Question 4
A force of 25 N is applied to push a box along horizontal ground. The frictional force acting on the box is 20 N. What is the unbalanced force acting on the box?
The right answer is the one marked above.
5 N along the direction of motion. The two forces act in opposite directions along the same line, so they can be subtracted:
unbalanced force = 25 N − 20 N = 5 N
The applied force is the larger, so the 5 N left over points the way it is being pushed, and the box starts to move.
Had the applied force been less than 20 N, friction would have balanced it, the resultant would have been 0, and the box would not have moved.