# Atwood Machine Inertia Friction and directional forces

• Raju1972
In summary: This is surprising. Could there be a problem with the experiment?In summary, the conversation revolves around an Atwood machine that the speaker has built. They are using simple pulleys without bearings, which may be affecting the results of their experiments. The speaker is considering using bearings in the pulleys and is seeking advice on the types of bearings to use. They also mention that the weight required on one side of the pulley to start acceleration is close to twice the weight on the other side, which they find surprising. The conversation ends with the speaker planning to try using bearings in their pulleys and thanking the other person for their help.
Raju1972
I have an Atwood Machine. I have an aluminium Pulley of radius 4 inch. A gym cable running over it. Not sure coefficient of friction but smooth movement. One end of rope, I attach wt. of 0.5 kg. I had to attach weight of 1 kg for acceleration to start. If I attach 2 kg, I had to attach 3.1 kg, for the system to start acceleration. If I attach 4 kg, I had to attach 6.1 kg to other end. Now if I take 2 pulleys and run rope over them and repeat experiment, I need even more weight differential for system to accelerate. Thus there are some forces acting which need to overcome to start system acceleration. I think apart from inertia of pulley and static frictional force, probably some x and y directional force components are coming into play. All Atwood problems talk about equation for acceleration but not able to find equation for "weight differential" required for the system to start accelerating. Also any suggestions on ways to reduce this weight differential to start system acceleration shall help.

Because you have measurements, it sounds as if you're describing the behavior of an Atwood machine that you've actually built. If so, that's pretty cool... and the construction details will matter. What did you use for bearings for each pulley?

Nugatory said:
Because you have measurements, it sounds as if you're describing the behavior of an Atwood machine that you've actually built. If so, that's pretty cool... and the construction details will matter. What did you use for bearings for each pulley?

As I am doing a prototype, to cut on cost, I used very simple pulleys. Hence my pulleys have no bearings. Now when I use wt of 0.5 kg, I had to put 1 kg. So I am assuming I had to put extra 0.5 kg to accommodate all the deficiencies of cheap materials used. So next time when I take 2 kg, I would be fine if I get acceleration when I attach 2.5 kg at other end (extra 0.5 kg for pulley/rope friction, rotational inertia of cheap material) But I had to put 3 kg to start acceleration. Similarly when I use 4 kg, I would be fine to see acceleration at 4.5 kg. But I had to put 6 kg. Things get worse when I use Atwood Machine with 2 pulleys. Do you still think this huge difference is only because I am using pulleys with no bearings? If yes, please suggest what kind of bearings/pulleys I should use.. I suspect that the so called inertia I am observing before acceleration is a function of masses attached probably due to vector forces. But I am unable to prove it. I am attaching images of my setup. One set of images is for "2 pulley Atwood machine with small pulleys" and other set of images is with big pulleys and 1 and 2 pulley Atwood machine. [Even though I did experiments for all combinations]

#### Attachments

• Double Big Pulley.JPG
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• Double Small Pulley.JPG
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• Single Big Pulley Atwood.JPG
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• Double Small Pulley Atwood.JPG
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I was right - what you are doing is pretty cool! :)
Raju1972 said:
As I am doing a prototype, to cut on cost, I used very simple pulleys. Hence my pulleys have no bearings.
That is a reasonable tradeoff to make, but the downside is that the physics of a rotating shaft in a sleeve is fairly complicated: You have to consider static friction, dynamic friction, and the way that the sideways loads on the pulleys will tend to jam the shafts against the sides and change the resistance of the pulleys - any or all of these may be large enough to overwhelm the effects that you're trying to measure.

You can try smoothing the shafts, using shafts that are as round as possible (a lathe would be great, but just chucking them in a power drill and turning them against sandpaper works), try for the smoothest possible tight fit in the holes, lubricate the shafts... Try to get as close as possible to the ideal frictionless rotation that a theoretical Atwood machine depends on.

But you'll probably be happiest if you can find some bearings in the trash somewhere. The wheel bearings in a scrapped bicycle would be good for the weights and sizes you're working with, and old hard disk drives contain extremely high-quality ball bearings but may be a bit small for your current design.

Nugatory said:
I was right - what you are doing is pretty cool! :)
the sideways loads on the pulleys will tend to jam the shafts against the sides and change the resistance of the pulleys - any or all of these may be large enough to overwhelm the effects that you're trying to measure.
But you'll probably be happiest if you can find some bearings in the trash somewhere. The wheel bearings in a scrapped bicycle would be good for the weights and sizes you're working with, and old hard disk drives contain extremely high-quality ball bearings but may be a bit small for your current design.

As I think more, I tend to agree with you. Initially I neglected bearings but bearings are probably must. As I increase weight, there is more tension in the rope and that in turn adversely affecting load on shaft. Having bearings shall probably help convert this load into rotational force, which is what is required. I am out of town this weekend but will try inserting bearings into my pulley and check the results. Thanks for your help.

I did few more experiments with bearings in the pulley. But not much of a difference. With latest experiments using 2 pulleys, I am seeing that wt. required on other side of pulley to start acceleration, is close to twice the wt. on one side. So if wt. is x kg on one side then I need to put ~2x on other side before acceleration starts.

## What is an Atwood Machine?

An Atwood Machine is a simple mechanical device that consists of a pulley, a string, and two masses. It is used to demonstrate the concepts of inertia, friction, and directional forces.

## What is the role of inertia in an Atwood Machine?

Inertia is the tendency of an object to resist changes in its state of motion. In an Atwood Machine, the inertia of the masses causes them to continue moving in the direction they were initially set in, unless acted upon by an external force.

## How does friction affect an Atwood Machine?

Friction is a force that opposes the motion of objects in contact. In an Atwood Machine, friction can cause the string to slip on the pulley, reducing the efficiency of the machine and affecting the acceleration of the masses.

## What is the significance of directional forces in an Atwood Machine?

Directional forces, also known as tension forces, play a crucial role in an Atwood Machine. They are responsible for transmitting the pulling force from one mass to the other through the string, allowing the masses to move.

## How does the direction of the pulley affect the motion of an Atwood Machine?

The direction of the pulley can affect the direction of the forces acting on the masses, thus affecting their motion. For example, if the pulley is inverted, the direction of the tension force will also be inverted, causing the masses to move in the opposite direction compared to a regular pulley configuration.

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