LAB : Using A Spark Timer

Transcription

1 LAB : Using A Spark Timer Read through the whole lab and answer prelab questions prior to lab day. Name: F1

2 Introduction A spark timer is used to make accurate time and distance measurements for moving objects. To put it simply, a spark timer is like a high tech stopwatch. It is basically a small box through which a piece of special tape is pulled through. Inside this box, a spark is made and repeats in a known amount of time. When the tape is pulled through the box, sparks marks are made on the tape. Sparks made here Tape pulled through Since we know how often a spark is made, we can count the number of marks on the tape to see how long it took the tape to move through. A finished tape would look something like this. Using the Timer As seen above, the timer will make a series of dots and therefore also a series of spacings or gaps between dots. It is these spacings that we want to focus on. The timer is set to produce these spacings at a known time interval. The timer can be set at either 1/60 of a second (60 Hz) or 1/10 of a second (10 Hz). When set at 1/60 of a second, this means that it takes seconds (1/60) to make a space. A setting of 1/10 would then mean each space is made in 0.10 seconds. We can easily measure a spacing with a ruler to find the distance. The time for the spacing is also known (1/60 or 1/10 sec per space depending on how the timer is set). We can therefore use this data to determine the speed at a given spacing by finding distance over time. Sample tape analysis: The spark timer is set at 1/60 and a motorized car pulls it through. The tape is shown below. Visual Analysis: Since the dots get farther and farther apart in the beginning, we can see that the car must be speeding up logically, if each dot is made in the same amount of time and the spacing between them is getting larger, the car must go faster for this to happen. In the second half of the tape, the dot spacing is constant and therefore we can conclude the car was moving at a steady rate (constant speed) Note: The spacing between each dot is what we are measuring here, not the dot itself. Therefore, we want to count the # of dot spacings to determine the amount of time for a given distance rather than counting the # of dots by themselves. F2

3 Initial and Final (Instantaneous) speed of car: Instantaneous speeds can be assumed to be average speeds over very short time intervals. The initial speed of the car will simply be the speed of the first spacing which can easily be found by using the distance and time of this spacing. Likewise, the final speed will be determined using the last spacing. For the sample tape above: The first spacing takes 1/60 sec to make and measures 0.5 cm (.005 m). This gives a speed of (v = d/t = m / (1/60) sec) = 0.3 m/s The last spacing takes 1/60 sec to make and measures 3.5 cm (.035 m) This gives a speed of (v = d/t = m / (1/60) sec) = 2.1 m/s Average Velocity of the car: The average velocity of the car is defined by the total distance traveled / total time. Using the tape, we measure the distance from the first dot to the last dot with the ruler. Then we can count the total number of spacings and add up to get the total time NOTE: The time used here is not 1/60 of a second, rather it is the total time. Measured Distance = 15.4 cm convert to m total distance = m Total # of dot spacings = 8 x (1/60 sec) per spacing total time = sec Avg Speed = dist / time = 1.16 m/s F3

4 Prelab question: A toy car has a spark timer tape attached to it and moves down a track. The timer is set to the 1/10 setting. Visually inspect the tape above and qualitatively (no numbers) explain what the tape tells you about the motion of the car. Explain your reasoning. Calculate the average velocity of the car, the initial instantaneous velocity of the car, and the maximum instantaneous velocity. (show all work with all equations and units) F4

5 Procedure Each person must produce and analyze their own tape. After running a tape, tape it to the attached timer tape collection sheet. Cut the timer tape as necessary to fit it on the page, but be sure to cut keep the tapes in order when pasting them. Fix the timer tape firmly to the collection sheet. YOU WILL LOSE POINTS IF YOUR TAPES ARE NOT NEATLY AFFIXED TO THE COLLECTION SHEET, ARE CRINKLED OR ARE HARD TO READ. PART A 1. Put the spark timer box flat on the lab table. On the face of the timer there is an arrow to show the proper direction of travel of the timer tape. Set the timer to 1/10 second (10 Hz) intervals so that each interval between dots represents this amount of time. Be sure the timer switch is off. Plug in the timer. 2. Take a roll of timer tape and note that the outside of the tape while it is on the roll is the part that should face up when it is put in the timer. Have each person rip off a piece of timer tape approximately 0.50 meters long. Measure 15 cm in from what you will use as the front end of the tape and put a line across it to mark a start position. back start front 15 cm 3. Feed the tape through the timer (outside up) so that the 15 cm starting mark is near the front of the spark timer as you feed it in (see diagram below). Use masking tape to attach the front end of the spark tape to the cart as shown in the diagram below, do not tape it to the wheel. Front 15 cm Start line DO NOT TURN ON THE TIMER OR START THE EXPERIMENT YET, FOLLOW DIRECTIONS EXACTLY, The Note below is for information purposes only, it does not tell you to start yet. NOTE: The 15 cm of space from front to start line is used as slack in this lab to get the cart moving before you start the stop watch. YOU WILL NOT START THE TIMER UNTIL THIS START LINE REACHES THE TIP OF THE SPARKING MECHANISM. Dots will be created in this first 15 cm, but are not relevant are only used to get the cart going. The only dots we will be looking at are the ones that form after we pass the start position. The first 15 cm is only used to get the cart going and we will ignore these dots when analyzing. 4. If using a lightweight plastic cart, put a 1 kg mass in the cart. F5

