When finding net force, why must you know the directions of the forces acting on an object? give two reasons.

Since, the force is a vector quantity. Hence, it depends not only on the magnitude but also on the direction.

Reason:

1) The effects of the forces on motion will change. For example: If the forces are acting in the opposite direction then one force wants to make the body move while the other wants to stop it. But if you don't know the direction then you won't be able to tell the effect.

2) While adding the forces, we need to add them using their direction. One cannot add simply 1 N + 2 N = 3 N without knowing the direction because it is possible the forces are not acting in the same direction. We need vector method to add the forces, which require directions.

Force is a vector quantity. The directions of the forces acting on an object are important to be known, so that the forces in the same direction can be added.  Thereafter, net force can be calculated.

The direction of the net force is also determined by the directions of the individual forces acting on the object.

t = 1/400 h   ⇔ 0.0025 h ⇔ 9s ⇔ 0.15 m

any of the answers are correct.