* Inertia: Objects in motion tend to stay in motion. This means that once an object is moving, it will continue moving at a constant speed and direction unless acted upon by a force.

* Newton's First Law: This law of motion states that an object at rest will stay at rest, and an object in motion will stay in motion at a constant velocity unless acted upon by a net force.

Here's a breakdown:

1. To start moving: You need a force to overcome inertia and get the object moving from a standstill.

2. To maintain speed: Once moving, you only need a force to overcome friction and other opposing forces (like air resistance) to maintain a constant speed.

3. To change speed: A force is needed to accelerate or decelerate the object.

Example:

Imagine pushing a box across a smooth floor.

* You apply a force to start the box moving.

* Once it's moving, you can reduce the force you apply, but the box will still continue moving forward (at a decreasing speed) due to inertia.

* To keep the box moving at the same speed, you need to apply just enough force to counter friction.

In short:

* You need a force to *initiate* motion.

* You don't need a constant force to *maintain* speed, but you may need a force to counteract friction.

* You need a force to *change* speed.