Newton's First Law

According to Newton's First Law of motion, an object remains in the same state of motion unless a resultant force acts on it. If the resultant force on an object is zero, this means:

  • a stationary object stays stationary
  • a moving object continues to move at the same velocity (at the same speed and in the same direction)

Inertia - Higher

The tendency of an object to continue in its current state (at rest or in uniform motion) is called inertia.

Examples of objects with uniform motion

Newton's First Law can be used to explain the movement of objects travelling with uniform motion (constant velocity). For example, when a car travels at a constant speed, the driving force from the engine is balanced by resistive forces such as air resistance and friction in the car's moving parts. The resultant force on the car is zero.

Other examples include:

  • a runner at their top speed experiences the same air resistance as their thrust
  • an object falling at terminal velocity experiences the same air resistance as its weight
The image shows a box with two arrows of equal size and length extending out from each side.If the forces acting on an object are balanced, the resultant force is zero

Examples of objects with non-uniform motion

Newton's First Law can also be used to explain the movement of objects travelling with non-uniform motion. This includes situations when the speed, the direction, or both change. For example, when a car accelerates, the driving force from the engine is greater than the resistive forces. The resultant force is not zero.

Other examples include:

  • at the start of their run, a runner experiences less air resistance than their thrust, so they accelerate
  • an object that begins to fall experiences less air resistance than its weight, so it accelerates
The image shows a box with two arrows extending, one from each side. The left arrow is much shorter than the right arrow, representing the unbalanced forces acting on the object.If the forces acting on an object are not balanced, the resultant force is not zero

Forces on a submarine

Water resistance pulls the submarine back, the engine force pushes the submarine forward, the weight pulls the submarine down, and the buoyancy force pulls the submarine up.

The submarine above has both vertical forces and horizontal forces acting on it. The horizontal forces will not affect its vertical movement and the vertical forces will not affect its horizontal movement.

The horizontal forces are equal in size and opposite in direction. They are balanced, so the horizontal resultant force is zero. This means that there is no horizontal acceleration. The vertical forces are equal in size and opposite in direction. They are balanced, so the vertical resultant force is also zero. This means that there is no resultant vertical acceleration.

The submarine will continue with the same motion, either remaining stationary or moving at a constant speed. If the submarine is moving, it is impossible to tell which direction it is moving from the forces alone, only that it will continue in the same direction at the same speed.