Resultant force — Physical education, 14–17 years
Several pushes and pulls can act on one object at once. The resultant force is their combined effect, and it tells us whether the object’s motion will change.
Adding forces
A force is a push or a pull with a size and a direction. When forces act on the same object, combine them as arrows: same direction adds, opposite directions subtract. The single arrow left over is the resultant force, which determines the object’s acceleration.
Why use one force?
An object may be pulled by gravity, pushed by a surface, and slowed by friction at the same time. Listing every force is important, but predicting motion becomes clearer when their combined effect is found. Newton’s laws grew from the need to connect measured pushes and pulls with changes in motion.
Two people pull a box
One person pulls a box right with 50 N, while another pulls left with 30 N. Choose right as positive. The resultant is 50 − 30 = 20 N to the right. If the box has a mass of 10 kg, Newton’s second law gives acceleration = 20 ÷ 10 = 2 m/s² to the right.
Balanced does not mean no forces
People often say that no forces act when an object is still. That seems natural because nothing appears to happen, but opposing forces may simply cancel. A book resting on a table has gravity downward and the table’s support upward; the resultant is zero, so its acceleration is zero.
Designing safe structures
Engineers draw force diagrams when designing bridges, lifts, bicycles, and buildings. They check whether loads are balanced and whether materials can withstand the largest resultant forces, including wind or sudden braking. The same reasoning helps explain why a seat belt must exert a force during a crash.
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