Newton's First Law of Motion  

Force is a push or pull.

An applied force does not always change motion of an object but does change the shape of the object. It may take sensitive instruments to detect deformation if it is a very rigid object.  Example - What happens when you squeeze a balloon?

Force can be measured using a spring scale. The length the spring is stretched or expanded is directly proportional to the applied force.

Galileo imagined an idealized world with no friction. If no force is applied to a moving object, it will continue to move with constant speed in a straight line. Friction is a force like ordinary pushes and pulls.

Newton’s First Law of Motion is similar

Every body continues in its state of rest or uniform speed in a straight line unless it is compelled to change that state by a net force acting on it.

This tendency is called inertia. Newton’s first law of motion is also called the law of inertia.

Newton - mass was a synonym for a quantity of matter.

Mass is a measure of the inertia of a body.

Kilogram - mass of a particular platinum-iridium cylinder kept at the International Bureau of Weights and Measures near Paris, France.

British System - Unit of mass is called slug.

When dealing with atoms and molecules - unified atomic mass unit (u) 1 atom ¹²C = 12 u

1 u = 1.6605 x 10 ^-27 kg.

Comparison of Mass and Weight

Mass is a property of the object.

An object has the same mass on earth and the moon. It is just as difficult to stop the object.

Weight is the force of gravity acting on a body.

An object has 1/6th the weight on the moon that it has on earth.