Value of frictional force applied

[Priyadarshini]

Homework Statement

The mass of A is 5kg and the mass of B is 10kg. If the acceleration of A is 2m/s^2 which is smaller than acceleration of B, where A is placed on top of B, then the value of frictional force applied by B on A is:
A) 50N. B) 20N. C) 10N. D) None of these

Homework Equations

a(max)=meu*g
frictional force=meu*normal force

The Attempt at a Solution

[/B]
normal force= 5*10=50N
meu= 2/10 = 1/5
frictional force= 10N
But I had to assume that 2m/s^2 was the maximum acceleration. Can I do that? Because otherwise there are too many unknowns.
Thanks in advance!

 

[Suraj M]

Where did you get the normal force from?

 

[Priyadarshini]

Where did you get the normal force from?

sorry, I forgot to mention the masses. The mass of A is 5kg and B is 10kg

 

[Suraj M]

You don't need to assume that 2 is the max acceleration, they've asked for the frictional force in this situation, not for the maximum frictional force that can exist, so you don't need to worry about that.

 

[Priyadarshini]

You don't need to assume that 2 is the max acceleration, they've asked for the frictional force in this situation, not for the maximum frictional force that can exist, so you don't need to worry about that.

But how would I do it if I don't assume that it is maximum? Because the a=meu*g formula is only for maximum a. So if I don't assume it to be the maximum how would I go about solving it?

 

[haruspex]

But how would I do it if I don't assume that it is maximum? Because the a=meu*g formula is only for maximum a. So if I don't assume it to be the maximum how would I go about solving it?

You are assuming you need to find mu, but you don't.
Forget B, forget friction. A mass m is undergoing a (horizontal) acceleration a in consequence of a net horizontal force F. What equation connects the three?

 

[Priyadarshini]

You are assuming you need to find mu, but you don't.
Forget B, forget friction. A mass m is undergoing a (horizontal) acceleration a in consequence of a net horizontal force F. What equation connects the three?

F=ma

 

[haruspex]

F=ma

Right. So apply that.

 

[UchihaClan13]

In this case it doesn't matter if 2m/s^2 is the maximum acceleration or not
When two blocks are moving relative to each other,the friction between them is always the maximum (when they start their motion or are sliding w.r.t. respect to each other)
If they have a certain relative acceleration,the friction acting is kinetic and is always a constant force

 

[UchihaClan13]

are the two blocks on the ground?Does the ground possesses friction?
is it frictionless??

 

[UchihaClan13]

assuming the ground /surface offers friction is going to make the problem too cumbersome/hard to solve(2 equations,3 unknowns!)

I'll try to do it assuming the ground is frictionless!:)

 

[UchihaClan13]

and yeah no implicit assumptions

 

[haruspex]

assuming the ground /surface offers friction is going to make the problem too cumbersome/hard to solve(2 equations,3 unknowns!)

I'll try to do it assuming the ground is frictionless!:)

As I posted (#6 and #8), you do not need to think about B at all. You know the mass of A, you know its acceleration, and you want to know the force acting on it.

 

[UchihaClan13]

As I posted (#6 and #8), you do not need to think about B at all. You know the mass of A, you know its acceleration, and you want to know the force acting on it.

truly said,haruspex
it's friction which acts forward and provides the acceleration for A
and thus F=m*a=5*2=10 N is the magnitude of the frictional force!:)
I was just complicating things,wasn't I,for the OP?
My bad
:)