LAB: FORCE AND MOTION

Transcription

1 LAB: FORCE AND MOTION Introduction In this lab we will apply a force to a cart and look at the motion that results. Therefore, we are asking the question: "How does the motion depend on the force?" More specifically, we will look at a particular motion variable. Which motion variable do you suppose it is? (Hint: Look at Newton's 2 nd law.) What is the variable that will not be changing (kept constant)? (Hint: It is also in Newton's 2 nd law.) Using different words than mine, and including the particular motion variable that is suggested by Newton's 2 nd law, write a purpose for the lab in the space below. Include the variables that we will be comparing. Make sure you also mention the things that will be kept constant. Purpose: Write a purpose using the information in the introduction. Materials You will need the following materials to complete this activity: Low-Friction Cart Frictionless Pulley Track (we will use PASCO tracks this time) 2 PASCO Photogates (1 pair) String Set of 50 or 100-Gram Masses Mass Hook Setting Up The Experiment 1. If it hasn t been already, attach the frictionless pulley to the end of the track. 2. Set the track on a lab bench so that the end with the pulley just hangs over the edge of the table. 3. Tie a loop at one end of the string, and attach the other to the low friction cart. (You will attach the mass hook to the loop at the end.) 4. Place the cart on the track, and run the string over the frictionless pulley. You will set up the photogates in a few moments Measuring the Variables You will need to work through the following questions before conducting the experiment.

2 Lab: Force and Motion page 2?? Setting up the Photogates The instant you let the mass go, the cart will move. This motion is what we need to look at. (Remember the purpose?) You will need to measure some motion information each time a different weight force is applied. You will use the photogates to take measurements that can be used to find the acceleration. To help you figure out how to come up with a procedure to find the acceleration, let s look at what you can measure. That is what you have to work with. The possible kinematic quantities are: Acceleration Position Time Velocity You can't measure acceleration directly. So what can you measure directly? Hint: What can you measure with photogates? What can you measure with the markings on the track??? Finding Acceleration a.) Let's assume that you have made a measurement. Finish the Motion Diagram below according to what you know before running the Cart. Then circle the values that you will know after making a run with the Cart. x v t a x v t b.) Use your kinematic equations to come up with a method to calculate the acceleration of the cart from the values that you can measure. Explain your method. Clearly show your equation. Make sure that you use this method later when calculating the acceleration from your measured data. Check with your instructor at this point.

3 Lab: Force and Motion page 3 Theory: You will need to write an experimental theory for this lab. Think about all the things discussed above and well as what you expect for an answer to your purpose. Be sure to cite any ideas you use from the text or another person s work. Procedure: You will have to make several decisions in order to complete the procedure. For example, how will you decide how far the cart can run? Your Track has a special stopping barrier that will need to be set up to prevent the Cart from going too far. Listen in class for directions on how to change the applied force. Make one sample run to check out everything and then summarize your procedure: Conducting the Experiment You are now ready to run the experiment. 1. Hold the cart at its starting position. Attach the mass hook (which has a mass of 50 g) to the string, and place the 50 and 100 g masses on the cart. 2. Let the cart go, and record the necessary data in a table.?? Observing the Motion Let s make sure that you are observing well and that you know how to do the calculations. a.) What is the total amount of mass that is accelerating? (Include uncertainty.) b.) What is the hanging mass? (Include uncertainty.) c.) What else was measured? (Include uncertainty.) d.) Calculate the force. e.) Calculate the acceleration. Check with your instructor before going to step 3 of the experiment. 3. Move one 50 g mass from the cart to the hanger and take data for another run. Continue to move 50 g masses from the cart to the hanger.

4 Lab: Force and Motion page 4?? Do you understand? Why do we change the masses this way, instead of simply adding mass to the hanger? Data What columns will you need in a data table for your measured data? What columns will you need in a data table for your derived data? Create a table for both measured and derived data. Be sure to label the columns with units. The measured data should also have a +/- range heading to the columns. Analyzing the Results Plot a Force vs. Average Acceleration graph.?? Analysis from your graph We will now look at the graphed results and come to an understanding of the relationship between force and acceleration from the graph. Give your observations below: a.) From your graph, write the equation of the best-fit line. Include all your slope calculations. Don't forget to include your slope units. b.) What does the shape of the graph tell you about how force and acceleration are related? Do not use numbers here - just explain using qualitative words like "more", "less", "up", or "down". c.) Now be more specific. What does the equation of your line tell you about the relationship? It should match the qualitative observation that you made above. d.) What is the physical meaning of the slope of the line? e.) What is the physical meaning of the value of the "y-intercept"?

5 Lab: Force and Motion page 5 Conclusion Write a short conclusion that answers the question you wrote in your Purpose.?? What if? Think about the following situation: What if you repeated the experiment with a bigger cart with twice the mass? You might get different data this time. If you then plotted a Force vs. Acceleration graph, how would it compare to the graph you got when you did the experiment with the smaller cart? Sketch a Force vs. Acceleration graph that shows your prediction. Include a sketch of the graph of your experimental result on the same sketch so that they can be compared. Explain your reasoning. Turn In: A Formal Lab Report, written in your own words and including everything suggested above.