Idea Transcript
SP211 Lab: Three Newton’s Laws Version: September 12, 2014
PHYSICS LAB 3 SP211
Newton’s Laws I. Introduction Newton (1642 ‐ 1727) was able to develop much of classical mechanics by developing the three laws of motion which now bear his name (Chapter 5 of our book).
Newton’s laws will be relevant around us out in the Fleet or even in the Marine Expeditionary Unit (for those that choose that option).
(Pictures taken from US Navy Enlisted Training Manuals)
The goal of this experiment is to find ways Newton's laws influence physical systems. In particular we will test his second Law using two different experiments. Page 1 of 8
SP211 Lab: Three Newton’s Laws Version: September 12, 2014
II. Objectives At the end of this activity, you should: 1. Be able to see that F = ma. 2. Be able to prove that the acceleration of an object on an inclined plane (the cart) = g * sin (theta) within uncertainty. 3. Be able to discuss the specific experimental examples of each of Newton’s three laws. 4. (Optional based on your instructor) Be able to use an Excel spreadsheet and specifically the Excel function “Linest,” a powerful linear regression tool.
III.Needed Equipment Your instructor will show you the experimental setup, which consists of a motion sensor (MS), a force sensor (FS), an inclined plane and a dynamics cart as well as the required calibration procedures. Force Sensor: The PASCO Force Sensor CI‐6618 is shown below. The FS detects a force (push or pull) on the small metal hook attached to one end. (The small metal hook can be replaced by different attachments such as a clay holder or spring.) The FS generates an electrical signal that is proportional to the force. Once the FS is calibrated (electrical signal correlated with a known force), the LabPro/computer combination translates any electrical signal into a force. Please be careful with the FS. It is a delicate, scientific instrument.
There is a small selector switch on the face of the FS. This sets the detection limits of the electrical equipment. It can detect a force (push or pull) of up to either 10 N or 50 N. We will primarily use the sensor in the ±50 N range. Please do not apply a force to the FS exceeding 50 N (about 11 pounds). It will be permanently damaged!
There may be a button on the side labeled Push To Tare. NEVER PUSH THE TARE BUTTON. All zeroing and calibrating will be done via computer.
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SP211 Lab: Three Newton’s Laws Version: September 12, 2014
IV. Turn in your Pre‐lab/homework problem if assigned. V. Discussion Your instructor will demonstrate the experimental setup, the required calibration procedures, and how to take data.
VI. Procedure
The experiment deals with the classic Newton’s Second Law (NSL) physics problem of an object on an inclined plane. In our case the object will be a cart. First we will deduce the force down the plane on a cart both theoretically and experimentally. Then in experiment two we will measure the acceleration of the cart down the inclined plane and compare it to the prediction of NSL. Experiment 1:
Experiment 2:
A.
Preliminary Data: A.1. Set your track at a ramp angle of about 5˚, and then measure the angle carefully by making a graph of the ramp’s height as a function of distance along its edge. Use the function in Excel called “Linest” to find sin and its uncertainty. A.2. Measure and record the mass of the cart along with its uncertainty. A.3. Remember that g = +9.810 +/‐ 0.005 m/s2
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SP211 Lab: Three Newton’s Laws Version: September 12, 2014
B.
Connect the two devices to the Lab‐Pro and set up Logger‐Pro Check that everything is plugged in correctly: the MS into DIG/SONIC2 and the FS into CH1.
Recurring steps: Using Logger Pro to program the LabPro: Known as the “Three Steps”
Experiment ‐> Set Up Sensors ‐> Show all Interfaces: Make sure all the right sensors show up in all the right holes.
Experiment ‐> Data Collection: Set the length of time data is to be taken and the rate at which data is to be taken.
File ‐> Settings for...: Check the checkbox to Show Zero on Toolbar, and make sure number of points for Derivative and Smoothing are both 7.
It is necessary to calibrate the FS any time that we start LoggerPro. In case you do not recall the procedure for calibration, it is as follows:
Attach the FS to the track. Place a “pulley” at the high end of the track and hang the 200g weight from it.
Under the Experiment menu, click on Calibrate. Then Select CH1: Dual Range Force and click on Calibrate now.
For Reading 1, wait for the voltage reading to become steady, change Enter Value to 1.96N (which is the weight of the 200g mass). Click the Keep button.
For Reading 2, hang the 500 g mass from the hook located on the Force Sensor. Wait for the voltage reading to become steady, and then change Enter Value to 4.9 (which is the weight of the 500g mass) and click the Keep button.
Click the Done button.
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SP211 Lab: Three Newton’s Laws Version: September 12, 2014
C.
