Can the only reaction force (to gravity) be friction?

[Jeviah]

Homework Statement

If you have an object say in a vertical pipe, which only has a gravitational force acting on it would the reaction forces only be friction forces?
If this is the case how would you calculate the forces? would you calculate the friction force using the friction coefficient or just take Fy as friction?

Homework Equations

Friction F=Friction coefficient * F(n)
Fy= F*sin(theta)

The Attempt at a Solution

I would say that all forces acting against gravity would be friction forces, also assuming the object is at equilibrium then static friction would act in an upwards direction.

 

[kuruman]

The force of static friction is what is necessary to provide the observed acceleration. It has a maximum value related to the coefficient of static friction, but this does not mean that it is always equal to this maximum value for the same reason that if you have you have a credit card with a limit of $5,000 it doesn't mean that every time you buy something you have to charge $5,000. Consider a magnet stuck to a refrigerator. If the magnet is at rest, the force of static friction is equal to the weight of the magnet, no more no less, i.e. what is necessary to provide zero acceleration.

On edit: This force of friction is not the reaction force to gravity pulling on the magnet. The reaction force to the magnet's weight is the gravitational force that the magnet exerts on the Earth.

 

[drvrm]

I would say that all forces acting against gravity would be friction forces, also assuming the object is at equilibrium then static friction would act in an upwards direction.

frictional forces have a way to act...it always act at the surface of contact and its

direction is against the direction of possible movement..

you can not pre fix its way to act.

 

[Jeviah]

The force of static friction is what is necessary to provide the observed acceleration. It has a maximum value related to the coefficient of static friction, but this does not mean that it is always equal to this maximum value for the same reason that if you have you have a credit card with a limit of $5,000 it doesn't mean that every time you buy something you have to charge $5,000. Consider a magnet stuck to a refrigerator. If the magnet is at rest, the force of static friction is equal to the weight of the magnet, no more no less, i.e. what is necessary to provide zero acceleration.

On edit: This force of friction is not the reaction force to gravity pulling on the magnet. The reaction force to the magnet's weight is the gravitational force that the magnet exerts on the Earth.

Thank you for the quick response i understand. I do have another question though, for the object not to move it has to be in equilibrium as you have said, if there are 3 points of contact with a surface, two on the right hand side and one of the left would the sum of friction force on the right hand side have to be relatively balanced with the contact point on the left? Or could the left be say 1N, the right hand side 1000N with the weight being a total of 1001N?

 

[CWatters]

I'm finding it difficult to understand the situation you are describing..

Homework Statement

If you have an object say in a vertical pipe, which only has a gravitational force acting on it would the reaction forces only be friction forces?

If something _only_ has gravity acting on it then it _only_ has gravity acting on it. No friction, no springs, no other forces acting on it, only gravity.

If the object isn't accelerating vertically then the net force acting on it must be zero because of Newton's law...

Fnet = mass * acceleration

That implies there must be another vertical force in addition to gravity to make the total zero, but it doesn't have to be friction. If the object is an air tight fit in the pipe then air pressure below the object might prevent it falling.

The problem seems badly specified.

 

[CWatters]

Thank you for the quick response i understand. I do have another question though, for the object not to move it has to be in equilibrium as you have said, if there are 3 points of contact with a surface, two on the right hand side and one of the left would the sum of friction force on the right hand side have to be relatively balanced with the contact point on the left? Or could the left be say 1N, the right hand side 1000N with the weight being a total of 1001N?

Newton's law also applies to rotation as well as translation.

If the object doesn't have angular acceleration then the net torque must be zero. If that's what you mean by the left and right forces being in balance then yes.

PS: Perhaps draw a free body diagram of the object.