- Computers are NOT needed for lab this week
- One or two electronic balances are needed this week
- Typically, they have not covered this material in class yet (especially the Monday and Tuesday groups), so a brief intro is needed
- Two independent experiments, both dealing with buoyant force:
- Pressure-Depth Relation: Calculate the pressure on a jar bottom some depth below the water surface
- Fall 2023: I replaced the metal cans with plastic jars. There may have been some improvement in the results, but it's not tremendous. Baby steps...
- Containers of sand are placed near the sink. The containers are placed within a box lid to keep sand out of the sink. Tell students to keep the sand dry, and try not to sink their jar!
- If sand does get wet, do not place it back in the containers with the other sand!
- The jars should be submerged about ½ their length. Students do this by marking the jars with a piece of tape, then putting sand in the jar to sink it to this level. Students should be encouraged to add sand and water near the sink to prevent spilling a lot of water on lab bench
- They will use a plastic ruler or the caliper to measure the depth of the jar. Note that the bottom of the jar is not located at the bottom of the lip of the jar!
- The results for this experiment jar vary widely, but they should be able to calculate the pressures to within ~5% of each other (but I'll accept anything up to ~10%). Anything more and they likely measured 'h' incorrectly. The jars are not cylindrical, and that can cause variation in results
- Common problems:
- Incorrect calculation of area
- Rounding errors
- Measurement error
- Archimedes' Principle: Used to calculate buoyant force
on an aluminum alloy cube; compare to measurement using Newton's Laws
- Instructor demo (optional): On the front sink there's an extra aluminum block, a container filled with water and a large-size force gauge. Show students the force reading while the block is suspended in air, and then with the block submerged under the water. This is useful if they haven't yet seen Archimedes in class
- The vernier calipers are the weak point here, since students don't remember how to use them. Show students that they can measure the diameter of the hole using the calipers properly
- Adjust the force gauge if it does not read 0 when hanging vertically as follows:
- Hold the force gauge vertically in your hand, pull and release the hook a couple of times to see how the needle settles
- With the force gauge vertical and at rest in one hand, hold the hook firmly with your other hand, then move the needle to zero
- Repeat previous two steps as necessary
- The density of the cube is calculated as a check on their volume calculation. Here are my measurements:
- L = 4.135 cm
- W = 5.370 cm
- H = 7.685 cm
- D = 2.000 cm (diameter of the hole)
- mblock = 421.7 g
- With these measurements, you'll get the following results
- Vblock = 1.577×10–4 m3
= 157.7 cm3
- ρblock = 2.674×103 kg/m3 (1% diff. from ρaluminum)
- I have a spreadsheet to quickly check student measurements of volume and density
- Common mistake: Students will use H to calculate the volume of the hole instead of L
- The force gauge is suspended from the pendulum clamp so it can be read without jiggling the gauge
- Many don't realize that they can lower the supporting pendulum clamp to immerse the block!
- The difference between the buoyant forces calculated by each method should be within ~5%
- The difference between calculations using Archimedes and Newton are explained during pre-lab. Don't expect the weak students to get it without a lot of explanation
- Many will immediately question the accuracy of the force gauge, and they are asked how they can check its accuracy. Many won't get it - there's an electronic balance in the lab, so they can check the calculated dry weight from the electronic balance mass to that measured on the force gauge. The gauges are pretty accurate!
- Note that many will still blame the force gauge, even though they showed it to be very accurate!
- Fall 2018: I found one force gauge where the needle was bent, causing it to hit the screw on the dial face. This caused the need to stop around the 4 N mark!
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