Friction problem of playground slide

[pttest]

Homework Statement

A child goes down a playground slide with an acceleration of 1.40 m/s2. Find the coefficient of kinetic friction between the child and the slide if the slide is inclined at an angle of 29.0 degrees below the horizontal.

Homework Equations

Normal force = force of gravity (f_g) cos theta
F_f kinetic = mu_k . normal force

The Attempt at a Solution

here weight of the child is not given. so could someone help me how to approach such problem?

Thanks in advance

 

[RoyalCat]

The mass is irrelevant.

$$\vec F=m\vec a$$

 

[pttest]

I am still confused how to approach the problem without knowing the mass?
could anybody explain little more?

Thanks in advance

 

[rock.freak667]

I am still confused how to approach the problem without knowing the mass?
could anybody explain little more?

Thanks in advance

Forget about the mass for the time being, start by drawing a free body diagram with all the forces acting.

 

[RoyalCat]

I am still confused how to approach the problem without knowing the mass?
could anybody explain little more?

Thanks in advance

Yeah, do what rock.freak suggested. Just assign the mass of the child an arbitrary value, $$m$$, and see where that gets you. You'll see soon enough what we meant by 'irrelevant.' It'll cancel out.

Remember, the coefficient of friction is a pure number. Unless you've got more than one mass that you can add up and divide one by another, it is dimensionally impossible to incorporate the mass of the child into your solution, and as such, the problem boils down to one of two cases. Either its unsolvable (It isn't) or the solution is independent of the mass.

Solving the problem will prove that the latter is correct.