Download Forces and Motion

Forces and Motion
What is a Force?
• A force is a push or pull.
• SI unit for force is Newton
Force
• A force is described by its strength(magnitude)
and by the direction in which it acts.
Combining Forces
• When forces act in the same direction they
add together.
• When forces act in opposite directions, you
subtract the forces and it moves in the
direction of the stronger force.
• Once forces are combined known as net force
Unbalanced forces
• Unbalanced forces acting on an object result
in a net force , and causes a change in the
object’s motion.
Balanced Forces
• Equal and opposite forces that result in no
change of direction or position.
Friction
• Friction is a force that 2 surfaces exert on each
other when they rub against one another.
• Friction depends on 2 factors:
– How hard objects push together
– Type of surfaces
4 types of friction
• Static Friction: friction on an object not
moving (ex: trying to move a desk)
• Sliding friction: friction from sliding (Ex: Skis)
• Rolling Friction: friction from rolling (Ex: Bike)
• Fluid Friction: Friction with liquid or gases.
(ex: Air resistance)
Gravity
• Force that pulls objects toward each other
• Universal Gravitation: all objects exert a
gravitational force.
2 factors Affect Gravity
• Mass of objects
• Distance between objects
Weight and Mass
• Weight- how much force is exerted on an
object due to gravity
• Mass- how much matter in an object.
Free Fall
• In free fall the only force acting on an object is
gravity, so object accelerates toward ground.
Air Resistance
• Objects falling through air experience an
opposite force known as air resistance.
Terminal Velocity
• Greatest velocity an object reaches is called its
terminal velocity.
Newton’s 3 Laws of Motion
The first Law of motion
• Newton’s first law states that an object at rest
will remain at rest, and an object moving at a
constant velocity will continue moving at a
constant velocity, unless it is acted upon by an
unbalanced force.
• VIDEO: CAR CRASH
• https://www.youtube.com/watch?v=y118jLg2
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Inertia
• Inertia is the tendancy of an object to resist a
change in motion.
Newton’s first law is called the law of inertia.
Inertia depends on mass. The more mass the
more inertia.
The Second law of Motion
• According to Newton’s second law of motion,
acceleration depends on the object’s mass
and on the net force acting on the object
Acceleration= Net Force
Mass
Newton’s third Law
• Newton’s third law states that if one object
exerts a force on another object , then the
second object exerts a force of equal strength
in the opposite direction on the first.
• https://www.youtube.com/watch?v=BiLq5Gn
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Action-Reaction Pairs
• When you jump you push off the ground with
your feet, this is an action force. The ground
pushes back on your feet with an equal and
opposite force, this is the reaction force.
Detecting Motion
• You can’t always detect motion.
• For example when you drop a pencil, you see
the pencil fall towards the ground, but you
don’t see the pull of the ground to the pencil
because the ground is much larger and has a
much greater inertia (resistance to move)
Action and reaction forces don’t cancel
out
• Because the 2 forces act on different objects
• Example: Girl hitting a volleyball a force on her
wrists and on the ball.
Momentum
• “Quantity of motion”
• Momementum = mass x velocity
• The more momentum an object has the
harder it is to stop.
Conservation of Momentum
• In the absence of outside forces, the total
momentum of objects that interact does not
change.
• The total momentum of any group is
conserved or remains the same unless outside
forces act on the objects.
• See page 358
Key term to know
• Centripetal force: Any force that acts on an
object to move in a circular path