When several forces act on an object, the resultant force is the single force that would have the same effect.

Along a line: forces the same way add; forces in opposite directions subtract. The resultant points the way of the bigger total.

1. A crate is pushed right with 200 N and 150 N; friction is 270 N to the left
2. right: 200 + 150 = 350 N
3. resultant = 350 − 270 = 80 N to the right

If the forces balance, the resultant is zero. The object stays still, or keeps moving at a steady speed in a straight line.

A free body diagram shows one object and every force on it, as arrows from the object. Arrow length shows size.

A car at a steady speed: up balances down and forwards balances backwards. If it speeds up, the driving force arrow is longer than the drag arrow.

Only draw forces acting on the object. The force the car exerts on the road belongs on the road's diagram, not the car's.

Two forces at an angle are added with a scale drawing. Pick a scale (say 1 cm = 10 N), draw both forces from one point at the right angle, complete the parallelogram, and measure the diagonal from the starting point. Its length gives the size; a protractor gives the direction.

The reverse job is resolving: splitting one force into two parts at right angles. Draw the force as the long side of a right-angled triangle; the other two sides are the components.

1. A 60 N pull at 30° above the horizontal
2. horizontal part = 60 × cos 30° = 52.0 N
3. vertical part = 60 × sin 30° = 30.0 N

If the components of all the forces balance in both directions, the object is in equilibrium.