Lesson Explainer: Newton's First Law Of Motion - Nagwa

Example 4: Defining Newton’s First Law of Motion

Which of the following statements most correctly describes Newton’s first law of motion?

1. An object will not change its velocity unless a net force acts on the object.
2. An object will not move unless a net force acts on the object.
3. An object will move until a net force acts on the object.
4. An object will change direction if a net force acts on the object.

Answer

Let us first determine whether it is correct to say that an object will move until a net force acts on it.

This is clearly incorrect as an object can be at rest and have no net force act on it.

Let us next determine whether it is correct to say that an object will not move unless a net force acts on it.

It is correct to say that an object that a net force acts on cannot remain at rest. This does not necessarily mean that an object that no net force acts on cannot be moving. The object could have been moving before the net force on it became zero. If so, the object would continue to move after the net force on it became zero. The velocity of the object would no longer change but would not become zero if it was not already zero.

Let us next determine whether it is correct to say that an object will change direction if a net force acts on it.

If the net force on an object is in a direction that is not the direction of the current velocity of the object, then the object will indeed change direction, as it will accelerate in the direction of the net force. If the net force acts in the opposite direction to that of the current velocity of the object, then, if it acts for enough time, it will reverse the direction of the velocity of the object. A net force can act in the same direction as that of the current velocity of the object, in which case it will not change the direction in which the object moves.

Finally, let us next determine whether it is correct to say that an object will not change its velocity unless a net force acts on it.

We can test the correctness of this statement by asking how an object would move if no net force acted on it. According to Newton’s first law of motion, the object will remain at rest if it was already at rest, and if it was moving at a velocity, it will continue to move at that velocity. In either case, the velocity of the object would not change. It is therefore correct to say that an object will not change its velocity unless a net force acts on it, and so this statement is the closest in meaning to Newton’s first law of motion of the four options.