no. recall the formula for static friction force. . Is it always equal to C_sf*N?physicos said:Homework Statement
A 3 kg block is on a plane inclined at 30.0° with respect to horizontal.Coefficient of static friction Csfbetween block and plane is 0.60.
What is normal force , and is the block moving ??
Homework Equations
Using second low of Newton (vectors):
Fg +f+N=ma.
So N=mg*Cos(30)
and we know that f=Csf *N = mg cos(30)* Csf
but considering the object not moving f= m*g*sin(30)
so the object must be moving
The Attempt at a Solution
IS IT CORRECT ?
PhanthomJay said:no. recall the formula for static friction force. . Is it always equal to C_sf*N?
No. Think about it. If the static friction force acting up the plane was greater than the component of the weight force acting down the plane, then the object would accelerate up the plane! Does that make any sense? Static friction force is not always the same as the limiting (C_sf)N. For example, if a block of mass m was sitting on a rough table with a static friction coefficient of C_sf, and the block was not moving, what would be the friction force?physicos said:yes it is ! I used a different method , as the object is given a static friction coefficient , I thought a=0 , and I calculated C-sf to see whether it is the same as given by the statement : It wasn't so I concluded the object was not static , is it correct ?
No. Check this outphysicos said:I can't find any helpful link and the teacher was not clear enough ! Do you mean that if f (Friction force ) is greater than a weight (Gravitational force) the object is moving ??
Because we know that f = Csf * N and N= mg*sin(30) which means that f>Wg and thus the object is moving
physicos said:The question is to know whether it is moving or not !
No, you are not catching onto the fact that the static friction force is often less than uN.physicos said:I have an idea : if the object was not moving , it means that :
vectors f+Fg+N=0 so mg sinθ = μs *N
and N = mg cosθ
so
Sinθ = μs *mg cosθ
so μs = tan θ = 0.57 , but μs= 0.60 <tan θ
So the object started sliding.
Is it correct now ?
Yes! Then it starts to slide down the plane and kinetic friction comes Ito play.physicos said:all I want to know now is that is it true that if μs<tan θ the object will start sliding ?
The normal force is the perpendicular force exerted by a surface on an object in contact with it. This force is directly related to the friction force, as it is the force that counteracts the downward force of an object and allows for the object to stay in place or move with a constant velocity.
The normal force can be calculated by multiplying the mass of the object by the acceleration due to gravity (9.8 m/s^2) and the cosine of the angle between the surface and the object. This can be represented by the equation FN = mgcosθ, where FN is the normal force, m is the mass of the object, and θ is the angle between the surface and the object.
The amount of friction force is affected by several factors, including the type of surface the object is on, the weight of the object, and the roughness of the surfaces in contact. Additionally, the coefficient of friction, which is a measure of the interaction between the two surfaces, also plays a role in determining the amount of friction force.
The coefficient of friction can be calculated by dividing the force of friction by the normal force. This can be represented by the equation μ = Ff/FN, where μ is the coefficient of friction, Ff is the force of friction, and FN is the normal force.
Friction force is always opposite to the direction of motion. This means that when an object is in motion, the direction of the friction force will be opposite to the direction of motion. Additionally, the amount of friction force can affect the acceleration of an object, as a larger friction force will result in a smaller acceleration.