Resultant Forces
Describe forces as vector quantities with both magnitude and direction, distinguish between balanced forces (zero resultant, no change in motion) and unbalanced forces (non-zero resultant, causes acceleration or deceleration)
What a learner can do afterwards
- Explains what a vector quantity is and why force is a vector
- Calculates the resultant force when two forces act in the same or opposite directions on an object
- Explains what happens to an object's motion when forces are balanced vs unbalanced
- Draws free body diagrams showing all forces on a stationary object (e.g. a book on a table)
The lesson
A force isn't just how hard you push or pull. It also has a direction. Anything with both a size and a direction is called a vector quantity. Force is a vector, so a 5 newton push to the right is not the same as a 5 newton push to the left, even though the size is equal.
Dog A pulls a sled forward with 40 newtons. Dog B pulls the same sled, same direction, with 30 newtons. Because both forces point the same way, you add them: 40 N + 30 N = 70 N forward. That total is called the resultant force.
To find a resultant force: if the two forces point the same way, add them. If they point opposite ways, subtract the smaller from the larger. The resultant points in the direction of the bigger force.
A book sits still on a table. Gravity pulls it down. The table pushes back up with a normal force of equal size. These two forces are balanced, so the resultant force is 0 N and the book doesn't move. If you then push the book sideways harder than friction can hold it, the forces become unbalanced and the book accelerates.
Same-direction forces add together; opposite-direction forces subtract. A zero resultant means balanced forces and no change in motion; a non-zero resultant means unbalanced forces and acceleration.
Watch it
Where it sits
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.