Why no net force when their is constant speed in an object?

Question

Say an object travels at constant speed, how come their is no net force?

Answer 1

Newton's first law states that an object stays in the same state of motion or rest if there is an 'equilibrium of forces' acting on it. There is an equilibrium of forces when a force is equaled by an opposite force (newton's third law of motion).Therefore, there is no resultant or net force.

When an object is moving at constant velocity, lets say it has mass m.

Therefore, according to Newton's second law of motion,

F=mass*acceleration. F is the net or resultant force. Since the object is moving at constant speed, it cannot have acceleration. So, simple mathematics, 0*a number= 0, right? So F=0.

Answer 2

Because of Newton's law, an object in motion continues in that motion unless acted on by an external force.

In the real world, there is the force of friction or air resistance that will slow the object down. Only in outer space will it continue in motion, as there is no friction or air.

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Answer 3

There can be net force even if there is constant speed. However, there is no net force when there is constant velocity. This is because, force is something which changes velocity. So, if there is no change in velocity, then there is no net force.

To understand why this is so, let us assume the opposite. Let us assume that an object is moving at constant velocity and we need to apply a net force on it to maintain its constant velocity. And if something is at rest, then we do not need to apply any force to maintain the state of rest.

But what do we mean when we say that something is moving or is at rest? Moving with respect to what? At rest with respect to what? My table is at rest with respect to Earth and is moving with respect to Sun. There is no meaning of absolute motion or absolute rest. Motion and rest are relative. So, if no force is needed to maintain state of rest, then no force is needed to maintain constant velocity.