- #106
alkaspeltzar
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Assume its isolated. Whether it be the Earth or plateA.T. said:Then it's not an isolated system, unless you include the entire Earth.
Assume its isolated. Whether it be the Earth or plateA.T. said:Then it's not an isolated system, unless you include the entire Earth.
That what I meant. Please respond to my earler question. Post 95jbriggs444 said:That's not Newton's 3rd.
It's equal and opposite and applied symmetrically between the same pair of objects.
alkaspeltzar said:A is fixed the ground so it isn't rotating in circles, it is the axis
If the entire thing with plate is isolated, then you cannot guarantee that A is inertial, unless you specify that A is the center of mass of the entire isolated system.alkaspeltzar said:Assume its isolated. Whether it be the Earth or plate
If you are using reference point A then there are two non-zero torques on the ground. One from the motor and one from the axle from gear B.alkaspeltzar said:Okay, if the gears like B are accelerating then how do you determine the torque about A( with the ground connected) such that you can determine how it rotates oppositely?
That's my question. If gear B is accelerating rotationally, what torque balances out the entries assembly( plus ground).
jbriggs444, hold on, i think you might have found my errorjbriggs444 said:If you are using reference point A then there are two non-zero torques on the ground. One from the motor and one from the axle from gear B.
If you are using reference point B then there are two non-zero torques on the ground. One from the motor's torque and one from the axle from gear A.
The standard approach to problems of this sort is to start writing down equations. For instance, for the angular acceleration of gear A based on the sum of torques on it divided by its moment of inertia.
Can you write down some equations for us? Feel free to make the ground infinitely massive so that the positions of the axles at the center of A and B do not move. [As pointed out previously, an infinitely massive ground can still act as a source or sink for angular momentum]. With enough equations in hand, we can solve for forces and accelerations.
You are still not showing any equations, any calculations and not even any list of torques.alkaspeltzar said:jbriggs444, hold on, i think you might have found my error
"If you are using reference point A then there are two non-zero torques on the ground. One from the motor and one from the axle from gear B."
I am using reference point A, but isn't there the reaction force from the axle on the Gear B. If you see my drawing in post #73 that is what i was trying to show. Then everytime i figure the torques about A, it would sum to zero.
Can i not use the reaction from the axle on B about A as part of the calculation? If not, can you tell me why?
I think this is what is creating the error
You evidently have the force on the plate at B facing the wrong direction and you also don't know the magnitude. The torque on the plate about A from that force and the motor torque do not sum to zero.alkaspeltzar said:jbriggs444, hold on, i think you might have found my error
"If you are using reference point A then there are two non-zero torques on the ground. One from the motor and one from the axle from gear B."
I am using reference point A, but isn't there the reaction force from the axle on the Gear B. If you see my drawing in post #73 that is what i was trying to show. Then everytime i figure the torques about A, it would sum to zero.
Huh?alkaspeltzar said:Contact forces at gear interface go to zero, sowe can ignore those
Those are wrong and you haven't really attempted to properly calculate them yet.Force on axle at B about A is 60inlbs
Force on B from axle about A is -60 inlbs
i cleaned up, see editsruss_watters said:Huh?
Those are wrong and you haven't really attempted to properly calculate them yet.
To get the angular acceleration of the plate you sum only the torques acting on the plate, not all torques that exist. You seem to have problems with applying N2L.alkaspeltzar said:When i sum all this up, it is zero. That is not right, as the plate/entire assembly about A should rotate opposite of the rotation of gear B
SOB, I was treating the force on gear B and the plate the same. There is force on B due the axle, but it doesn't create a torque about A for the entire plate.A.T. said:To get the angular acceleration of the plate you sum only the torques acting on the plate, not all torques that exist. You seem to have problems with applying N2L.
Again: Those are wrong and you haven't really attempted to properly calculate them yet.alkaspeltzar said:Force on axle at B, which is 10, creates a torque about A that is 60inlbs
Force on B from axle, which is 10, creates a torque about A that is -60 inlbs
You should use the exploded view in your FBDs to avoid confusing yourself:alkaspeltzar said:I just applied it wrong in my freebody...fml!
jbriggs444 said:So we are talking about the drawing in #73. And you are summing torques around point A at the center of the driven gear A.
But you are responding to a post by @A.T. where he speaks of an "isolated system". There is no isolated system in post #73.
We can turn this into an isolated system composed of three entities. There is the ground, there is gear A. There is gear B.
We wish to identify the torques on gear B about the reference point at the center of gear A.
We begin by listing the forces on gear B. There are two. The force from the teeth on gear A where they mesh with gear B. And the force of the axle where gear B is fixed to the ground. The two forces are equal and opposite. The moment arms are not equal. So the torques are not equal and opposite. So the angular momentum of gear B changes over time.
We can apply Newton's third law for forces and identify the third law partner forces: The force from the teeth on gear B on gear A and the force of gear B on the ground at its axle.
Both force pairs are contact forces. The points of application of the two members of the one force pair are co-located. The points of application of the two members of the other force pair are also co-located. Newton's third law for torque follows trivially in this case. The moment arm for the two members of the force pair are equal. The forces are equal and opposite. So the torques will be equal and opposite.
One can see that the total torque on the three pieces of the isolated system will necessarily sum to zero as long as only contact forces are involved. For every torque in the sum, there is an equal and opposite torque somewhere else in the sum.
Hey we got there guess that's all that matters nowA.T. said:You should use the exploded view in your FBDs to avoid confusing yourself:
https://peer.asee.org/the-exploded-...pproach-to-teaching-the-free-body-diagram.pdf
Propellor driven (drawn?) aircraft with big engines have a tendency to flip over as a result of the "equal and opposite reaction. Pilots have to be careful not to apply too much power until they are traveling fast enough for the ailerons to counteract it.alkaspeltzar said:Summary:: There are similar 1st and 2nd laws for torques as there are for forces, but it doesn't seem like this applies to N3L?
True, but its harder to see when there are multiple axis which is why I was trying to think/form the gear example.Shane Kennedy said:Propellor driven (drawn?) aircraft with big engines have a tendency to flip over as a result of the "equal and opposite reaction. Pilots have to be careful not to apply too much power until they are traveling fast enough for the ailerons to counteract it.
Another example is a planetary gearbox. If just one part is fixed, then the other parts will share the power. In the case of the planets being fixed, the inner and outer will rotate in opposite directions with the opposite (to input direction) torque being experienced by the gearbox mountingalkaspeltzar said:True, but its harder to see when there are multiple axis which is why I was trying to think/form the gear example.
But in the end, torque is about a constant axis or point of rotation and I mixing them up. Also if the gears are attached to Earth then it too isn't a closed system so we typically ignore Ang conservation and equal/opposite forces etc as they don't apply
Thanks for good example
alkaspeltzar said:This is what my physics books says. It says that thru N3L, internal torques on a system should sum to zero. But doesn't say there is a N3L strictly for torque