# Centripetal force question

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#1
Hello, in the question a sock is being pushed around the walls of a drum of a spin dryer, please may you tell me why the resultant force at the top of the drum isn't = weight of the sock? 🙂

If I've not been specific enough, please tell me 🙂
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2 years ago
#2
The resultant force is mv^2/R, which is equal to the weight normal contact force.
Last edited by Levi23317; 2 years ago
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#3
(Original post by Levi23317)
The resultant force is mv^2/R, which is equal to the weight - normal contact force.
But for an ideal situation, why is there a normal contact force? 🙂
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2 years ago
#4
There is always a normal contact force when an object is resting against another.
In that sort of case of circular motion, at the top the weight of an object contributes to the force acting toward the centre of the circle. This does not mean there isn't any normal contact force, but the force toward the centre of the circle must be greater than the normal contact force, so that the resultant (the weight of the sock minus the normal contact force) still acts toward the centre of the circle.
I don't know if that's helpful, but hopefully some of it is.
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#5
(Original post by tautologica)
There is always a normal contact force when an object is resting against another.
In that sort of case of circular motion, at the top the weight of an object contributes to the force acting toward the centre of the circle. This does not mean there isn't any normal contact force, but the force toward the centre of the circle must be greater than the normal contact force, so that the resultant (the weight of the sock minus the normal contact force) still acts toward the centre of the circle.
I don't know if that's helpful, but hopefully some of it is.
It's a bit helpful, although, why does the normal contract force act upwards? 🙂
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2 years ago
#6
(Original post by Freedom physics)
Hello, in the question a sock is being pushed around the walls of a drum of a spin dryer, please may you tell me why the resultant force at the top of the drum isn't = weight of the sock? 🙂

If I've not been specific enough, please tell me 🙂
Why resultant force need to be equal to the weight of the sock?

If the sock undergoes vertical circular motion, the resultant force is generally equal to the weight of the sock and the normal reaction of the sock.

When the sock is at the top of the circular motion, the normal force on the sock is toward the centre of the circle and the weight of the sock is also towards the centre of the circle.

(Original post by Freedom physics)
But for an ideal situation, why is there a normal contact force? 🙂
Not sure what do you mean by ideal situation. To undergo vertical circular motion, the sock need to move at minimum speed. If the sock moves at such speed, the resultant force is equal to the weight of the sock where the normal contact force is zero.

(Original post by Freedom physics)
It's a bit helpful, although, why does the normal contract force act upwards? 🙂
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2 years ago
#7
It doesn't, it acts downwards at the top of the spin. The unbalanced force is mv2/R but this equals mg plus the normal reaction.
So in theory the mv2/R could just be provided by mg if it is spinning at the perfect speed. In reality the drum will need to provide some reaction force to keep it moving in a circle if it is spinning any faster than this ideal.
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