Newton’s First Law of Motion Definition
Newton’s First Law of Motion states that a body continues to travel at a constant velocity as long as there is no resultant force acting on the body.
Since velocity is a vector, Constant velocity means that the body has the same speed and direction for a given period of time. This could either mean that an object is at rest continues to stay at rest (constant velocity = 0) or that a body moving at a certain speed continues to move at the same constant speed along a straight line. If the body changes direction, even if the speed is constant, there’s an acceleration and the forces on the body are not balanced. For example, if you swing an object in a circle at constant speed, the object is still accelerating because it is changing its direction of motion.
Newton’s First Law of Motion and Inertia
The tendency for a body to maintain its state of motion is called inertia. If a bus suddenly applies breaks, for example, the passengers on it may continue to move forward and they collide with the seat in front of them. When the bus applies breaks more gently, the force of friction between the passengers and the seat may be sufficient to stop the passengers from falling off their seats.
If you kick a ball along the ground, surely, it doesn’t continue to move forever with the same speed. This is because, on the Earth, the resultant force on the ball is not 0. Friction acts between the ball and the ground, causing the ball to decelerate. A puck used in ice-hockey experiences much less friction and so it continues to move for a considerably longer period of time. Spacecraft, once they are in space, also experiences a very little force. So they continue to travel with almost no change in speed. They do experience gravity when they travel closer to planets or stars, and their paths bend. Scientists actually make use of this effect, and by doing prior calculations, they are able to plan out the trajectories of the spacecraft carefully. When the trajectory of a spacecraft gets curved as it travels around a massive object (e.g. a planet), they are said to slingshot around the body.
Air Resistance and Terminal Velocity
On the Earth, falling objects may travel with constant speed if they achieve terminal velocity. This happens, for example, when an object is falling through the air. As the object accelerates, the air resistance on the body would increase, while the weight of the body remains the same. Eventually, the air resistance may become equal to the weight of the object. In this case, the weight and the air resistance, now having the same sizes and acting in opposite directions, would cancel each other out, making the net force on the object 0. Then, the object’s velocity will no longer change until it reaches the ground. This constant velocity achieved by the object is referred to as terminal velocity.
Example of Newton’s First Law of Motion
A skydiver, with a mass of 65 kg, is falling at terminal velocity. Find the size of air resistance experienced by the skydiver.
Since the skydiver is falling at a constant velocity, according to Newton’s first law, the forces on the skydiver should be balanced. Weight acts downward, and this has a magnitude of 
