Mechanics Made Easy: Newton’s Laws of Motion

Physics Tutorial

Mechanics Made Easy: Newton's Laws of Motion


Newton’s First Law of Motion reads “An object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force.” This is where we get the idea of inertia: an object’s tendency to keep on its current course of movement.

This law can be more directly interpreted as “without a force, an object will maintain the same velocity in all observable instances.”

Newton’s Second Law of Motion reads “The acceleration of an object as produced by a net force is directly proportional to the magnitude of the net force, in the same direction as the net force, and inversely proportional to the mass of the object. Stated simply in equation form as:

F = ma

Where F represents the net force on an object, m represents the mass of the object, and a represents the acceleration the object is undergoing.

Lastly, Newton’s Third Law of Motion reads “For every action, there is an equal and opposite reaction.”

Sample Problem

On Earth, ball of positive mass m is at rest upon the ground. The ball’s observed velocity is an unchanging zero meters per second. Explain why this is in terms of Newton’s Laws of Motion.


Newton’s First Law of Motion tells us an object at rest stays at rest unless acted upon by an outside (net) force. Newton’s Second Law of Motion tells us:

F = ma

Since the ball has mass and its velocity is not changing, the net force must be zero Newtons. Since we know the net force of the ball is zero, but that the ball is on Earth and thus experiencing Earth’s force of gravity, yet still is not accelerating: Newton’s Third Law of Motion tells us that there is an equal and opposite force being exerted on the ball by the ground it rests upon.

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