Frictional Force Problem

1. Jan 27, 2016

1. The problem statement, all variables and given/known data
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)

2. Relevant equations

f= meu*N
a=meu*g
3. 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.

2. Jan 27, 2016

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 $μ$

3. Jan 27, 2016

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

4. Jan 27, 2016

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

Last edited: Jan 27, 2016
5. Jan 27, 2016

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.

6. Jan 27, 2016

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.

7. Jan 27, 2016

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

8. Jan 27, 2016

Suraj M

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.
(PS- Look at us 3 Indians at this hour :-) sorry off subject- ignore this)

9. Jan 27, 2016

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

10. Jan 27, 2016

Suraj M

Sorry my bad, down here we just omit the "darshini"
No offence, force of habit

11. Jan 27, 2016

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

12. Jan 27, 2016

physicsisnotboring

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

friction = 30 N