Lab Changing Motion

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Physics

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Dec 6, 2023

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Save your work frequently! Name: Katen Gulati Section 2:30-4:20 LAB: CHANGING MOTION Learning goals:  Translate among the following representations of motion: verbal, motion diagram, and graphical: For example, given a verbal description of motion, create x - t , v - t , and a - t graphs; given a motion diagram, describe the motion in words, etc.  Given a v - t graph, determine average acceleration during a given time interval, and construct an a — t graph.  Determine change in velocity during a given time interval from an a — t graph.  Qualitatively and quantitatively reason with definition of acceleration.  Represent a physical quantity with a vector.  Add and subtract vectors graphically.  Use vector subtraction of velocity vectors and the definition of acceleration to determine the direction of acceleration. Materials:  Computer  Motion detector  Experiment files  Cart  Track  Fan unit In the previous lab, you looked at position-time and velocity-time graphs of the motion of your body and a cart at a constant velocity. The data for the graphs were collected using a motion detector. Your goal in this lab is to learn how to describe various kinds of motion in more detail. You have probably realized that a velocity-time graph is easier to use than a position-time graph when you want to know how fast and in what direction you are moving at each instant in time as you walk (even though you can calculate this information from a position-time graph). It is not enough when studying motion in physics to simply say that "the object is moving toward the right" or "it is standing still." When the velocity of an object is changing, it is also important to describe how it is changing. The rate of change of velocity with respect to time is known as the acceleration. To get a feeling for acceleration, it is helpful to create and learn to interpret velocity-time and acceleration-time graphs for some relatively simple motions of a cart on a smooth ramp or other level 1

Save your work frequently! surface. You will be observing the cart with the motion detector as it moves with its velocity changing at a constant rate. Investigation 1: Velocity and acceleration graphs In this investigation you will be asked to predict and observe the shapes of velocity-time and acceleration- time graphs of a cart moving along a smooth ramp or other level surface. You will focus on cart motions with a steadily increasing velocity. 1-1: Speeding Up In this activity you will look at velocity- time and acceleration- time graphs of the motion of a cart, and you will be able to see how these two representations of the motion are related to each other when the cart is speeding up. This could be done by moving the cart with your hand, but it is difficult to get a smoothly changing velocity in this way. Instead you will use a fan or propeller driven by an electric motor to accelerate the cart. 1. Set up the cart on the ramp, with the fan unit and motion detector as shown below. Be sure that the ramp is level. 2

Save your work frequently! 2. Open the experiment file called Speeding Up (L02Al-1) to display the axes that follow. 3. Use a position graph to make sure that the detector can "see" the cart all the way to the end of the ramp. You may need to tilt the detector up slightly. 4. Hold the cart with your hand on its side, begin graphing , switch the fan unit on LOW and when you hear the clicks of the motion detector, release the cart from rest. Do not put your hand between the cart and the detector. Be sure to stop the cart before it hits the end stop. Turn off the fan unit. Repeat, if necessary, until you get a nice set of graphs. Adjust the position and velocity axes if necessary so that the graphs fill the axes. Use the features of your software to transfer your data so that the graphs will remain persistently displayed on the screen . Also save your data for analysis in Investigation 2. (Name your file SPEEDUPl.XXX, where XXX are your initials.) 5. Insert your position and velocity graphs on the next page. Label the graphs "Speeding Up 1." (Ignore the acceleration axes for now.) 3

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Save your work frequently! Question 1·1: How does your position graph differ from the position graphs for steady (constant velocity) motion that you observed in Lab 1: Introduction to Motion? The position graph for a steady graph has a constant change in position over time, while this graph did not have a constant slope but was changing position by more and more each time. This is different as the graphs from Lab 1 because they both have constant velocity which means a constant change in motion. Question 1-2: What feature of your velocity graph signifies that the motion was away from the motion detector? It has a positive velocity, which means it is moving right. A positive velocity means that it is speeding up. Question 1-3: What feature of your velocity graph signifies that the cart was speeding up? How would a graph of motion with a constant velocity differ? The velocity had a positive slope which means the speed is increasing and it was positive, so it travels faster and faster each time in turn speeding up. Question 1-4: During the time that the cart is speeding up, is the acceleration positive or negative? How does speeding up while moving away from the detector result in this sign of acceleration? (Hint: Remember that acceleration is the rate of change of velocity. Look at how the velocity is changing. It takes two points on the velocity-time graph to calculate the rate of change of velocity.) When the cart is speeding up, the acceleration is positive, when velocity is increasing, the change in velocity is increasing which means it is positively increasing. Question 1-5: How does the velocity vary in time as the cart speeds up? Does it increase at a steady (constant) rate or in some other way? Velocity increases at a steady constant rate which causes the position to increase exponentially. Question 1-6: How does the acceleration vary in time as the cart speeds up? Is this what you expect based on the velocity graph? Explain. 5

Save your work frequently! The acceleration is constant because the velocity had a constant speed at which it was increasing, so there is a constant slope that is causing to increase which is the acceleration. For instant, the slope of the velocity graph is the constant value for acceleration. Question 1-7: The diagram below shows the positions of a cart at equal time intervals as it speeds up. Assume that the cart is already moving at t 1 . At each indicated time, sketch a vector above the cart that might represent the velocity of the cart at that time while it is moving away from the motion detector and speeding up. To the right and a positive slope. Question 1-8: Show below how you would find the vector representing the change in velocity between the times 2 and 3 s in the diagram above. ( Hint: Remember that the change in velocity is what you need to add to the initial velocity to give the final velocity.) The position is increasing at a greater rate, so velocity increases and is moving to the right. Based on the direction of this vector and the direction of the positive x axis, what is the sign of the acceleration? Does this agree with your answer to Question 1-4? Acceleration is positive. This is because if velocity is increasing in a positive direction, then acceleration must be positive to be the slope for it to increase in a positive manor. Activity 1-2: Speeding Up More Prediction 1-1: Suppose that you accelerate the cart at a faster rate. How would your velocity and acceleration graphs be different? Draw your predictions in Logger Pro. Your velocity would have a greater slope, which means a greater change in position per second. 6

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