- There’s something I find pretty interesting in this video, Immediately after it’s hit, the billiard ball is moving, but not turning.It starts turning after a time.
- Yo-yo physics: They drop a yo-yo and analyze the motion with Tracker. They do it with pennies stuck right at the center of the yo-yo and also evenly distributed on the outer edge. The trial with the pennies distributed on the outer edge have a larger moment of inertia, and so fell slower. It is a cool video, but the audio isn’t so good, so I didn’t post it on educanon
- This video has a way cool scene on it (in my opinion). Check out the video at 2 minutes, 30 seconds. There is a collision that seems to create kinetic energy. There is a strong magnetic reaction that sends a ball off with higher speed than the incoming ball, and three balls move backwards. Jacob analyzed the video and saw in this analysis that momentum is conserved, but kinetic energy is greatly increased in the collision. Of course, two magnets come closer together, so there is magnetic potential energy turned to kinetic energy. This is just as if two planets come closer together. They would lose gravitational potential energy (going deeper into a gravitational potential well), and this would turn to kinetic energy.
- These folks got the right answer, but I think this was just lucky, as they made several mistakes in the process. Here is what I wrote to them: I think you made two pretty bad mistakes that canceled each other out!. The acceleration in the direction it’s moving is g*sin(theta). Convince yourself of this: at the top, theta = 0, and sin (0) is zero, and the acceleration is zero. acceleration is g at 90 degrees. You let centripetal acceleration is v^2/R, and the tangential acceleration is g*cos(theta). You substituted them for each other. They are not the same. If you did a force diagram, you’d say that gravity + the normal force = acceleration. You need to recognize that what is holding the cart in the circular path (and against the circle) is the radial component of g: or g*cos(theta), which you mistakenly said was the tangential acceleration, and then mistakenly substituted it for the centripetal acceleration… which it actually IS…. In any case, you got the right answer, but I think it wasn’t because you did the calculation correctly.
- There were some problems with this analysis. However, the video at the end taken at BYU was worth watching the whole thing! This is what I wrote to them: You made some mistakes with tracker. You have a speed of 1100 m/s. Did you consider how fast this is? the speed of sound is 300 m/s. Do you suppose that the units were actually cm/s? Also, if you look at the trajectory of the stone, in the air it is not parabolic. In fact it goes straight up and then gets a boost to the right when it is in the air!. In looking at your slow motion, I think that tracker picked up the position of the splash which largely went up. If you look at the picture more closely, I think you’ll see that tracker made a mistake here. You might want to reanalyze. However, I still like your video.
- Nicely done video. It all seems reasonable up until you try to slow down. You find the force of friction on the wheels necessary to slow it down. But this isn’t the only force on the wheels… in fact, the force of friction on the wheels is trying to speed the wheels up as you are trying to slow the wheels down… likely by applying a break. Also there is some torque from gravity, so the slowing down dynamics is pretty complicated. I think in order to solve this, you’d want to assume some breaks supplying the torque… then you’d just do another energy analysis that you lose more potential energy, turning into kinetic energy, and it all turns to heat, and heat is distance times force. From this force, you can get your coefficient of friction.
- This was correct, but what does it mean? What must be the coefficient of friction? We see that the centripetal acceleration is about 4 gravities (I’ve heard that it’s tough to be a driver). That means that the coefficient of friction must be at least 5… that’s too high. What’s going on? The normal force is much greater than the force of gravity because the air foil forces air upwards, no? … additionally, the curves are a little bit banked… but not much. In any case, you did the calculation correctly, but it would be a good idea to ask yourself what this number means.
Pete Schwartz, Cal Poly Physics
Shared Public Resources