Sound (102M) | 4 Lab - Mechanics, Heat, and Sound (102M) NAME: ______________________________________________ Figure 1 Friction from the floor acts against us as we push on a box. A skateboard and wheels can reduce some of this resistance. The goal for this experimental inquiry is to address the question: how do we characterize the resistive force objects experience, either while in motion or not in motion? To investigate this question, we introduced the two models: static friction and kinetic friction. Procedure The Procedure section describes the lab activity. Questions posed within this section should be addressed in the corresponding space on the worksheets at the end of this document (pgs. 7-11). Be sure to record any thoughts, sketches, or data in the appropriate locations on the worksheets. For Experiment 2, you should expect to spend approximately 70-90 minutes to complete the investigation outlined below. For this part of the experiment, you will investigate the conditions of no relative motion between an object and a surface. Specifically, the goal is to determine the coefficient of static friction between two surfaces; that is, the ratio of the friction force to the normal force just at the point where friction no longer prevents the object from beginning to slide. There are numerous ways in which you can do this. For example, you can place a block on an adjustable inclined plane and measure the angle at which it begins to slide. Alternatively, you can pull on a block using a spring scale and measure the scale reading just as the block moves. All of these are acceptable approaches, and, depending on the equipment that you have available, some may be more desirable than others. The suggested procedure below requires a block, a pulley, and a set of hanging masses. Part 1: Investigating Static Friction

Lab – Mechanics, Heat, and Sound (102M) | 5 Lab - Mechanics, Heat, and Sound (102M) NAME: ______________________________________________ Whether you choose to follow this procedure or use another set-up, draw a free-body diagram for the set-up that you are using. Use Newton’s Laws to determine a mathematical relationship among the force of friction, the normal force of the surface, and the force that is being applied (in this case, that would be the force of the hanging mass). Record this work in the space provided on the data sheet. Figure 2. A simple diagram of a block at rest on a horizontal track connected to a hanging mass via a light string that passes over a pulley. With the materials listed for this experiment, you can measure the coefficient of static friction by applying a known external force to a block parallel to the surface on which the block rests (see Figure 1 ). Either complete the following steps, or design your own experiment to accomplish the same goal. Record all relevant data in the tables provided. You will submit this data in Canvas at the completion of your experimental inquiry. 1) Place a block on a horizontal surface. Be sure to clean the table on which your block is resting so it is free of any dust or oils that may have accumulated on it. 2) Attach the block to a hanger using a string. The hanger will hang vertically over the edge of the table, connected to the block through a pulley (as shown Figure 1 ). 3) Add masses to the hanger in small increments. When adding a mass to the hanger, attempt to place it gently (avoiding dropping the additional mass on the hanger). 1 As the block just begins to move, the mass on the hanger is slightly greater than the maximum force due to static friction. 4) Record all the measurements that are needed to determine the coefficient of static friction, and repeat this experiment three to five more times. 5) Applying the model of static friction introduced in Equation L2.2, calculate and record the coefficient of static friction for each trial. Do the trials agree with one another? 1 To build some intuition about why, imagine the strength required to hold a bowling ball that someone gently places in your hands compared to the strength required to catch a bowling ball that someone drops from above – if your intuition is that a falling ball would be harder to stop (requiring your arms to provide a larger upward force), you’re right! In this case, the block and hanger are a bit like your arms, in that they are providing the upward force to “catch” the mass.

Lab – Mechanics, Heat, and Sound (102M) | 6 Lab - Mechanics, Heat, and Sound (102M) NAME: ______________________________________________ Digging a bit deeper, we will examine some of the simplifications made within our model for static friction. We will find whether (1) the mass or (2) the surface area of contact makes a difference to the coefficient of static friction between two surfaces. First, choose to investigate either mass or surface area; if you try to investigate both changes at the same time and something comes out differently, you won’t be able to tell which change played a role! Then, consider why you think this may or may not really matter when it comes to friction. On the handout included here, write down 1-2 sentences summarizing your ideas. For this part of the experiment, you may repeat the procedure that you carried out in Part 1, but alter either the amount of mass on the block (by adding weights on top of it) or turning the block so that the surface area between the block and the table is a different size from Part 1. Note this choice on your data sheet. At the completion of this procedure, calculate a new coefficient of static friction and compare it to the value you found previously. Are they the same (or close)? Part 2: Does surface area matter? Does the mass of the block matter?

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Lab – Mechanics, Heat, and Sound (102M) | 7 Lab - Mechanics, Heat, and Sound (102M) NAME: ______________________________________________ Assessment This section summarizes information regarding the assessment of Post- Lab 2 . The worksheets for this lab should be scanned or photographed and uploaded as part of the post-lab assignment in the college Canvas course. The Post-Lab is worth 100 points, distributed as follows: - 40 pts for Canvas-based Post-Lab questions, and - 60 pts for uploaded work, including parts of the worksheets assessed using the rubrics. Please make sure that you are familiar with each scientific ability that we are looking for, and check that your work addresses them. Part 1: Static Friction (10 pts) Space for free-body diagram and equations:

Lab – Mechanics, Heat, and Sound (102M) | 8 Lab - Mechanics, Heat, and Sound (102M) NAME: ______________________________________________ Use the data table below to record your data. Add vertical lines to separate the table into as many columns as needed based on your measurements. Make a column for each type of data that you measured (be sure to include units!), and record values for each trial. The top row should be used as a header for the quantities written below. The table is assessed using the following rubric (10 pts). Scientific Ability Adequate (10 pts) Needs some improvement (7 pts) Inadequate (3 pts) Missing (0 pts) Decides what quantities are to be measured, and identifies independent and dependent variables The measured quantities are relevant and appropriately labelled as independent or dependent variables. The measured quantities are relevant, but independent and dependent variables are not properly identified. Only some of the measured quantities are relevant to the experiment goal(s). The measured quantities are irrelevant to the experiment goal(s). Trials 1 2 3 4 5 (10 pts) Average coefficient of static friction: ________

Lab – Mechanics, Heat, and Sound (102M) | 9 Lab - Mechanics, Heat, and Sound (102M) NAME: ______________________________________________ Modified Procedure for Part 1 (if needed) Use this space to describe the step-by-step procedure that you used to measure the coefficient of static friction, if it was different from the procedure provided. Any additional information provided here gives context to the data collected and summarized above, increasing your chances of earning full credit.

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Lab – Mechanics, Heat, and Sound (102M) | 10 Lab - Mechanics, Heat, and Sound (102M) NAME: ______________________________________________ Part 2: Does mass or surface area really matter? Which do you choose to investigate? Choose one option: ° Mass of Object ° Contact Surface Area Use the data table below to record your data. Add vertical lines to separate the table into additional columns. Make a column for each type of data that you measured (be sure to include units!), and record values for each trial. The top row should be used as a header for the quantities written below. The table is assessed using the following rubric (10 pts). Scientific Ability Adequate (10 pts) Needs some improvement (7 pts) Inadequate (3 pts) Missing (0 pts) Decides what quantities are to be measured, and identifies independent and dependent variables The measured quantities are relevant and appropriately labelled as independent or dependent variables. The measured quantities are relevant, but independent and dependent variables are not properly identified. Only some of the measured quantities are relevant to the experiment goal(s). The measured quantities are irrelevant to the experiment goal(s). (10 pts) Do you think that increasing this property will increase, decrease, or have no impact on the coefficient of static friction? Why?

Lab – Mechanics, Heat, and Sound (102M) | 11 Lab - Mechanics, Heat, and Sound (102M) NAME: ______________________________________________ Trials 1 2 3 4 5 (10 pts) Average coefficient of static friction: ________