Objects move or stay still because of forces and a property called inertia, which together are summed up in Newton’s first law of motion.

Core idea in one line

  • If the forces on an object are unbalanced , it speeds up, slows down, or changes direction.
  • If the forces are balanced (or zero), it keeps doing what it was already doing: either staying still or moving in a straight line at constant speed.

Inertia: why motion (or rest) continues

  • Every object has inertia , its built‑in “stubbornness” to change how it’s moving.
  • More mass means more inertia, so it is harder to start or stop a heavy object than a light one.
  • In deep space, a pushed object just keeps gliding forever at the same speed and direction, because nothing is there to slow it down or speed it up.

Imagine a hockey puck on nearly frictionless ice: once you give it a push, it slides a long way because there is very little to oppose its motion.

Forces: what actually cause changes

Key forces that decide whether something moves or stays still include:

  • Gravity (pulls objects toward Earth or other masses).
  • Contact forces like pushes, pulls, and normal force from a surface (the table pushing up on a book).
  • Friction and air resistance (forces that oppose motion).
  • Tension, electromagnetic forces, and others in more advanced situations.

Whenever the sum of all these forces (the net force) is not zero, the object’s motion changes (it accelerates). If the net force is zero, its motion does not change.

Why objects stay still

Objects stay still when all the forces on them balance each other.

  • A book on a table: gravity pulls it down, the table pushes it up equally. Net force is zero, so the book stays at rest.
  • A hanging sign: gravity pulls down, the chain’s tension pulls up with equal strength, so it doesn’t move.

Even though multiple forces act, they cancel out, so there is no change in motion.

Why objects move (and keep moving)

Objects start moving when an unbalanced force acts on them, and they keep moving at constant speed if no unbalanced force interferes.

  • You pushing a box on a floor: your push to the right may be stronger than friction to the left, so the box speeds up.
  • A car cruising with the engine providing just enough forward force to balance air resistance and friction: its speed stays steady.
  • A spacecraft coasting after its engines switch off in space: with almost no opposing forces, it keeps its same speed and direction for a very long time.

Putting it together (quick claim for your post)

Objects move or stay still because of the balance of forces acting on them and their inertia. When forces are balanced, an object keeps its current state (rest or steady motion). When forces are unbalanced, they cause the object to speed up, slow down, or change direction, changing its motion.

Information gathered from public forums or data available on the internet and portrayed here.