Download Circular Motion - Appoquinimink High School

Types of Forces
Friction
• What are the factors that determine the
amount of friction between two surfaces?
• 1) The types of surfaces in contact (μ)
• 2) The amount of contact between the two
surfaces (FN)
• f = μFN
Static vs. Kinetic Friction
• If I push the table with a small amount of
force, it will not move. Why not?
• Static friction prevents objects from sliding.
• Objects at rest require a minimum force to
start moving. The force applied must be
greater than static friction first.
Minimum
force to start
moving
Moving at a
constant speed
Not moving
Kinetic Friction
• Once the object starts moving, there will still
be friction between the surfaces (Kinetic
Friction)
• CHECK FOR UNDERSTANDING:
• Which is a greater force: Static Friction or
Kinetic Friction. Use Newton’s Laws of motion
in your explanation.
Apply
• A 75kg block is resting on a surface. The
coefficient of static friction between the block
and the surface is 0.3. A person applies a
force to the block to try to move it.
• 200N of force applied… will it move?
• 600N of force applied… will it move?
• Minimal force needed to move the block?
Investigate
• Using an Atwood Machine, determine the
coefficient of kinetic friction between a block
and the surface of the track.
Centripetal force
• Spinning an object on a string over your head.
• How do you keep the object rotating?
• In order for an object to be in revolution,
there must be a force pulling it towards the
center of the circular path.
Centripetal force
• Definition: any force that is at a right angle to
the path of a moving object that produces a
circular motion.
• Example: The Sun exerts a centripetal force
on the Earth
Cars going around Bunker Hill Circle
• What force keeps a car from leaving the traffic
circle in front of our school?
• Friction is what keeps the car from leaving the
circle, therefore friction is the centripetal
force.
Washing machines
• During the spin cycle, the tub in the washing
machine rotates at a high speed.
• The inner wall exerts a centripetal force (like a
normal force) on the clothes keeping them in
a circular path.
• The holes in the tub allow for the water to
escape.
• The water escapes not due to a force, but due
to inertia.
Centripetal vs. Centrifugal
• Centripetal means “center-seeking”
– This is a force that keeps the object in a circular
motion
• Centrifugal means “center-fleeing”
– This is the tendency of the objects to want to
break away from the circular motion due to
inertia.
Misconception Alert!
• Consider a ladybug in a can that you are
spinning around your head.
• There is only one force acting on the ladybug.
– Centripetal force holding the bug in circular
motion.
• The lady bug pushes against the edge of the
can due to inertia or centrifugal tendency.
• The outward push is NOT due to a force, it is
due to the lady-bug’s MASS (inertia)
Roller coaster
• Describe what happens to a person as they are
riding a roller coaster that goes upside-down.
• Why don’t the fall out of the coaster when they
are upside down?
– Inertia “pushes” the person to their seat.
– The inertia is caused by the speed of the roller coaster.
• What keeps the person in the circular motion and
not leaving the circle?
– The normal force exerted by the track.
Centripetal force Mathematically
• Acceleration = v
2
r
Cars around a turn
• A 1000 kg car rounds a curve on a flat road
with a radius of 50m at a speed of 14 m/s.
Will the car make the turn or will it skid? The
pavement is dry and has a coefficient of static
friction of 0.6.
FLYING PIG!!!
Elastic Forces
• To stretch or compress an object requires a
force.
• Back in the 1600’s, British scientist Robert
Hooke came up with a relationship between
the distance an object is displaced from its
equilibrium and the force required to do so.
• F = kx
Investigation
• Determine the spring-constant (k) of the
colorful springs.