- Computers are NOT needed for lab this week
- An electronic balance is needed this week
- Two separate experiments, both related to forces on an object. The order in which the experiments are performed does not matter, so have some groups do the elevator first (since there aren't enough scales for everyone to use at once):
- Radial force:
- Stuff Jeff will (hopefully!) do before lab:
- Set up a couple of electronic balances
- Check the balance between Bob and counterweight
- Level the base of the apparatus (using the round bubble level)
- Spray WD-40 on the pulley to reduce friction (and hence the load required to put Bob in the correct position). Note that pulley friction might still be a source of error
- The apparatus has been leveled in its current location on the bench. Students should check the round bubble level to make sure it's still level. The apparatus shouldn't be moved on the table, since the tables are not level and changing position on the bench will screw up everything. Also be sure that the bench does not interfere with the string supporting the extra mass, m2 (static part)
- Bob and the counterweight should be balanced: loosen the screw holding the 'yardarm' in place, and make sure that it pivots gently on this point. It's important to make sure this screw is very tight before the apparatus is rotated; if the yardarm slips, the radius will not be the same as what they measured
- Students should keep their linear measurements in units of cm until the force is calculated
- If students will make a mistake, it will be with measuring the radius of the orbit: some never figure out how to read the vernier calipers (for diameter of pointer and rotating shaft), and they frequently read from the wrong side of the caliper jaws on the ½-meter stick. The radius will be 16 to 18 cm; after students tell you their measurement, quickly confirm using a plastic ruler
- I created this map to keep track of the radius measurements for each setup. This is especially handy when grading. I'll try to take measurements (with a plastic ruler) during the equipment setup and will make copies for each lab instructor
- Measured values:
- Dshaft = 1.270 cm; Dpointer = 0.318 cm; mBob ≈ 445 g; T ≈ 0.7 s
- If they measure T ≈ 1.5 s, they probably had the photogate in Pend mode so they actually measured 2T (like a group I just had). Fortunately, the data are still salvageable; dividing the period by 2 should give the correct result
- Problems:
- Is the orbit truly circular? Nope. The apparatus tends to wobble slightly as it rotates. Students want to blame the fact that they're turning the shaft by hand, but the difference between their min and max period is usually very small, typically less than 5% (although some suck at this, getting 10% or higher. *sigh*). Be sure to ask students about the assumptions that are made for this experiment (it appears in boldface, italics and double underlined on p. 1 of the instructions!)
- Rarely, someone will ask about the mass of the spring holding Bob, and if it contributes to the uncertainty in the force. The spring mass is 15.7 g, which is 3.5% of Bob's mass; I'm guessing that entire spring mass doesn't count towards the measurement of the spring force, so it likely isn't an issue
- Riding an elevator:
- In the past we've used regular bathroom scales that read in pounds. Converting to kilograms shouldn't be difficult, but it's usually a sticking point for weaker students. Metric bathroom scales were purchased in Canada for use beginning with the Fall 2016 semester. It remains to be seen if this improves things
- Since only a couple of scales are available, encourage some groups to do the elevator part before the rotating Bob section. That will minimize waiting around for a scale to become available. This is especially important if you have a large lab section
- You can have two groups go at one time, using the data for one person on the scale
- Only one person in the group needs to have their weight measured on the scale
- Place a wood drafting board under the scale; the elevators are carpeted, and this can affect the scale reading
- A separate worksheet is printed for them to keep track of their measurements. One sample calculation should be worked out in their report. Worksheet notes:
- Here are my solutions for the worksheet (details below). Bewkes Hall results are in green, Johnson Hall in violet
- The top half is for an "up" trip, the bottom half is for "down". Students should first measure themselves on the bathroom scale in the lab, then collect measurements when the elevator starts moving upward, and stops moving upward (same for down). It's important that students collect more than one measurement for each trip!
- They should draw vectors on the FBD for each trip, and the length of each vector should be scaled properly relative to the other to help emphasize the direction of acceleration. Many get confused about whether the vectors change or remain constant. I always ask: "Did your mass change? Did you throw up on the elevator?" Gross, but effective
- Some just don't get the concept, that the acceleration comes from the vector addition of the two forces. This will be readily apparent when they calculate that the acceleration was on the order of g. Good luck
- Elevators won't accelerate the same upward as downward. Students assume that they should be the same
- The elevator in Johnson Hall has much smoother motion, so the accelerations calculated will be much more consistent. Values will be on the order of ±0.5 m/s2
- The Bewkes Hall elevator is much older and crankier, and results can vary somewhat from year to year if there has been recent repairs. Of particular note is the fact that while stopping on the "up" trip, the elevator has a tendency to 'lurch' upwards, significantly increasing the acceleration to as much as 0.5 g!
- My data:
- Bewkes Hall (2004!):
- Starting up: +0.46 m/s2
- Stopping up: –8.4 m/s2 (!!) - This is not a typo!
- Starting down: –0.46 m/s2
- Stopping down: +0.91 m/s2
- Johnson Hall (student data, ~2016):
- Starting up: +0.82 m/s2
- Stopping up: –0.25 m/s2
- Starting down: –0.23 m/s2
- Stopping down: 0.80 m/s2
- In case anyone asks, a Google search (2018) shows that the fastest elevator in the world is in Shanghai. It travels 121 stories, hitting a maximum velocity of 73.8 km/h (45.8 mph)!
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