Transcript Physics - bsparrow

Physics
Describing Motion
Aristotle
• 384-322 BC
• One of the first to study motion
• Stated there were two kinds of motion
– Natural Motion
– Violent Motion
Natural Motion
• Due to “nature” of object –
– the combination of the 4 elements:
– Objects wanted to get to “natural place”
• Ex: Smoke rises to be with air; Rocks fall to be with
Earth
• Straight up and Down (on Earth)
• Circular (in space)
– Believed different rules applied to the heavens,
which were made of quintessence (perfect
unchanging substance)
Violent Motion
• Due to pushes and pulls (FORCES)
– Imposed motion
– Ex:
Person pushing a cart
Wind blowing a ship
– EXTERNAL CAUSE, not due to “nature” of object
Galileo
•
1564-1642, showed Aristotle was wrong
1. Showed heavy and light objects fall at same rate
2. Forces are not needed to keep objects in motion
– Inertia: Tendency of objects to resist change in motion
– Objects at rest tend to stay at rest, Objects in motion tend to
stay in motion until another force acts on it.
•
Tested his ideas through experimentation
– End of philosophizing about ideas
– Beginning of modern science
Mass – A Measure of Inertia
• Amount of Inertia (resist in change of motion) depends
on amount of mass
– Mass: Amount of matter in an object; measured in kilograms (kg)
– DIFFERENT THAN WEIGHT!
• Weight: Force on an object due to gravity
– Weight and mass are directly proportional
• Double the Mass, Double the Weight!
Which bucket would be
harder to push? WHY?
Newtons
• Newton (N) = standard unit of force
– 1kg = 9.8N on Earth
– 1kg = 2.2 pounds (lb)
9.8N was
rounded to 10
Density
• Measure of “compactness” – how much
mass is squeezed into a given space
Density = mass / volume
What
more
mass:
Whichhas
liquid
is the
most dense?
1kg rocks or 1 kg feathers?
Which
solid
is the
most dense?
What has
more
weight:
1kg rocks or 1 kg feathers?
Which liquid is the least dense?
What is more dense:
1kg rocks or 1 kg feathers?
Which solid is the least dense?
Net Force
• Net Force: Combination of all forces on an
object
• Vector Quantities: Forces shown by arrows
– Have both magnitude (how much)
and direction (which way)
The Equilibrium Rule
• When the net force on something is equal to zero
Tension = Stretching Force
Is this diagram in equilibrium?
Support Force
• The force that supports an object against
gravity – often called normal force
If the Normal Force is
equal to the Weight, is the
object in equilibrium?
If the Normal Force is less than
the Weight, what happens?
Equilibrium of Moving Things
• Equilibrium = state of no change
• An object moving at a constant speed in a
straight line is in equilibrium
• Friction: Force that occurs opposite of motion
when objects are in contact
Friction
• Occurs for Solids, Liquids, and Gases
• ALWAYS acts opposite the direction of
movement
– Ex: Air resistance: Force of friction acting on an
object as it moves through the air
Causes of Friction
• Due to surface bumps and “stickiness” of the
atoms on the surfaces of materials
– Ex:
Ridges in fingers allow you to grab things
• When your hands are wet, water fills the ridges, you
can’t hold on to things as well
Shooting Stars
• Meteors are heated to the point of burning when
entering our atmosphere due to hitting gas molecules
Earthquakes
• Occur when the friction that is holding rock slabs
together is overcome and plates slide past each other
Vector Addition
• Parallelogram Law:
• Triangle Method (always head to tail)
Vector Practice
Free-Body Diagram: A sketch showing
only the forces on the selected particle.
Speed and Velocity
• Speed: Distance covered per amount of time traveled
– Units: km/hr; mi/hr; cm/min; km/day
• Preferred unit: m/s
• Instantaneous Speed: Speed at a given instant
– Ex: Speedometer
• Average Speed: averages all of the instantaneous
speeds
• Velocity: Speed AND Direction
– Ex: 60mi/hr North; 30km/hr West
Is it possible to have constant
speed but NOT constant velocity?
Motion is Relative
• When we discuss speed or velocity, we
mean RELATIVE to something else
Relative Velocities
• Define coordinate system
• Direction of bus = +x
+x
+y
19
Relative Velocities
• Relative to street vbus= +8m/s
• Relative to bus vyou = +3m/s
20
Relative Velocities
• Vector Diagram:
vyou=3m/s
vbus= 8m/s
vyou, relative to street=11m/s
21
Relative Velocities
• Relative to street vbus= +8m/s
• Relative to street vyou = -3m/s
22
Relative Velocities
• Vector Diagram:
vyou= -3m/s
vbus= 8m/s
vyou, relative to street= 5m/s
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Acceleration
• Rate at which velocity changes with time
– Magnitude (speeding up OR slowing down)
– Direction
– Units: m/s2
– Ex: Gravity = 9.8m/s2