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.

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

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