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Chapter 4
Newton’s First Law of Motion:
Inertia
• Force – any push or pull
• Friction – is the name given to the force that
acts between materials that touch as they
move past each other
• Galileo showed that a force was not required
to keep an object in motion. Two surfaces
that come in contact will create some degree
of friction, even very smooth surfaces.
• If friction were absent, a moving object would
need no force to remain in motion
• Galileo also demonstrated
that an object resists changes
to its state of motion which
we call Inertia
Inertia
Inertia – the tendency of an object to resist a
change in its motion
• Mass is proportional to inertia
Newton’s First Law
• Law of Inertia – Every object continues in a
state of rest, or of motion in a straight line at
constant speed, unless it is compelled to
change that state by forces exerted upon it.
Restated:
• An object at rest (or in motion) tends to stay
at rest (or in motion) unless acted on by an
outside force
Mass vs. Weight
Mass - The amount of matter contained in an object
• Mass is NOT Volume
Example: A bowling ball usually has more mass
than a large beach ball
• Mass is NOT Weight
Example: an astronaut has the same mass in
space, but he/she is weightless
Mass vs. Weight Continued…
• The SI unit of mass is the kilogram
(the base unit is the gram)
• Weight is mass x gravity
• Pounds are the English measurement of weight
(mass x gravity)
• The Newton is the SI unit of weight
The Newton
Newton = mass x gravity
• Newton = Kilograms x 9.81 m/s2
Net Force
Net force – the combination of all forces acting
on an object
Example 1: What is the net force acting on the box?
10 N
5N
Net Force = 5 Newtons to the right
Example 2: What is the net force acting on the box?
10 N
14 N
4N
Net force = 0 Newtons
Net Force Continued:
Normal force (also called the support force) – the upward
force that balances the weight of the object. The normal
force always acts perpendicular to the plane that the
object is resting on.
200 N (Support force in Newtons)
200 N (Weight of box in Newtons)
Vector Addition of Forces
Example: A box is suspended by a rope
10 N
10 N
The tension in the rope is equal to the weight of the
box
5N
5N
10 N
If two ropes are attached to the same box, then the
tension is each rope is half of the weight of the box.