Question

Three people are pushing a 500 kg of box in the same direction. applied forces are 30 n, 20 n, and 10 n respectively. if the acceleration of the box is 0.02 m/s2, what is the magnitude of a force created by friction?

Answer 1

The total force applied by the three people is:
$F=30 N+20 N+10 N=60 N$
This force is pushing toward the direction of the motion, while the  frictional force $F_f$ points in the opposite direction.
We can write Newton's second law applied to the block: the resultant of the two forces must be equal to the product between the block's mass and its acceleration
$F-F_f = ma$
We know the mass of the block, m=500 kg, and the acceleration, $a=0.02 m/s^2$, so we can find the friction:
$F_f = F-ma=60 N - (500 kg)(0.02 m/s^2)=50 N$