Answer:
The mass of the crate is 5kg.
We know that the force of friction can be obtained by:
F = N*k
where k is the coefficient of friction, where we use the static one if the object is at rest, and the kinetic one if the object os moving. N is the normal force
If we tilt the base making an angle of 30° with the horizontal, now the normal force against the plank will be equal to the fraction of the weight in the direction normal to the surface of the plank.
Knowing that the angle is 30°, then the fraction of the weight that pushes against the normal is Cos(30°)*W = cos(30°)*5kg*9.8m/s^2 = 42.4N
The fraction of the force in the parallel direction to the plank (the force that would accelerate the crate downwards) is:
F = sin(30°)*5k*9,8m/s = 24.5N
now, the statical friction force is:
Fs = 42.4N*0.5 = 21.2N
The statical force is less than the 24.5N, so the crate will move downwards, then the force that acts on the crate is the kinetic force of friction:
Fk = 42.4N*0.4 = 16.96N
Then, the total force that acts on the crate is:
total force = F - Fk = 24.5N - 16.69N = 7.54N and the direction of this force points downside along the parallel direction of the plank.
