PHY111-161 Online Lab #2 Forces and Motion CR (1)

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PHY111/161 ON-LINE LAB #2 Forces and Motion Lab #2 Forces and Motion NAME: Caden Robbins NAU User ID: car692 Introduction This lab will be helping you to understand those key elements of classical physics, Forces and Motion . A simple definition of force is a push or a pull, and the net force that acts on an object is related to how its motion changes. In this lab, we will be looking more closely at Newton’s first and second laws of motion. They are: Newton’s First Law: Every object continues in a state of rest or of uniform speed in a straight line unless acted on by a nonzero net force. Newton’s Second Law: The acceleration produced by a net force on an object is directly proportional to the net force, is in the same direction as the net force, and is inversely proportional to the mass of the object: a = F m Overview This lab is composed of three tasks utilizing PhET (Physics Education technology) simulations. You will be using this interactive web-based program to complete this lab. If you have questions or problems with the interactive website, be sure to ask your TA for assistance. Instructions : Download and save this document to your computer. Answer the questions directly in this document. When you are done, SAVE the file as “PHYSICS LAB 2”, and return it to your TA via BB Learn. Please contact your TA with any questions or other issues. Go to the web page https://phet.colorado.edu/en/simulation/forces-and-motion-basics . Click on the Forces and Motion: Basics icon. This will take you to the following page. Task #1 will utilize the Net Force simulation, task #2 will utilize the Motion simulation, and task #3 will utilize the Friction simulation. . 1

PHY111/161 ON-LINE LAB #2 Forces and Motion Task #1 : Click on the first icon Net Force for the following simulation . In the top right hand corner of the simulation are three check boxes. Check the boxes Sum of Forces , Values , and Speed . Now, by clicking and dragging, place the two small blue men on the left side of the cart and the big man on the right side of the cart. Press GO! To begin. Questions 1. What is the sum of the forces and in what direction does the arrow point? 2. What do you notice about the velocity of the cart as the simulation proceeds? Now, take some time trying various combinations of men on either side of the cart. Analyze different situations in which the left and right forces are balanced, and when they are not balanced. Take special note of how the velocity of the cart changes compared to the net force. 2 The sum of forces is 50N and the arrow is pointing to the right, where the big, red man is. The velocity of the cart accelerates as the simulation proceeds.

PHY111/161 ON-LINE LAB #2 Forces and Motion Questions 3. What is the correct, technical term that we use to describe a change in velocity ? 4. What do you notice about this change in velocity as it relates to the Sum of Forces? 5. Which one of Newton’s Laws describes this relationship? 6. Try to get the cart to move when the left and right forces are balanced. Hint: You can add men to the rope when the cart is moving . Describe the steps you took. What is happening to the speed in this case? 3 The correct, technical term used to describe change in velocity is acceleration. We know force on an object , Force= (mass x acceleration), Sum of forces = mass x (change in velocity/ time). So, then Acceleration= (change in velocity/ time) The 2nd law of Newton states or describes this relationship. F=ma F= mass x (change in velocity/ time) I rebooted the program and positioned hefty individuals on either side of the carriage. Afterward, I pressed the "Go" button and observed the simulation in progress. It became evident that the cart remained stationary in both directions, indicating an absence of speed or velocity due to the equilibrium of forces acting on either side. The total force in this simulation equated to zero, resulting in a speed of zero in this scenario.

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PHY111/161 ON-LINE LAB #2 Forces and Motion Task #2 : Click on the Motion icon at the bottom of the page. Spend a few moments familiarizing yourself with the controls for this simulation. You will notice that the slide bar on the bottom allows you to change both the amount of applied force and the direction. Once you are familiar with the controls, check all of the boxes in the upper right hand corner. Select the 200 kg refrigerator, use the slide bar to continuously apply a force for a few seconds, and note how the speed changes. Release the mouse to bring the applied force back to zero and note how the speed changes. Questions: 1. What happens to the velocity of the cart as you are applying the force? 2. What happens to the velocity of the cart when you discontinue applying the force? 3. Are these observations consistent with Newton’s first and second laws? Explain. 4 When there is force being applied to the cart, the velocity of the cart continues to accelerate, until you stop applying force to the cart. When I stopped applying the force to the cart, then the cart stays the exact same speed. Indeed, these observations align with Newton's first and second laws of motion. The cart exemplified the first law as it remained at rest until an external force acted upon it. Moreover, when the force ceased, the cart maintained a constant speed unless another force acted to halt it. Additionally, the cart exemplified the second law as it exhibited accelerated motion when subjected to a force, achieving a higher rate of acceleration.

