Ch 4 Newton's Laws of Motion

Physics Classroom's Newton's Laws of Motion - check it out!

In this chapter we will study newton's three laws of motion and Newton's law of gravity. These laws are at the heart of classical mechanics and describe motion, in the scale of the world we live in every day, quite well. We will find that the motion of objects is related to the forces acting on the objects.


A force is simply a push or a pull on an object. The force that is acting on the object can be in contact or at a distance depending on the nature of the force.

Contact Forces:

friction, push or pull directly

Forces at Distance:

gravitational fields, electric fields, and magnetic fields

Newton's First Law

An object remains in constant straight line motion unless an unbalanced force acts on it. An object at rest remains at rest unless an unbalanced force acts on it.
The first law is also known as the Law of Inertia. Unbalanced forces produce a NET force that is NOT zero. IMPORTANT- when there is a NET force on an objects, the object changes its motion- it accelerates!

Mass and Inertia

Inertia is simply the object's resistance to a change in its motion. Not understanding the direct relationship between mass and inertia is one of the common mistakes made by students in physics- the amount of INERTIA an object has is directly related to the amount of MASS it has. More mass, more inertia... Less mass, less inertia.

Newton's Second Law

Newton's second law is the primary law that will be used when calculating motion quantities. In words it states that the acceleration of an object is proportional to the ration of force to mass.

In a much simpler equation form the law is written as: a = F/m
acceleration is equal to the ratio of force to mass

OR it can be written F = ma

We measure mass in kilograms and force in Newtons

1 N is defined as the force required to accelerate a 1kg mass 1 m/s^2 F = ma or 1N = 1kg (1 m/s^2)

1 Newton is about 1/4 pound or 1N = 0.225 lb

Newton's Third Law

This is the tough one... forces always come in PAIRS! This is the action-reaction law:

For every action, there is an equal and opposite reaction.

For example- as you sit on a chair, your weight is the force pushing down on the seat of the chair, but the chair has a reactive force pushing opposite your weight holding you in place. The reactive force in this case is called the NORMAL force.