Experiment 1: Stationary Cart with Newton's First and Second Laws.
C.1. REMEMBER to calibrate your force sensor using the procedure demonstrated by your instructor. That is: Place the FS on the track, note how it is not reading zero due to its orientation (the hook is now pressing the sensor) then zero it before placing the cart on the track. C.2. Draw a free body diagram (FBD) for the Cart, identifying the forces acting on it. C.3. If the cart is not moving, the net force acting on it must be zero. From your FBD, add the three vectors together to show that the force applied by the cart on the FS is Fcart on FS = mg sin. C.4. Calculate the expected force on the FS from the cart from the preliminary data. Show your work and calculate uncertainty on your prediction. C.5. Measure the force using your LabPro for a few seconds. Use Analyze‐> Statistics to find the best estimate of this force and its uncertainty. C.6. In a short paragraph, discuss whether or not the force you predicted the cart would exert on the Force Sensor is consistent with what you actually measured. That is, do they agree?
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SP211 Lab: Three Newton’s Laws Version: September 12, 2014
D.
Experiment 2: Moving Cart and Newton's Second Law.
D.1. Without the force sensor in place, the cart will accelerate down the plane. Do the experiment (no crashes, please!) and use the MS to measure the motion of the cart. You may want to push the cart up the plane and then have it roll down under gravity instead of just releasing it. Create a graph showing the motion as a function of time, and use it to determine the cart’s acceleration, and then the net force on the cart. Don’t forget uncertainty! D.2. Apply Newton’s Second Law to a free body diagram of the ramp on the cart to predict the net force on the cart. D.3. In a short paragraph be sure to address the following points:
a) Do your predicted and measured net forces agree?
b) What does this say about Newton's Second Law? c) With respect to the setups of Experiments 1 and 2, describe one example of each of Newton's three laws.
Ask you instructor if she/he wants you to continue with the LAB or proceed to section VII (Lab Report).
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SP211 Lab: Three Newton’s Laws Version: September 12, 2014
E.
(Time permitting) Experiment 3: Atwood style setups.
E.1. Mount the force sensor on the dynamics cart and mass the system of cart and force sensor as m1. Now take the mass of one of the hanging weights (0.1 kg) and record it as m2. Create the following experimental setup where the string from the hanging mass ties onto the hook on the force sensor. Don't forget to re‐zero the force sensor
E.2. Draw a free body diagram showing the three forces acting on m1 (normal force from the track, force of gravity on the cart, tension of the rope from the hanging mass). Two of them sum to zero. Name them and give a physical reason for why we know they sum to zero. E.3. The tension in the rope comes from the hanging mass m2, which is influenced by gravity and feels a force. This force accelerates the system. Start a data collection and release the cart two seconds after the motion sensor starts working, collecting the velocity vs. time from the motion sensor (and then fitting a line to get the acceleration) and also the average force over the trip. Perform statistics on the force before and after you release the cart, fit a quadratic to the position vs. time graph and fit a line to velocity vs. time. Print this graph. E.4. The hanging mass m2 is falling under a gravitational force. Discuss why the mass is not accelerating at 9.8 m/s2? E.5. Compare the tension in the rope before it is released to the tension in the rope after it is released. E.6. Within the uncertainty, is the tension in the rope equivalent to the acceleration of the cart? Page 7 of 8
SP211 Lab: Three Newton’s Laws Version: September 12, 2014
VII.
Lab Report to Hand In: A. Your predicted value of g*sin(); A Spreadsheet (Part A and C.4) is probably the quickest and cleanest way to present this. B. Data/graphs from Parts C, D, and E above (as applicable) with discussions written on graphs or on the back. Write the names of all of your lab partners on your lab report.
VIII. Clean‐Up A. Golden Rule: “Do unto others as you desire them to do unto you.” This applies as much here in the lab as it does in the Fleet. As future Naval Officers, how can you expect your enlisted sailors to maintain a clean work area if your stateroom, work areas, mess area, etc is a “pig sty?” So as officers it is imperative that we clean up after ourselves not only to follow the Golden Rule, but also to lead by example for the enlisted personnel under our charge. 1. End of Lab Checkout: Before leaving the laboratory, please tidy up the equipment at the workstation and quit all running software. 2. The lab station should be in better condition than when you arrived and more importantly, should be of an appearance that you would be PROUD to show to your legal guardians during a “Parents Weekend.” 3.
Have your instructor inspect your lab station and receive their permission to leave the Lab Room.
4. You SHALL follow this procedure doing every lab for BOTH SP211 and SP212! Many thanks to Dr. Huddle for his assistance in producing this Laboratory procedure; specific references can be supplied on request. LCDR Timothy Shivok
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