What is the Frictional Force Acting on a Block on a Rough Incline Plane?

[Priyadarshini]

Homework Statement

A block of mass 10kg is released on a rough incline plane which is inclined at 30 degrees. Block start descending with acceleretion 2m/s^2. Kinetic friction force acting on the block is:
A) 10. B) 30. C) 50. D) 50*3^(1/2)

Homework Equations

f= meu*N
a=meu*g

The Attempt at a Solution

meu= 2/10=1/5
N=mgcostheta
= 50*3^(1/2)
frictional force= 50*3^(1/2) / 5
which is 10*3^(1/2) and is not in the options.

 

[Suraj M]

I think you should start off by determining the forces acting on the object first
Equate through forces, you don't even need to bring in the variable $μ$

 

[Priyadarshini]

I think you should start off by determining the forces acting on the object first
Equate through forces, you don't even need to bring in the variable $μ$

50-friction=ma
50-friction= 20
f=30N
But why doesn't my initial working give me this answer?

 

[Suraj M]

How did you get your coefficient of friction to be 1/5?
It's not in plane with the ground hence you can't just take 2/10 you have to take their components

 

[Priyadarshini]

How did you get your coefficient of friction to be 1/5?
It's not planar with the ground hence you can't just take 2/10 you have to take their components

What do you mean by "planar with the ground"? The f=N*meu formula has conditions? I thought it could be applied to any situation.

 

[cnh1995]

Which force will pull the block down and which force will oppose the motion? Draw FBD. As Suraj said earlier, you don't need μ.

 

[Priyadarshini]

Which force will pull the block down and which force will oppose the motion? Draw FBD. As Suraj said earlier, you don't need μ.

Thank you, the answer matched. However I do not completely understand why my initial method did not give me the same answer.

 

[Suraj M]

What do you mean by "planar with the ground"? The f=N*meu formula has conditions? I thought it could be applied to any situation.

Not conditions Priyadarshini, your normal force is ?
You've taken 2/10
That denominator is the normal acceleration right? But that's not the normal acceleration in this case because the plane is not parallel to the ground
So you'll have to take a component of this.
FBD please
(PS- Look at us 3 Indians at this hour :-) sorry off subject- ignore this)

 

[Priyadarshini]

Not conditions Priya, your normal force is ?
You've taken 2/10
That denominator is the normal acceleration right? But that's not the normal acceleration in this case because the plane is not parallel to the ground
So you'll have to take a component of this.
FBD please
PS- Look at us 3 Indians :-) sorry off subject- ignore this

Ohhh. I see! Thank you!
Haha, true. The name is Priyadarshini by the way, not Priya.

 

[Suraj M]

Ohhh. I see! Thank you!
Haha, true. The name is Priyadarshini by the way, not Priya.

Sorry my bad, down here we just omit the "darshini"
No offence, force of habit
Hope you got your answer[emoji106]

 

[Priyadarshini]

Sorry my bad, down here we just omit the "darshini"
No offence, force of habit
Hope you got your answer[emoji106]

None taken, a lot of people do that.
Yup, thanks a lot.

 

[physicsisnotboring]

mgsin(30)-friction = ma
10*10*(1/2) - friction = 10*2

friction = 30 N