- #36
russ_watters
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Did you not read my post #28?andyrk said:Then what does changing gears even do?
(Or, to answer a different way: exactly what AT just said and nothing more.)
Did you not read my post #28?andyrk said:Then what does changing gears even do?
You seem to be hazy on the concept of mechanical advantage.andyrk said:I don't understand why the rear wheel accelerates more, by reducing the rpm of our legs/where we pedal.
andyrk said:It has a velocity at the bottom-most point
Have you pedaled a bike before? What happens to your ability to apply force to the pedals as the rpm gets too high? Or: is it easier to apply a large force when your legs are moving fast or slow?andyrk said:I don't understand why the rear wheel accelerates more, by reducing the rpm of our legs/where we pedal.
I don't understand why you think this must be the case.andyrk said:I don't understand why the rear wheel accelerates more, by reducing the rpm of our legs/where we pedal.
I get the feeling that you have stopped thinking about what your actual experience has been of riding a bike and latched yourself in a totally 'verbal' mode of approach. You expect too much if you insist on an answer that fits totally into your requirements for an 'explanation'.andyrk said:I don't understand why the rear wheel accelerates more, by reducing the rpm of our legs/where we pedal.
So what does more torque make the rear wheel do? Make it spin faster?russ_watters said:A bicycle begins to accelerate (from a stop or a constant speed) because more torque is applied to the back wheel
The increased torque at the pedals is delivered to the rear wheel by the chain, which exerts an increased torque on the rear gear. The rear gear is rigidly affixed to the wheel. So the increased torque on the rear gear is accompanied by an increased torque on the rear wheel. This causes it to exert a larger backward force on the ground, which, in turn, exerts the larger forward force on the wheel. This larger forward force is transmitted through the wheel to the axle, and from that, to the frame. This causes the body of the car to accelerate. The increasing linear velocity of the body is accompanied by an increasing rate of rotation of the wheel.andyrk said:So what does more torque make the rear wheel do? Make it spin faster?
Yes. And because of friction with the ground, instead of just spinning free and slipping, it accelerates the bike forward as it spins faster.andyrk said:So what does more torque make the rear wheel do? Make it spin faster?
Have you looked into the physics of simple levers? Gears are levers working continuously.andyrk said:Also, when we change gears, why does it become easier to pedal(less effort, pedals feel lighter) or difficult to pedal (more effort, pedals feel heavier), depending on the gear we shifted to? Does this have any physics related to it?
What does it have to do with physics?A.T. said:Have you looked into the physics of simple levers? Gears are levers working continuously.
Physics describes levers quite well.andyrk said:What does it have to do with physics?
Do you understand it for levers? Pushing down at different points of a playground seesaw?andyrk said:I don't understand how the fact that it becomes harder or easier to pedal, can be explained by physics.
Yes I know it becomes tougher or easier. But how do you relate it to circular motion and what part of circular motion would it use? Torque? The relation between angular velocity and tangential velocity? What?A.T. said:Do you understand it for levers? Pushing down at different points of a playground seesaw?
I would start with a static case, and try to understand the relation between applied forces (that's what you "feel").andyrk said:But how do you relate it to circular motion and what part of circular motion would it use?
Do you see a connection between gears and levers (and all other machines, in fact)? Have you actually read any basics of this or are you trying to get your education just by asking questions here on PF. That is a very inefficient method of learning.andyrk said:Yes I know it becomes tougher or easier. But how do you relate it to circular motion and what part of circular motion would it use? Torque? The relation between angular velocity and tangential velocity? What?
This:andyrk said:Also, when we change gears, why does it become easier to pedal(less effort, pedals feel lighter) or difficult to pedal (more effort, pedals feel heavier), depending on the gear we shifted to? Does this have any physics related to it?
Also: https://en.wikipedia.org/wiki/Mechanical_advantageA.T. said:It changes the ratios of angular velocities and torques, between back wheel and pedals.
They don't. Again, read this:andyrk said:Why should two gears connected by the same chain rotate with the same torque?
A.T. said:It changes the ratios of angular velocities and torques, between back wheel and pedals.
As Russ said, the torques are not the same. You would already know this if had followed my suggestion in post #58 and drawn the free body diagrams that I indicated. This is a very straightforward system to analyze if you use the free body diagrams. Show us what you can do. On the other hand, if you feel that you have advanced beyond the point where you need to use free body diagrams, I think you will now agree that you haven't.andyrk said:Why should two gears connected by the same chain rotate with the same torque?
No I can understand it by drawing the FBD. But I am studying this just for fun and I am lazy, so I don't want to bother with FBD and instead understand it just by discussion. But if that won't make me understand this completely, then I would have no other option than to draw the FBD. But I am just waiting till then.Chestermiller said:As Russ said, the torques are not the same. You would already know this if had followed my suggestion in post #58 and drawn the free body diagrams that I indicated. This is a very straightforward system to analyze if you use the free body diagrams. Show us what you can do. On the other hand, if you feel that you have advanced beyond the point where you need to use free body diagrams, I think you will now agree that you haven't.
Chet
You've already waited too long. Your admitted laziness has cost those of us who are trying to help you lots of our valuable time. Doesn't that matter to you?andyrk said:No I can understand it by drawing the FBD. But I am studying this just for fun and I am lazy, so I don't want to bother with FBD and instead understand it just by discussion. But if that won't make me understand this completely, then I would have no other option than to draw the FBD. But I am just waiting till then.