When i think about it, the moment you lift the weight into position , there is tension or a pulling force on the rope also the stand, the stands and wooden board are very light with hardly any friction. The sheer weight of the weight is intially pulling the stand towards it before it can swing. I imagine the weight droppping vertically (a very tiny bit) due to gravity, causing the stand to be pulled towards it, before it swings, but a some time X the forces and friction acting on the stand is greater than the weight, therefore allowing it to swing back and forth.
Thank you for your comment! We hinted at this in the video, but this is a great way to approach the problem! Instead of using conservation of momentum, you could certainly draw a free-body diagram and use Newton's second law to analyze the motion, as you are describing in words. We typically use this Quick Quiz during a conservation of momentum unit, but it would be interesting to use it earlier in the typical sequence and see if students arrive at your reasoning as well.
This is a common issue with 3d printers too! As the printing head moves back and forth it shakes the table. This causes uneven and alternating offsets in the print lines. Solution? Increase the mass of the table! By the conservation of momentum, the table velocity will be lower, so less effect on the printing quality. For my 3d printer, I place a stack of two concrete pavers on the bottom shelf of the table. They're very heavy and reduce the shaking back and forth substantially
Initially, the table movement is 180 degrees out of phase with the pendulum. If allowed to continue, eventually the pendulum will come to rest in the lab frame and the table will be moving. Then the table moving will cause the pendulum to move - 180 degrees out of phase and out of phase with it's original motion.
Great illustration of the principle of an application (becoming mature) of the "Mass Damper" used in tall structures in Japan and other earthquake prone areas today.
whats interesting is how quickly the system seems to cancel out the normal pendulum (i.e. the arc length of travel reduces within 1 or 2 back and forth)
There is definitely a decent bit of loss in this set-up! In principle the cart and the pendulum would continue oscillating out of sync forever, but the primary loss comes from the rods twisting under the wood. Once they are no longer parallel, they jam and slow things down substantially. This whole set-up could be optimized, but we hope the essential points were clear!
Love the demonstration and content!! I'd like to offer a pointer in the edit/graphics. You recorded the video in HDR, but treated the graphics as SDR...This brought in the white slides at 100% HDR (which is blinding) they ought to be at 50% HDR (which is 100% SDR). Alternatively...and easier: Just take the camera out of HDR mode ;-)
Thank you for the kind words! Thus far, we have always recorded at 4K, 30fps on an iPhone. The animations are made at that same spec in Adobe AE and exported as such. Two questions: 1) how could you tell the difference between HDR/SDR and 2) is there any downside to shutting the HDR off in the camera settings? Thank you very much for your feedback! We are really trying to learn as we go.
To get the correct solution, it's best to start at rest with a single force vector to represent the pendulum tension, break it into horizontal and vertical components, then draw the normal forces to counteract the vertical. Since there's no outside horizontal, there must be a horizontal retained by the bar apparatus, which is why it starts moving.
Thank you for sharing your analysis! Newton’s second law is a valid way to approach the problem and nicely complements the use of conservation of momentum, which we are targeting in this particular Quick Quiz!
Before I see the result. My hypotesis is that the pendulum on it's first swing it's going to push the table to a side, then that momentum of the table is going to counter the second swing and so on. I think the table is going to move a little bit to a side and then wobble with the pendulum.
When i think about it, the moment you lift the weight into position , there is tension or a pulling force on the rope also the stand, the stands and wooden board are very light with hardly any friction. The sheer weight of the weight is intially pulling the stand towards it before it can swing. I imagine the weight droppping vertically (a very tiny bit) due to gravity, causing the stand to be pulled towards it, before it swings, but a some time X the forces and friction acting on the stand is greater than the weight, therefore allowing it to swing back and forth.
Thank you for your comment! We hinted at this in the video, but this is a great way to approach the problem! Instead of using conservation of momentum, you could certainly draw a free-body diagram and use Newton's second law to analyze the motion, as you are describing in words. We typically use this Quick Quiz during a conservation of momentum unit, but it would be interesting to use it earlier in the typical sequence and see if students arrive at your reasoning as well.
This is a common issue with 3d printers too! As the printing head moves back and forth it shakes the table. This causes uneven and alternating offsets in the print lines. Solution? Increase the mass of the table! By the conservation of momentum, the table velocity will be lower, so less effect on the printing quality.
For my 3d printer, I place a stack of two concrete pavers on the bottom shelf of the table. They're very heavy and reduce the shaking back and forth substantially
Thank you for sharing this even more interesting real world application, and a great, cheap fix for the problem!
Initially, the table movement is 180 degrees out of phase with the pendulum. If allowed to continue, eventually the pendulum will come to rest in the lab frame and the table will be moving. Then the table moving will cause the pendulum to move - 180 degrees out of phase and out of phase with it's original motion.
Great illustration of the principle of an application (becoming mature) of the "Mass Damper" used in tall structures in Japan and other earthquake prone areas today.
That is another great example of a real world application for this simple system! Thank you for watching!
whats interesting is how quickly the system seems to cancel out the normal pendulum (i.e. the arc length of travel reduces within 1 or 2 back and forth)
There is definitely a decent bit of loss in this set-up! In principle the cart and the pendulum would continue oscillating out of sync forever, but the primary loss comes from the rods twisting under the wood. Once they are no longer parallel, they jam and slow things down substantially. This whole set-up could be optimized, but we hope the essential points were clear!
Love the demonstration and content!!
I'd like to offer a pointer in the edit/graphics. You recorded the video in HDR, but treated the graphics as SDR...This brought in the white slides at 100% HDR (which is blinding) they ought to be at 50% HDR (which is 100% SDR). Alternatively...and easier: Just take the camera out of HDR mode ;-)
Thank you for the kind words!
Thus far, we have always recorded at 4K, 30fps on an iPhone. The animations are made at that same spec in Adobe AE and exported as such. Two questions: 1) how could you tell the difference between HDR/SDR and 2) is there any downside to shutting the HDR off in the camera settings?
Thank you very much for your feedback! We are really trying to learn as we go.
To get the correct solution, it's best to start at rest with a single force vector to represent the pendulum tension, break it into horizontal and vertical components, then draw the normal forces to counteract the vertical. Since there's no outside horizontal, there must be a horizontal retained by the bar apparatus, which is why it starts moving.
Thank you for sharing your analysis! Newton’s second law is a valid way to approach the problem and nicely complements the use of conservation of momentum, which we are targeting in this particular Quick Quiz!
Before I see the result. My hypotesis is that the pendulum on it's first swing it's going to push the table to a side, then that momentum of the table is going to counter the second swing and so on. I think the table is going to move a little bit to a side and then wobble with the pendulum.
Thank you for your engagement! We also encourage you to list your confidence alongside any claims. Did the result match what you expected?
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