What is Net Force? – Definition, Magnitude & Equations

Definition of a Net Force

When you kick a soccer ball, the ball takes off and moves through the air. At that moment, there is a net force acting on the ball. When the ball starts to come back to the ground and eventually stops, there is a net force acting on the ball as well. Newton’s Second Law says that when a net force is acting on an object, then that object must be accelerating, that is, its speed changes from second to second. When you first kick the soccer ball, it accelerates, and when the soccer ball begins to slow down to a stop, it is also accelerating.

A net force is defined as the sum of all the forces acting on an object. The equation below is the sum of N forces acting on an object.

There may be several forces acting on an object, and when you add up all of those forces, the result is what we call the net force acting on the object. If the net force adds up to zero, then the object is not accelerating, therefore it moves with a constant speed. If the net force adds up to a non-zero value, then the object is accelerating.

Magnitude and Equation

According to Newton’s Second Law, when an object accelerates, then there must be a net force acting on it. Conversely, if a net force acts on an object, that object will accelerate. The magnitude of the net force acting on an object is equal to the mass of the object multiplied by the acceleration of the object as shown in the formula below.

If the net force acting on an object is zero, then the object is not accelerating and is in a state that we call equilibrium. When an object is in equilibrium, then two things can be true: either the object is not moving at all, or the object is moving with a constant speed. The formula for equilibrium is shown below.

Examples

Consider a hypothetical situation in space. You are doing a spacewalk and are fixing something on your shuttle. While working on the issue with a wrench, you get angry. You throw the wrench away from you. What happens? Well, once the wrench leaves your hand, it will continue moving with the same speed that you gave it when you threw it. This is an example of a zero net force situation. The wrench will move with the same speed and will not accelerate in space. If you throw the same wrench on Earth, the wrench will fall to the ground and eventually stop. Why did it stop? There is a net force acting on the wrench causing it to slow down and stop.