Download Newton`s 1st Law

Newton’s 1st Law
Inertia
Force
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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
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Forces can be measured with a spring scale
Read in a unit called the Newton (N)
Forces are Vectors
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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
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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
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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
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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
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A school bus comes to a sudden stop, and all of the
backpacks on the floor slide forward. Explain.
Inertial Reference Frame
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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
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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