6 READ STEP 5 COMPLETELY BEFORE PROCEEDING. 5. Description of task One person is to pull and move the cart at a moderate constant speed, not too fast and not to slow. You will be counting the spacings so if its too slow there will be too many and if its too fast there will not be enough. When the start line reaches the part of the spark timer where the dots are being made, the other person will begin timing with a stopwatch for the rest of the pull and stop it when the tape leaves the spark timer (Note: A stopwatch is being used to investigate the accuracy of the spark timer s timing mechanism, but technically a stopwatch is not needed to get the time, you could just use the dot spacings on the tape) Procedure a) Turn on spark timer b) Begin pulling car c) Start stopwatch when start line on spark tape reached the spark creating location of the timer. d) Stop the stopwatch when the spark tape comes out. e) Turn off spark timer Recorded Stopwatch Time: 6. Take the spark tape and use scotch tape to attach it to the collection sheets at the end of the lab. Cut the tape when needed and continue it below the first piece taking care to make sure you know where the start and end of the tape is. Write PART A above this tape that you attach on the sheet. PART B Do not use the stopwatch in this part Tear off a length of timer tape about 40 cm long and thread it through the spark timer. Keep the spark timer flat on the desk and move it to the edge of the lab with the tape overhanging the side of the table. Set the spark timer to 1/60 (60 Hz) of a second so that each interval between dots represents this amount of time. Attach a 50 g weight to the end of the timer tape and place a book on the floor where the weight will hit when it is dropped. Turn on the spark timer and drop the weight. Turn off the spark timer. Inspect your tape. The spark timer can sometimes skip a dot. You should look at your tape to see that a pattern exists and that it does not appear that a dot was skipped. If there is a big messy clump of dots at the beginning of the tape, you can discard that part of the tape. We want to only use the part of the tape where the mass was falling freely and interference from your hand was not causing stray dots. Call your teacher if your tape looks strange. Repeat the above steps so that each lab partner has his or her own tape. Take the spark tape and use scotch tape to attach it to the collection sheets at the end of the lab. Cut the tape if needed and continue it below the first piece taking care to make sure you know where the start and end of the tape is. Write PART B above this tape that you attach on the sheet. F6

7 Spark Timer Lab When turning in the lab, only turn in from this page forward. The prior pages are for your reference only Name: 1. Remember, for Part A, the dots prior to the start line are irrelevant. Count the number of dot intervals (not the # of dots, but how many spaces between dots are on the tape) and use that number of dot intervals to find out how much time the spark timer recorded for the trip. (Remember that the timer was set to 1/10 second intervals) Part A Pulled Cart (calculated time using the spark timer tape) Number of Dot Intervals Spark Tape Time Stopwatch Time Time Calculation: (show work and explain how you got the time) 3. Assume the tape time is the actual (accepted) value and find the percent error of times. (show formula and work) (For questions 4 8 refer to the introduction of this lab if you need help to perform the calculation) 4. Based on the pattern of dots shown on the tape, does it appear to have been pulled at a constant speed. Explain your answer. F7

8 5.) Calculate the average speed of the pulled cart over the whole distance recorded by the spark timer. Show work clearly, not just numbers. 6.) Calculate instantaneous speeds at 4 random locations to check whether the speed was constant or not. Label and circle the locations you choose A,B,C,D on the tape and show you work clearly for each one. State your conclusion based on these calculations F8

9 Part B Dropped Weight Remember for this part, if there is a clump of dots at the beginning, this should be ignored. Use the first distinguishable dot that begins the dot pattern as the starting point. 7.) Calculate the average speed of the dropped weight over the recorded distance. Show all measurements, calculations and data clearly including formulas and units 8.) Calculate the initial speed of the weight just after it started moving and the final speed of the weight as it hit. (clearly show all work and steps used to calculate values, including formulas and units) Summary Question 1. How can the dots on a timer tape tell whether an object is speeding up, slowing down or moving constant? F9

10 F10