# Friction on a particle on rough horizontal rotating disc

• koliko987
In summary: So, in summary, the question is asking for the minimum and maximum values of tension in an elastic string attached to a rotating rough disc, where the string is shorter than the radius of the disc and the particle moves in uniform circular motion. The minimum tension occurs when the particle is moving slowly and friction acts opposite the tension, while the maximum tension occurs when the particle is moving quickly and friction acts in the same direction as the tension. The direction of friction changes with different speeds because the net force must act radially towards the center of the circle, and the string also exerts a force.
koliko987
A particle is attached to an inextensible string. The other end of the string is attached to the centre of a rotating rough disc. The string is shorter than the radius of the disc so the particle remains on the disc and moves in uniform circular motion.
I don't remember the quantities but the question was to find min and max values of tension in the string(for different speeds of rotation).
Apparently min tension is when the particle is moving the most slowly and at his point friction is acting opposite the tension so Tmin - Fr = mv^2/r.
Similarly the max tension is when speed is greatest and at this speed friction acts in the same direction as tension so Tmax + Fr = mv^2/r.
I really don't understand why the direction of friction changes with different speeds and how can the friction act outward of the circle, wouldn't that be a non existent centrifugal force? I though in uniform circular motion friction acts tangentially and toward the circle and nowhere else.
Can someone help me understand this? Any help is greatly appreciated. Cheers.

Edit: I found the question. It's actually an elastic string. The string is extended and it's asking for min and max values of angular speed without changing the extension. So tension is constant and angular speed is changing. Sorry about that.

Last edited:
koliko987 said:
I really don't understand why the direction of friction changes with different speeds and how can the friction act outward of the circle, wouldn't that be a non existent centrifugal force?
What if the speed was zero (or at least very low)? Which way would the forces act on the particle? What if the speed was very high?

koliko987 said:
I though in uniform circular motion friction acts tangentially and toward the circle and nowhere else.
In uniform circular motion, the net force must act radially (not tangentially) towards the center of the circle. If friction were the only force acting, then it would act toward the center. But that's not the case here. The string also exerts a force.

Edit: I found the question. It's actually an elastic string. The string is extended and it's asking for min and max values of angular speed without changing the extension. So tension is constant and angular speed is changing. Sorry about that.

That explains a lot :-)

## 1. What is friction?

Friction is a force that opposes motion between two surfaces in contact. It is caused by the irregularities in the surfaces and the interlocking of the microscopically rough surfaces.

## 2. How does friction affect a particle on a rough horizontal rotating disc?

Friction on a particle on a rough horizontal rotating disc can cause the particle to experience a tangential force that opposes its motion. This can result in the particle moving at a slower speed or even coming to a stop, depending on the amount of friction present.

## 3. What factors affect the amount of friction on a particle on a rough horizontal rotating disc?

The amount of friction on a particle on a rough horizontal rotating disc can be affected by several factors, including the roughness of the surfaces, the weight of the particle, and the speed of rotation of the disc. Additionally, the type of material the surfaces are made of and the presence of any lubricants can also impact the amount of friction.

## 4. Can friction be beneficial in this scenario?

Yes, friction can be beneficial in certain cases. For example, if the particle needs to be held in place on the rotating disc, friction can provide the necessary force to keep the particle from sliding off. Additionally, friction can also be used to slow down or stop the rotation of the disc.

## 5. How can the effects of friction be reduced in this scenario?

The effects of friction on a particle on a rough horizontal rotating disc can be reduced by using lubricants to reduce the roughness of the surfaces and create a smoother contact. Additionally, reducing the weight of the particle and decreasing the speed of rotation of the disc can also help to reduce friction.

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