Newton’s 1st 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