How to Calculate Unbalanced Force

Written by john brennan
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How to Calculate Unbalanced Force
This skydiver is experiencing unbalanced force. (Stockbyte/Retrofile/Getty Images)

When forces are balanced, there is no net acceleration. If you are lying on the ground, the ground pushes up on you with a force equal to your weight; the forces are balanced, and you move neither up nor down. If you are elevated to 40,000 feet then dropped from a plane, by contrast, the forces acting on you are now unbalanced; gravity is pulling you down, but there is nothing to push you up, so you fall. Calculating unbalanced force is a simple exercise you'll encounter frequently in introductory physics. Here's how to solve these kinds of problems.

Determine the magnitude of the two forces acting on an object in opposite directions. If one of the two forces is Earth's gravity, you can approximate its magnitude by multiplying the object's weight by 9.81 meters per second squared. A 100-kilogram mass, for example, would experience a force of 100 x 9.81 = 981 newtons in Earth's gravitational field. If other forces are involved, their magnitude will be specified in the problem.

Determine the magnitude of the two forces relative to each other. If the two forces are equal and in opposite directions, they are balanced. If one is greater than the other, they are unbalanced. In the example of the 100 kilogram mass, for instance, suppose that the 100-kilogram mass is strapped to a rocket booster, which creates an upward force of 1000 newtons. This force is larger than the force of gravity, so the forces are unbalanced here.

Subtract the smaller of the two forces from the larger. This will give you the net force on the object. In the case of the 100-kilogram object, the smaller force (981 newtons) subtracted from the larger (1000 newtons) yields a net force of 19 newtons pointing directly upwards.

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