is it possibe for the static friction to be bigger than the applied force?

Question

if an object is pushed by an applied force, but the object doesn't move... what is the static friction?
a) Fs>Fa
b)Fs=fa
c)Fs=m*g*µ

all of the three answers can be correct but which is the best answer? and please explain your reasoning, i know most people would say that the friction force is equal to the applied force, because if friction force was bigger than it move the other way. i don't think that would work because of the fact that friction force would cancel it self out. i think the answer should be a) because the formula for friction force is always constant, and the mew is always constant so it doesn't matter what the applied force is because friction is always the same force.

Answer 1

Newton's law states F = ma where F is net.

so you were actually correct to say if friction was greater it would move in the opposite direction.

Look in any "good" textbook and you'll see that static friction equation is not an equality equation by rather

an inequality equation; or F is less than or equal to umg.



Kinetic friction is equal to umg though. For a level even surface that is.

Answer 2

No, static friction is always equal but opposite to the applied force. If it weren't, the object would start moving. So the answer is b) Fs = Fa.



However, the static friction force does have a maximum value of µ Fn (with Fn the normal force, usually m g, and "mu" a constant). Note that this is the maximum value, the friction force can be less than that if the applied force is less. But as soon as the applied force is larger, the static force will not be enough to cancel it and thus the object will slip (after which kinetic instead of static friction applies).



The formula for static friction does *not* say it is constant. It is an inequality: Fs ≤ µ Fn.

Answer 3

As the object "doesn't move," its acceleration is A = 0. And we have Fa - Fs = f = MA = 0. Which means if the object is static (static friction) it's not moving. And b) Fs = Fa is correct. ANS.



a) can never happen as Fs <= Fa is a reaction force. So it cannot exceed the action force, which is the applied Fa. In fact...



c) is incorrect in general as Fs = mgu is true if and only if Fa >= Fs. When the applied force is fa < Fs, that friction force is fa = fs < Fs = mgu, less than mgu. In other words, c is true only if the applied force is at least mgu in value.

Answer 4

b) is the best choice, because (a) Fs> Fa is only if you can calculate Fs and compare to Fa, and in this Fs = n*Ks. but at this point the only thing is knowing is Fa



therefore the best choice is b)



the object is not moving means it is in static Equilibrium means al the force should add up to zero

Applying N. law then Fa must = on Ops direction Fs at this point



Fa+Fs=0 ====>



Fa +(-Fs since it is ops direction) =0



Fa - Fs= 0 ======> Fa=Fs