# Mechanics: Chapter 5 – Newton’s Laws of Motion

Law 1

If there is no net external force acting on an object in an inertial reference frame, the object will stay at rest if it was at rest and if the object had an initial velocity, it will continue moving at that constant velocity.

Law 2

The net external force on an object equals the product of the mass of an object and the rate of change of velocity of that object. In other words,

• $F_{net} = ma$

Law 3

If object A exerts a force on object B, object B will exert a force of equal magnitude and opposite direction on object A, thereby creating an action-reaction pair.

• $F_{A on B} = -F_{B on A}$

Other definitions

The weight of an object is the force exerted by gravity on that object.

• $w = F_{g} = mg$

The normal reaction force is the force that prevents solid objects from passing through each other. It acts perpendicularly outward from a surface (hence ‘normal’). If the object is not accelerating, the magnitude of the normal reaction force is equal to the magnitude of the force pushing the object towards the surface.

The tension is the force with which a rope, string etc. pull the object they are attached to. In a mass-less rope, the tension is the same at every point on the rope even when passing through a mass-less, friction-less pulley.