PHY111/161 ON-LINE LAB #2 Forces and Motion Now, reset the simulation, check the boxes in the upper right corner, and work through the following cases. To measure times, you can use the stopwatch app on your phone or some other timer that you may have available to you. Case 1: Place the 40 kg child on the skateboard. Continuously apply a 500 N force to the right until the skateboard is moving at 40 m / s . Note the time it takes to do this and record it in the Time column of the table below. Case 2: Repeat case 1 but use a force of 100 N to the right. Note the time it takes to do this and record it in the Time column of the table below. Case 3: Repeat case 2 but use the 200 kg refrigerator instead of the child. Note the time it takes to do this and record it in the Time column of the table below. Case 4: Repeat case 3 but choose your own object (NOT the present) and your own applied force. Note these values in the Mass and Applied Force columns of the table. Note the time it takes to do this and record it in the Time column of the table below. Data Table Cas e Mass Applied force Time (in s ) Calculated Acceleration (in m / s 2 ) Calculated Velocity or Speed (in m / s ) 1 40 kg 500 N 3.23 12.5 40.375 2 40 kg 100 N 15.5 2 2.5 38.8 3 200 kg 100 N 80.06 .5 40.03 4 80kg 200N 15.75 2.5 39.375 Questions 4. For each case, use Newton’s second law ( a = F m ) to calculate the acceleration of the skateboard. Record your value in the Calculated Acceleration column of the table above. 5. When an object starts from rest, its velocity (speed) is determined from the equation v = at . For each case, multiply the time (second column) by the acceleration (third column) to get the calculated velocity and record your value in the Calculated Velocity column of the table above. 5

PHY111/161 ON-LINE LAB #2 Forces and Motion 6. How do your calculated velocities compare to the actual velocity (speed) displayed on the dial of 40 m / s ? Where do you suppose the error is coming from in the experiment? 7. Reset the simulation and place the present on the skateboard. As carefully as you can, measure the time it takes to reach a velocity (speed) of 40 m / s with an applied force of 100 N. Time = 19.85 s 8. For straight-line motion, the average acceleration is equal to change in the velocity (speed) divided by the change in time. If the skateboard’s velocity changes by 40 m / s , use your measured time to calculate the average acceleration of the skateboard. Average acceleration = 2.015 m / s 2 9. Using the average acceleration and the force with which the skateboard is pushed, we can use Newton’s second law to determine the mass of the present through the equation m = F a . Using the applied force of 100 N and your average acceleration calculated in question 8, determine the mass of the present. Mass of present = 49.625 kg In several ways, this simulation may not seem realistic. In particular, as we all know, when we push a cart or anything else on a flat surface, once we stop applying a force it will eventually stop, even on wheels. Does this fact defy Newton’s Laws? No, because there is always force working in the opposite direction to the one we are applying, the Force of Friction. Even a rolling cart has frictional force working on it. 6 In Trial 1, the calculated velocity measures 40.375, just 0.375 units shy of the target of 40, indicating a close approximation to the actual velocity. Trial 2, on the other hand, yields a calculated velocity of 38.8, deviating by 1.2 units from the desired

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PHY111/161 ON-LINE LAB #2 Forces and Motion TASK #3 : Click on the Friction icon at the bottom of the page. Spend a few moments familiarizing yourself with the controls for this simulation. You will see that this simulation is nearly identical to the previous one. The only difference is the addition friction. Once you are familiar with the controls, check all of the boxes in the upper right hand corner. Slide the Friction selector all the way to the left or None. Apply a continuous force for a few moments and you will notice that the box will continue to move even after you discontinue applying the force. Reset the simulation and set the friction force to the center of the slide bar. Slowly start applying a force to the box. It should take a bit to finally get it moving. Also, try out different masses, and note the force required to get the crate to move. Questions 1. What do you notice about the Applied Force compared to the Frictional Force before the crate begins moving? 2. What do you notice about the Applied Force compared to the Frictional Force after the crate begins moving? 3. Does the mass of the object affect the Frictional Force ? 7 As the simulation initiates the application of force to the box, both the applied force and the frictional force are equal in magnitude, measured in Newtons. Once the crate begins moving, the applied force exceeds the frictional force. Indeed, the object's mass does have an impact on the frictional force.

