Unit 5 Guided Work Sheet Sci 701 NAME: 1) Define the following key terms. Acceleration DC motor Direct current (DC) Force Power
Shaft Speed Torque Work Wrench flat
1. Determine free wheel speed and stall torque of the vex motor Attach a zip tie to the 4 shaft between the tank tread pulley and the motor. The zip tie serves as a visual reference when counting motor revolutions. Plug the motor into port 6 of your Microcontroller. Plug the Transmitter into port Rx1 of the Microcontroller. Plug the 7.2V battery into the Microcontroller. Turn both your Transmitter and Microcontroller on. Using the yellow buttons on the back of the Transmitter, you should be able to control the rotation of the pulley. Perform five tests, where you run the test stand for one minute. During each test, count the number of times the zip tie passes a reference point. For each test, record this number in the chart below. Calculate the average of your five tests. This number is your experimental determination of the free Wheeling RPM of a VEX Motor. Trial #1 #2 #3 #4 #5 # of Cycles Average ( ) 2. Determining stall torque. Remove the zip tie that was attached in the Free Speed section. Wrap the rope around the pulley, and pass the end of it through the rope guide. Stall torque is the amount of torque required to prevent the motor from spinning. Torque is defined as: Torque = Force x Radius Where force is the force of gravity downwards measured in pounds (lbs.), radius is the radius of the rotating object measured in inches (in.), and torque is the torque on the motor measured in inch pounds (in-lbs.). To determine the stall torque, you need to determine what force or weight will prevent the motor from spinning. To do so, you load the test stand with a weight, and keep increasing it until the motor can no longer spin. Set up the test between two tables, with the string hanging between them. Attach an initial weight to the string.(try approximately 3 lbs to start with) Using the Transmitter, rotate the pulley and lift the weight.
Keep on increasing the weight attached to the string by small amounts and try and rotate the pulley. Continue increasing the weight until the motor can no longer rotate. Record this amount of weight in the table below. Measure the radius of the pulley. Now using the force in pounds and the radius of the pulley in inches, you can calculate the stall torque of the motor in inch-pounds. Max Weight (lbs) Pulley Radius (in) Stall Torque 3. The actual free Wheeling RPM and stall torque of a VEX motor are approximately 100 rpm and 6.5 in-lbs. a) Do your results match these? b) Explain what factors could cause a variance between the actual results, and the results you obtained experimentally. c) How could these tests be improved to ensure a greater degree of accuracy?
4. a) Some amusement park rides pull riders slowly to the top of a large tower, and then send them plummeting in a free fall to the bottom again. This can be achieved with a large motor that rotates a shaft with a pulley and cable attached. Why is this big drop so thrilling? Why don't you feel the same rush flying across the country in an airplane, even though the plane moves much faster than the ride? Does your body feel speed? Does it feel acceleration? b) As you picture an amusement park ride in which you are pulled to the top and then do a feel fall, explain how the following physics concepts come into play: power, speed, acceleration, energy, force, and work. c) Is it possible to get a lighter load (fewer people) to the top faster than a maximum load? d) If you move a lighter load of people to the top at a faster speed, what happens with motor torque? e) When attempting to lift a load to the top of a drop ride, when might you reach stall torque? f) Why would an amusement park designer want to avoid a situation involving stall torque? g) What might happen to the motor if you were to reach stall torque?
5. Given equation: Force = Mass * Acceleration. For example, a force of 12 N (that is, Newtons) could accelerate a 3 kg mass at 4 m/s2, because 12=3*4. Or, it could accelerate a 6 kg mass at 2 m/s2, because 12=6*2. When you apply the same force to double the mass, you get half the acceleration. a) Suppose you were building a drop ride, and you want it to lift 16 people at a time. If we plan for an average mass of 95 kg per person, how much force would the ride's motor need to apply in order to exactly counteract acceleration due to gravity (9.8 m/s2)? b) How much force to accelerate the riders upward at 3 m/s2? (Note: your answer to the second question should be greater than your answer to the first. Think about why this might be, and discuss with a teammate.) c) If 16 football players (average mass: 140 kg) decide to try the drop ride, how fast can the ride accelerate them upwards with a force of 25000 N? Remember to account for gravity.