41 Name________________________Date______________Partners________________________________ University of Virginia Physics Department Modified from P. Laws, D. Sokoloff, R. Thornton PHYS 635, Summer 2009 Supported by National Science Foundation and the U.S. Dept. of Education (FIPSE), 1993-2000 LAB 3: VELOCITY AND ACCELERATION A cheetah can accelerate from 0 to 50 miles per hour in 6.4 seconds. –Encyclopedia of the Animal World A Jaguar can accelerate from 0 to 50 miles per hour in 6.1 seconds. –World Cars OBJECTIVES • To discover how and when objects accelerate. • To understand the meaning of acceleration, its magnitude, and its direction. • To discover the relationship between velocity and acceleration graphs. • To learn how to find average acceleration from acceleration graphs. • To learn how to calculate average acceleration from velocity and position graphs. OVERVIEW 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. You also looked at the acceleration-time graph of the cart. 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 simple motions of a cart on a smooth, level ramp. You42 Lab 3 - Velocity & Acceleration University of Virginia Physics Department Modified from P. Laws, D. Sokoloff, R. Thornton PHYS 635, Summer 2009 Supported by National Science Foundation and the U.S. Dept. of Education (FIPSE), 1993-2000 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, level ramp. You will need the following materials: • motion detector • motion cart without friction pad • 2-m motion track • fan unit attachment with 4 batteries and 1 dummy cell • level • small screwdriver (to help remove batteries) ACTIVITY 1-1: MOVING AWAY AND 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 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. Make sure the fan switch is off, then place three batteries and the dummy cell in the battery compartment of the fan unit. Place the extra battery in the clips on top of the fan unit. We want to make sure the fan cart has a constant mass. To preserve the batteries, switch on the fan unit only when you are making measurements 2. Set the cart on the ramp, with the fan unit and motion detector as shown below. Verify that the ramp is level and that the fan blade does not extend beyond the end of the cart facing the motion detector. [If it does, the motion detector may collect bad data from the rotating blade.] Make sure the end stop is present in order to prevent the cart from going off the end of the track. 3. Put the switch on the motion detector to narrow beam.Lab 3 - Velocity & Acceleration 43 University of Virginia Physics Department Modified from P. Laws, D. Sokoloff, R. Thornton PHYS 635, Summer 2009 Supported by National Science Foundation and the U.S. Dept. of Education (FIPSE), 1993-2000 4. Open the experiment file called L03.A1-1 Speeding Up. Check “Setup”. 5. Start taking data and 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 or down slightly. Clear all data runs before moving on. 6. Hold the cart with your hand on its side, switch the fan unit on and Start taking data. 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 the program from taking data if it has not already shut off. Turn off the fan unit. If the batteries are weak, the cart may not accelerate. Contact your Instructor if this happens. Repeat, if necessary, until you get a nice set of graphs. The acceleration graph will have several ups and downs because of the instrument resolution and small bumps in the track. Adjust the position and velocity axes if necessary so that the graphs fill the axes. 7. Print out one set of graphs for your group report. Do not erase your data. Remember what data set this is. Label these graphs “Speeding Up 1”. Question 1-1: What feature of your velocity graph signifies that the cart was moving away from the motion detector and speeding up? How would a graph of motion with a constant velocity differ? Question 1-2: During the time that the cart is speeding up, is the acceleration mostly 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.) End Stop Motion Detector 20 cm44 Lab 3 - Velocity & Acceleration University of Virginia Physics Department Modified from P. Laws, D. Sokoloff, R. Thornton PHYS 635, Summer 2009 Supported by National Science Foundation and the U.S. Dept. of Education (FIPSE), 1993-2000 Question 1-3: How does the acceleration change in time as the