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Chapter 12. Forces & Motion. Forces. “a push or a pull” A force can start an object in motion or change the motion of an object. A force gives energy to an object. Representing forces. We often use arrows. Larger the arrow, the greater the force Combined balanced forces equal zero - PowerPoint PPT Presentation
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Forces “a push or a pull” A force can start an
object in motion or change the motion of an object.
A force gives energy to an object
Representing forces We often use arrows. Larger the arrow, the
greater the force Combined balanced
forces equal zero Normal force = the
force acting perpendicular on an object in contact with another object.
No change in motion!
Balanced forces will keep an object moving at a constant velocity
ENGINEFRICTION
Friction:
One of the most common forces. Always acts in a direction opposite
the direction of motion. There are four main types of
friction.
Static Friction The force that must one must overcome to
get an object to move. Examples: Trying to move a heavy object Tires on pavement when a car is moving Walking
Sliding Friction Two surfaces
sliding over each other.
Caused by two factors:
1. Weight of moving object
2. Types of surfaces in contact
Rolling Friction An object rolling
over a surface Not as strong as
sliding Can be reduced
by using wheels and ball bearings.
Fluid Friction When an object is
moving through a fluid (either gases or liquids)
ex. a fish swimming under water, air resistance
How fast do objects fall on earth?
Galileo 16th century Dropped two metal
spheres of different sizes off the Leaning Tower of Pisa
Which hit first? They hit at the same
time.
Acceleration due to gravity All objects fall at the
same rate, regardless of their mass or weight
The acceleration due to gravity on earth is about 9.8 m/sec2.
This value will change with elevation and location on earth.
Terminal Velocity After a period of
free fall, the force of gravity will be canceled by the force of air friction and objects will travel at a constant velocity
Projectile motion
Any object thrown in the air becomes a projectile.
The object will move forward due to inertia, and downward due to gravity.
Always a curved path.
Aristotle 384 B.C. – 322 B.C. Greek scientist and
philosopher First to work with
force. Incorrectly stated that
a force is required to keep an object moving at constant speed
Newton’s First Law of motion An object at rest will remain at rest and
an object in motion will remain in motion unless acted upon by an outside force.
Often referred to as the Law of Inertia.
(the property of matter that resists any change in motion)
Newton’s Second Law of Motion The force of a moving object is directly
proportional to the object’s mass and acceleration.
The most important of Newton’s laws.
How do we label force???
F = ma Mass is measured in kg Acceleration is measured in m/sec2
If we multiply these two units :
1 kg m/sec2 = 1 newton (N)
Weight and Mass
MASS The measure of inertia
of an object. The amount of matter
in an object Stays constant with
changes in location A scalar
WEIGHT A measure of the force
of gravity acting on an object.
Measured in newtons. Changes with changes
in location. A vector
Newton’s Third Law of Motion For every action there is an equal and
opposite reaction, or All forces occur in pairs examples:
Momentum
All moving objects have momentum. It is the product of an object’s mass and
velocity. Which has more momentum? A moving car
or a moving train? Not enough information. A 1500 kg car traveling 50 m/sec? Or a
48,000 kg train traveling 2 m/sec?
Nuclear forces
STRONG Responsible for
holding the nucleus together
Strongest known force
WEAK Very small
attractive force in the nucleus
Newton’s Law of Universal Gravitation Every object in the universe attracts
every other object. Depends on two factors:
1. Distance between the objects
2. Mass of the objects
The Earth, Moon and Tides
Earth’s gravitational pull on the moon combined with the inertia or momentum of the moon keeps the moon in orbit around the earth.
The pull of the moon and sun causes the tides
Satellites
Satellites must be placed at a specific height above the earth’s surface in order to stay in orbit.
If it is too close, or moving too slow, it can spiral into the earth’s atmosphere and burn.