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Newton's first law can be used to explain the movement of objects travelling with uniform motion. If an object is travelling with uniform motion, it is moving in a straight line at a constant speed. For example, when a car travels at a constant speed, the driving force from the engine is balanced by the resistive forces such as air resistance and frictional forces 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

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 changes, the direction changes, 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

Forces on a submarine

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.