PHY111/161 ON-LINE LAB #2 Forces and Motion Now, reset the simulation and check all the boxes in the upper right corner. Select a crate (50 kg) and work through the following cases, filling out the data table as you go. Case 1: Drag the friction slider to about one-third of its maximum value. Slowly increase the Applied Force until the crate just begins to move (use the single arrow buttons to increase the applied force by 1 N and the double arrow buttons to increase the applied force by 50 N). Note that you want the smallest applied force that gets the crate to move from rest ! Record that force in the second column of the table below. If you continue to apply this force, the crate will accelerate . As the crate is moving, slowly decrease the Applied Force until it moves at a constant velocity. Record the Applied Force for which this occurs in the fourth column of the table below. Case 2: Repeat case 1, but with the friction slider at about half of its maximum value. Case 3: Repeat case 1, but with the friction slider at about two-thirds of its maximum value. Data Table Case Force needed to move crate from rest (in N) Coefficient of static friction Force needed to move crate at constant speed (in N) Coefficient of kinetic friction 1 73 .146 54 .108 2 126 .252 94 .188 3 179 .328 134 .268 Friction explained While the details of friction are rather complicated, it turns out that we can develop a model that works quite well in describing its macroscopic effects. Experimentally, for pushing an object along a flat surface, the frictional force is proportional to the weight of the object. The proportionality constant is known as the coefficient of friction, denoted by μ . To calculate it, we simply divide the force of friction by the weight of the object. We can do this for cases where the object is not moving (obtaining a static coefficient of friction μ s ) and for cases when the object is moving (obtaining a kinetic coefficient of friction μ k ). The weight of the crate is equal to W = 500 N . For each case, divide the frictional force (for both the static and kinetic cases) by 500 N to get the friction coefficient. Record your values in the table above. Note that the coefficient of friction is a number (no units) between 0 and 1. The smaller the coefficient of friction, the smaller the force of friction. The actual value of μ s and μ k depend on what types of surfaces are in contact with each other. Open the following link: http://hyperphysics.phy-astr.gsu.edu/hbase/frict2.html 8

PHY111/161 ON-LINE LAB #2 Forces and Motion You are not expected to read the entire page, but please look at the graphs showing the interplay of static friction and kinetic friction . In the first two graphs there are boxed captions pointing to specific locations on the graphs. Please read these. Questions 1. What happens at the Threshold of motion to the total amount of frictional force? 2. Once past the Threshold of motion, is there any static friction ? Explain. 3. Before the Threshold of motion, is there any kinetic friction ? Explain. Static friction is essential for you or me to be able travel anywhere on the surface of the Earth . That statement may sound counter intuitive when you first read it, but it is quite true. Watch the following video for a brief explanation: https://www.youtube.com/watch?v=CTLXubXOTUQ Question 4. Describe two examples of static friction talked about in the video. 9 At the moment when motion is about to begin, the frictional force decreases below the magnitude of the applied force, indicating that the applied force exceeds the frictional force, thus resulting in motion. No, static friction ceases to exist after the threshold because static frictional forces, generated by the interlocking of surface irregularities, intensify to prevent relative motion until the point where motion begins. Therefore, beyond the threshold, there is motion, and static friction no longer applies. No, there is no kinetic friction occurring prior to the threshold of motion because kinetic friction is defined as the frictional resistance that remains nearly constant when two surfaces are in relative motion, even at low speeds within a wide range. The force between the tires of a car and the road. The object on the slightly tilted brick because there is no motion between the brick and the object. The force required for us to walk. In order to walk across the floor, the floor has to push you forward and your shoes are pushing backward on the floor.

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PHY111/161 ON-LINE LAB #2 Forces and Motion Conclusion Take a moment to reflect on all the simulations and tasks you’ve done with this lab. In the space below, briefly discuss any new insights into Forces and Motion that you’ve gained. Was there a particular simulation, website, or video that helped to solidify your conceptual understanding? How so? 10 The video unquestionably enhanced my comprehension of the distinction between static and kinetic friction. It provided relatable examples, particularly the one involving foot and ground interaction. This specific example allowed me to personally experience the concept by feeling the resistance of pushing against the ground and the ground's reciprocal push, thereby deepening my grasp of static friction.