What the resultant force is
- The resultant force on an object is the single force that has the same effect as all the individual forces added together, direction included
- It determines:
- the direction the object will move (or accelerate) in
- the magnitude of the net push or pull the object actually experiences
- The resultant alone matters for the motion; once you have it, you can ignore the individual forces
Adding forces along a line
- Forces along the same straight line combine by simple addition, taking sign into account:
- Forces pointing the same way are added together
- Forces pointing in opposite directions are subtracted
- A final answer is only complete when both magnitude and direction are stated, in newtons and as a compass-style direction or left/right/up/down
Example — a crate is dragged across the floor. Three pushes act along the same horizontal line: 15 N to the right, plus 4 N and 5 N both to the left. Calculate the resultant force.
- Total force to the right = 15 N
- Total force to the left = 4 + 5 = 9 N
- Resultant = 15 − 9 = 6 N
- The right-hand forces win, so the resultant force is 6 N pointing to the right
Friction inside the resultant
- Friction is the force that opposes motion of an object across or through another material
- On a free-body diagram, a friction arrow points opposite to whichever way the object is sliding
- Drag and air resistance are both special cases of friction: drag inside any fluid, air resistance inside air specifically