Newton’s 1 st Law Inertia

Force  Any push or pull acting on an object  Most forces require contact between two objects (Contact Forces) Ex. Motor lifts an elevator; hammer hits a nail; wind blows a paper  A few forces do not require contact Ex. Gravity pulls everything on Earth down  If an object is at rest, a force is needed to start it moving  If an object is already moving, a force is needed to speed it up, slow it down, or change its direction

Measuring Force  Forces can be measured with a spring scale  Read in a unit called the Newton (N)

Forces are Vectors  Both magnitude and direction are important  Force exerted in a different direction has a different effect  What would happen if you add more force to the right?  What would happen if you add a force to the left?

Early History  Aristotle (350 BC) believed that the natural state of an object is rest - in the absence of a force an object will be at rest - a force is required to keep an object moving or speed it up - the greater the force on a object the greater its speed  2000 years later Galileo disagreed

Galileo’s Experiment

Inertia  Galileo reasoned without friction ball would keep going  Also reasoned that ball initially at rest would stay that way unless something caused it to move  To explain why an object appears to need a force to stay at a constant velocity of Earth he introduced the idea of friction (a force opposing motion)  The force needed to keep an object sliding is equal to but opposite in direction of the frictional force – the vector sum is zero  Inertia - what Galileo called the tendency of an object to maintain its initial state

Law of Inertia or Newton’s First Law  Object at rest will remain at rest or object in motion will remain in uniform motion in a straight line as long as no net force acts on it

Example  A school bus comes to a sudden stop, and all of the backpacks on the floor slide forward. Explain.

Inertial Reference Frame  The law of inertia is only true for reference frames that are not accelerating (so either at rest or at constant velocity)  The Earth acts as an Inertial Reference Frame so Newton’s Law holds true

Newton  Related mass to inertia  More massive objects resist change more (have more inertia)  Mass is the measure of the inertia of an object  When thinking of mass in this way its called Inertial Mass  Standard of unit of mass is the kilogram (kg)  Weight is a force related to gravity’s pull  Mass can be measured by using its weight with a balance  Mass measured this way is called the Gravitational Mass  Less accurate because gravity varies with location