My daughter showed me something that is curious. My son has a toy bulldozer (Tonka-sized) with treads, of course. Recognize that, for any one tread, there is a top portion and a bottom portion for it is just like a conveyor belt. That said, try this:
Suspend a toy bulldozer in mid-air and move the tread facing you. Naturally, the whole tread goes around and around as expected. The top portion of the tread moves in the opposite direction relative to the bottom portion.
Now, place the bulldozer on the ground. On the tread facing you, place one finger on the bottom portion of the tread and advance the bulldozer. Your finger clearly doesn’t move, and it appears that the bottom portion of the tread did not move…and yet, you know it must, right? Or, not right? :dubious:
Maybe I’m just too sleepy, but can someone explain what I’m seeing.
Sorry, I don’t understand your word-picture. It sounds like the first part you’re keeping the truck still and moving the tread, but in the second part your keeping the tread still and moving the truck. Can you explain where I’m confused? Any way to upload it to YouTube?
The tread is moving in both cases. In the air, the truck isn’t moving though. So if you put your finger on the bottom of the tread as it turns, your finger will have to move. On the ground your finger isn’t moving, so the truck has to move. Your finger and the truck are moving relative to each other in both cases.
As such, it looks like the bottom tread does not move while the top portion clearly is. Yet, we know it all MUST move together. Not sure if I will get time to post a video…maybe this weekend I can mess with it. Or, if someone else can re-create my scenario…feel free to post a video of your own experiments.
The tread doesn’t move relative to the ground while it’s touching the ground. It can’t, if it did it would be sliding/skidding. It’s no different then when a real bulldozer runs over a rock.
In fact, it’s no different then a regular tire, it’s just oval shaped. If car tire ran over your foot, you’d have the same effect.
I completely understand what you’re saying though, it is visually confusing.
Skip to 1:13 to see the effect Jinx is referring to. You can see clearly that the treat doesn’t move with respect to the ground while it’s in contact with the ground.
Think of it this way. If the bottom of the tread was moving relative to your finger (i.e. relative to the ground), then the tread would be slipping.
Same thing with a normal wheel. If you think about it, the bottommost part of a wheel cannot be moving relative to to the ground, otherwise the wheel would be slipping. The points on a wheel do not, in fact, go round and round, they go hop hop hop.
The bottom of the tread is not moving, the carriage/wheel truck/whatcha call it is moving at the same speed as the tractor (of course), and the top of the tread is moving forward at double speed.
It’s no more mind-bending than looking up at the stars on a clear night.
The section of the tread that is in contact with the ground is not moving due to friction. Imagine that you went up to a stationary bulldozer and put a small dab of paint where the frontmost section of the bottom tread is touching the ground and then stuck a stake in the ground next to it. Then the bulldozer started up and began moving. Your paintmark would remain next to your stake. The bulldozer would move forward over your paintmark and eventually the paintmark would reach the end of where it contacts the ground. Then it would rise up and make the transit across the top tread.
This may help visualize it. Notice that, as mentioned already, the wheel appears to move at different speeds depending on where you look at it. The part touching the ground is briefly at rest to the ground. A bulldozer’s track is just a very long wheel so the part that is at rest is much easier to notice. Also easier to see on a bulldozer is the fact that the top track is moving forward twice as fast as the bulldozer itself.
Really, the only difference is that when a specific part makes contact with the ground it stays there for much longer and gives you a chance to notice it.
Another way to think about it is when to think about someone (or yourself running). Imagine, your foot that’s touching the ground is the bottom part of the tread and the foot that’s not touching the ground is the top part. When you bottom foot is touching the ground it isn’t moving WRT to the ground and by doing that it’s propelling your body forward.
Ok, everyone has pointed out in different ways what’s actually going on. But your asking about why it looks funny. I think that’s because you’re eye is focused on your finger, at a fixed spot on the ground which is also a fixed spot on the tread. But if your finger isn’t there, the tread would move over the ground the same way, but you’d notice the edge of the tread moving relative to the tractor all around. Or if you focus on a point on the tractor, and move your point of focus to keep up with the tractor while it moved, your finger would seem to be moving away from that point on the truck.
A similar effect can occur on a train in the station. You look out the window and see the train on the next track over, then it looks like that train has started moving. Then a post on the platform seems to move across your view, and you realize the train you are on is the one that’s moving.
In your case, you’ve thrown in the tractor in the air. Since the tractor doesn’t move relative to the point in the air, the tread has to. If you did the same thing on the ground, the truck wouldn’t be moving, and the tread would be skidding across your finger.
In the air:
Bulldozer doesn’t move
Top tread moves with velocity V (towards the front of the dozer)
Bottom tread moves with velocity -V (towards the back of the dozer)
On the ground relative to the Bulldozer:
Top tread moves with velocity V (towards the front of the dozer)
Bottom tread moves with velocity -V (towards the back of the dozer)
But the Bulldozer itself is moving with velocity V relative to your finger, so:
Top tread moves with velocity 2*V = V+V relative to your finger
Bottom tread moves with velocity 0 = -V+V relative to your finger
This of course ignores relativistic effects, so is not to be used if you put your Bulldozer in a particle accelerator.
Guys, I’m so confused. When I sit in my chair and pick my feet up, I can kick them through the air. They move just fine. But when I stand on my feet and start walking, the down foot doesn’t move! It’s a mind-bending optical illusion! What’s going on here? I’m freakin’ out, man…
Think of it this way. If the bottom of the tread was moving relative to your finger (i.e. relative to the ground), then the tread would be slipping.
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Exactly.
I get a lot of milage out of asking if the top and bottom of a dozer’s tracks are moving at the same speed.
For most folks it’s hard to see at first thought that the track on the ground has to be stopped for the vehicle to move. You have to realize that it isn’t the track that moves the vehicle, it’s the rollers rolling over the track. Exactly the same thing as a train.
