IGCSE Physics / Forces and shape
Resultant forces and free-body diagrams revision notes
Edexcel International GCSE Physics (4PH1), chapter 2: Forces and shape, specification point 1.15, 1.11. Free lesson.
Watch the video lesson · IGCSE Physics past papers
A resultant force is the single force with the same effect as all the forces on an object combined. Finding the resultant and drawing free-body diagrams helps you work out whether an object's motion will change.
What a resultant force means
Several forces can act on an object at once. The resultant force is the single force that has the same effect as all of them acting together. Force is a vector, so you must give both its size and its direction. For example, write 400 N forwards, not just 400 N.
How to draw a free-body diagram
A free-body diagram shows one object and every force acting on it, with nothing else. Draw the object simply, then add ruled arrows that start on it. Each arrow should be labelled and its length should match the force's size. For a motorboat, the weight is 2000 N down and the upthrust is 2000 N up, so their arrows are equal in length. The thrust is 1200 N forwards and the drag is 800 N backwards, so the drag arrow is shorter. Label the downward force weight, not gravity.
How to find the resultant along a line
Work along one line at a time. Forces in the same direction add together. Forces in opposite directions subtract, and the resultant acts in the direction of the larger force.
What balanced and unbalanced forces do
Balanced forces have a resultant of zero. They do not change an object's velocity, which means its speed and direction stay the same. This does not mean the object must be stopped. Unbalanced forces have a non-zero resultant, so the object's velocity changes. An object at rest with balanced forces stays at rest. With an unbalanced force, it starts moving in the direction of the resultant. A moving object with balanced forces keeps the same speed and direction. An unbalanced force can make it speed up, slow down or turn. A lift moving upwards at a steady speed has balanced forces, so…