Experiment 3 Newton s Second Law and the Atwood Machine This experiment will demonstrate the mechanical laws of motion using a simple assembly named the Atwood machine that is similar to that used by Rev George Atwood in 1784 to verify Newton s Second Law Materials Masking Tape 2 Paperclips Pulley 5 N Spring scale Stopwatch String Tape measure 15 Washers Procedure Part 1 1 Support the pulley so that objects hanging from it can descend to the floor Do this by tying a short piece of string to one of the pulley hooks Use a piece of masking tape to secure the string to a table top or door frame so that the pulley hangs plumb Figure 5 Note A higher pulley support will produce longer time intervals which are easier to measure 2 Thread a piece of string through the pulley so that you can attach washers to both ends of the string The string should be long enough for one set of washers to touch the ground with the other set near the pulley You may attach the washers using a paperclip or by tying them on 2014 eScience Labs LLC All Rights Reserved 3 Use the spring scale to weigh the set of fifteen washers Divide the total mass by fifteen to find the average mass of a washer Record the total mass of the washers and average mass of one washer in Table 5 4 Attach seven washers to each end of the string 5 Observe how the washers on one side behave when you pull on the washers on the other side 6 Add the remaining washer to one end of the string so one side of the string has seven washers M1 and the other has eight washers attached to it M2 Answer Post Lab Question 1 based on your observations 7 Place M1 on the floor Use the tape measure to measure the height that M 2 is suspended while M1 is on the floor Measure the distance M2 will fall if you release the light set when it is in contact with the floor Record the distance in Table 5 8 Time how long it takes for M2 to reach the floor Repeat Steps 7 8 four more times five times total recording the values in Table 5 Calculate the average time 9 Calculate the acceleration assuming it is constant from the average time and the distance the washers moved Part 2 1 Transfer one washer so that there are six on one end of the string M 1 and 9 on the other M2 2 Determine the mass on each end of the string 3 Place the M1 on the floor Measure the height that M2 is suspended at while M1 is on the floor Measure the distance M2 will fall if you release the light set when it is in contact with the floor 4 Time how long it takes for the heavy set of washers to reach the floor Repeat Steps 34 five times recording the values in a table and then calculate the average time 5 Calculate the acceleration assuming it is constant from the average time and the distance the washers move Table 5 Motion Data Mass of 15 Washers kg 054 kg Average Mass of Washer kg Procedure 1 Height m 0 56 m Trial Time s 1 1 28 s 2 1 38 s 3 1 46 s 2014 eScience Labs LLC All Rights Reserved 0 0036 kg 4 1 41 s 5 1 44 s Average 1 394 s Average Acceleration m s2 0 288 m s2 Procedure 2 Height m 0 57 m Trial Time s 1 1 00 s 2 0 91 s 3 0 91 s 4 1 09 s 5 1 03 s Average 0 998 s Average Acceleration m s2 0 57 m s2 Post Lab Questions 1 When you give one set of washers a downward push does it move as easily as the other set Does it stop before it reaches the floor How do you explain this behavior 2 Draw a free body diagram for M1 and M2 in each procedure Procedure 1 and Procedure 2 Draw force arrows for the force due to gravity acting on both masses Fg1 and Fg2 and the force of tension FT Also draw arrows indicating the direction of acceleration a 3 Use Newton s Second Law to write an equation for each of the free body diagrams you drew in Question 2 Note Be sure to use the correct signs to agree with your drawings Solve these four equations for the force of tension FT You answer should be in variable form 2014 eScience Labs LLC All Rights Reserved 4 Set the two resulting expressions for the force of tension equal to one another as long as the string does not stretch the magnitude of the acceleration in each equation is the same Replace Fg1 and Fg2 with M1 and M2 respectively Solve the resulting equation for a Then go back to Question 3 and solve for the FT 5 Calculate the acceleration for the two sets of data you recorded and compare these values to those obtained by measuring distance and time using percent error What factors may cause discrepancies between the two values 6 Calculate the tension in the string for the falling washers From these two values and the one where the masses were equal what trend do you observe in the tension in the string as the acceleration increases 2014 eScience Labs LLC All Rights Reserved
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