Micheal is pulling a mass of 30kg up an incline which makes an angle of 26° with the horizontal.
The mass has an upward acceleration of 2.5m/s2 and the coefficient of kinetic friction between the mass and the incline is 0.58.
What is the magnitude of the pulling force Micheal is exerting?
Solving the problem
As usual, we begin by drawing a sketch of what we think is happening.
We draw an incline that makes an angle of 26° with the horizontal, a mass on it, and indicate that the mass is pulled upwards, has an acceleration directed upwards, and is subject to the force of friction:
Looking at the sketch, we can infer that 4 forces are acting on the mass:
We then find the values of Rx and Ry by decomposing R, and substitute them in place of Rx and Ry in the equations that we found in the previous step.
Finally, we use the resulting equations to find F.
Let's begin with the first step of our strategy, by drawing the coordinate axes on our free-body diagram. For convenience, we choose the x axis to be in the direction of motion. Then, we determine the x and y components of all the forces that act on the mass:
Fx = F
Ffx = −Ff
Nx = 0
mgx = −mgsin26°
Fy = 0
Ffy = 0
Ny = N
mgy = −mgcos26°
And we find the components of the resultant force as a sum of the components of all the forces:
Rx = Fx + Ffx + Nx + mgx
Rx = F + (−Ff) + 0 + (−mgsin26°)
Rx = F − Ff − mgsin26°(1)
Ry = Fy + Ffy + Ny + mgy
Ry = 0 + 0 + N + (−mgcos26°)
Ry = N − mgcos26°(2)
The next step is to determine Rx and Ry by decomposing R:
First of all, R has the same direction as the acceleration of the mass, i.e. the positive x direction:
Rx = R = ma
Ry = 0
Now, let's substitute these values in Eq. (1) and Eq. (2), respectively:
Rx = F − Ff − mgsin26°
Ry = N − mgcos26°
ma = F − Ff − mgsin26°(3)
0 = N − mgcos26°(4)
We have two equations with 3 unknowns: F, Ff, N.
To find F, we need to reduce the number of unknowns to 2.
We can do that by remembering that the magnitude of the friction force is:
Ff = μN
We already know the value of the coefficient of kinetic friction μ. Therefore, Eq. (3) becomes:
ma = F − Ff − mgsin26°
ma = F − μN − mgsin26°(5)
At this point, we have two equations, Eq. (4) and Eq. (5), with two unknowns, F and N, so we can easily solve them.
Hence, Micheal is pulling the mass with a force of 360N.
Tips & Tricks
Even though it is good practice to find the value of only those unknowns that are required by the problem, sometimes you have to deal with long equations that have lots of variables in them. In that case it is acceptable to find the values of some unknowns that are not asked by the problem, so that you can keep your equations simple and short (e.g. instead of substituting N with mgcos26°, you can just calculate the value of N, and keep it in the equation as such). In this problem though, we didn't do this, and our equations got a little bit complex, but you should feel free to simplify your equations whenever you think it makes sense.