The Spoke Blur Effect

Man it is a rainy, lazy day here in Los Angeles and I am tirey-wheeled. That’s okay. I can keep it together and stay awake and you should too because this is episode two of the Learning Lounge. Today we are going to talk about the Spoke Blur Effect, something I learned about recently and have since become dangerously obsessed with. When I’m driving on the road, I find myself watching not the road ahead of me but the tires of the cars near me, just to see the effect happen. Now if you have ever seen a rolling wheel, whether by itself, or as part of a car, or bike or sandwich, you may have noticed that no matter what direction the wheel is rolling in, the spokes at the bottom are clearer, less blurry than those at the top. Next time you see a rolling wheel pay attention because this effect is everywhere but why does it happen? Wow the rain is really coming down right now but that’s not going to stop me. I get inside your brain rain or shine. Now look, when things move quickly they can become blurry like the spokes on this wheel while the wheel is turning. Right now, the spokes are blurry everywhere all over the wheel but when the wheel rolls, the bottom region is at all times less blurry than the top. I can make the most clearly distinguishable be on the top, the right, the left, or anywhere in between. Look closely and follow the region with the clearest spokes and note that region’s location relative to the direction I’m translation the wheel. Now from your frame of reference, at rest with me and this room, every part of a rotating disc that’s not the very center of rotation is accelerating it’s velocity. That is it’s speed and direction at any given moment of time is different than it was the moment before. For example, the point represented by this red dot may at one moment be traveling due left. Then a bit left and up. Then due up. A bit up and right. Due right. Down and right. Due down. A bit down and left and over and over again. The speed of its motion is what causes it to be blurry. If I rotate the disc fast enough, before the camera or my eye can resolve a clear image of the dot and the region it’s in it will have moved somewhere else. The blur inducing motion doesn’t have to be from rotation. I could also just translate the disc back and forth fast enough to fuzzify it. But what if I did both? What if I rotated the disc while simultaneously translating it. Well then the motion from both of those transformations would combine. If during some brief amount of time a part of the wheel rotates two centimeters to the left but translates two centimeters to the right it’s position after that interval will be the same as before. It will have been motionless for us and motionless means not blurry. And as it turns out this is exactly what happens during rolling. Pure rolling is a special combination of rotation and translation in which a rotating thing translates across a surface such that the point where the rotating thing contacts the surface is instantaneously motionless relative to the surface. I mean if that wasn’t true. If the instantaneous contact point had some velocity that was non-zero relative to the road while touching it the wheel wouldn’t be purely rolling. It would be sliding as well. If a wheel is rolling at any given moment parts of the bottom of the wheel are moving down or up with a little backwards motion due to rotation which is opposite the forward translational motion of the wheel and it’s vehicle. The point at the very bottom of the wheel is in fact only moving directly opposite the forward motion of the vehicle and with the same speed so the two velocities cancel out making it relative to the road or you standing on the road watching instantaneously motionless. So if you are not sliding, no matter how fast your speedometer says you’re going, the part of your tire that happens to be at any specific instant in time touching the road isn’t moving across the road at all. On the other hand parts of the top half of the wheel all have a horizontal velocity in the same direction of the car. The car’s forward motion and their rotational forward motion combine into a velocity faster than what your car’s speedometer says. In fact again when not sliding, only purely rolling, during the brief moment that a part of your tire is at the top it is flying past the road at twice the speed of your car. Let’s look at this effect again. While a spoke is in a position where the horizontal component of its velocity due to rotation and translation are in the same direction they combine into a faster overall velocity so it becomes blurrier but while the spoke is in a position where its horizontal velocity due to rotation is in the opposite direction as its horizontal velocity due to translation it moves more slowly relative to us watching and therefore is more clear, less blurry. It’s very spinteresting. Ahh it almost sounded like the rain was gonna die down just in time for us to be done. Well anyway point is we have arrived at a life hack. That’s right the next time you are pulled over for speeding by the police remind the officer that while yeah sure maybe your vehicle was traveling faster than the law allows at every moment during your journey the point of your tire that was in contact with the road was not speeding at all. In fact it was completely motionless. If you bring that up you are sure to still be fined but you will be much more annoying and as always thanks for watching

Channel: D!NG

Views: 1,121,969

Rating: 4.9629703 out of 5

Keywords: vsauce, michael stevens, velocity, motion blur, wheel, rotation, translation, physics, rolling, roll, bicycle, illusion, puzzle, vectors, vector addition, ding, d!ng, dingsauce

Id: jLV4NMsWxUc

Length: 6min 41sec (401 seconds)

Published: Fri Feb 01 2019

Reddit Comments

[u/MaxSupernova]

I love his videos, but that was incredibly unclear.

I think there were simpler ways that could be described.

[u/10gallon_mouth]

What he's describing is much more obvious and easily pronounced when watching the treads on caterpillar/tank tracks, while in motion. The treads on the bottom touching the ground remain stationary until they are rolled back up, while those at the top reel forward including the speed of the tank's velocity.

[u/zxcsd]

He's really bad at explaining, could've made it much clearer and taken 20 seconds.