Download Chapter 9 Motion and Energy

Chapter 9 Motion and Energy
MOTION
•When an object changes position over
time in relation to a reference point.
Reference point
• The object that appears to stay in place
• Earth is commonly used as reference,
buildings, trees
• Reference point can also be moving (Earth)
• Relative Motion depends on Reference Point
Chapter 9 Motion and Energy - Describing Motion
• Distance
– Total length of the actual path between two points
• Displacement
– Length and direction between a straight line from
starting to end point
Chapter 9 Motion and Energy
• Displacement is a vector
– Has magnitude (size) and direction
– Use arrows. Longer arrow, larger magnitude
for displacement
Rate at which an object moves is
speed
•
•
•
•
Distance/ time
m/s
Average speed
Total distance/total time
Chapter 9 Motion and Energy
When an airplane travels through the air, it produces sound waves. If the plane is traveling slower than
the speed of sound (the speed of sound varies, but 700 mph is typical through air), then sound waves can propagate
ahead of the plane. If the plane breaks the sound barrier and flies faster than the speed of sound, it produces a sonic boom when it flies past.
The boom is the "wake" of the plane's sound waves. All of the sound waves that would have normally propagated ahead of the plane are
combined together so at first you hear nothing, and then you hear the boom they create.
It is just like being on the shore of a smooth lake when a boat speeds past. There is no disturbance in the water as the boat comes by,
but eventually a large wave from the wake rolls onto shore. When a plane flies past at supersonic speeds the exact same thing happens,
but instead of the large wake wave, you get a sonic boom.
•
The Fastest Things on Earth
• Speed of Light: 300,000 km/s or 186,000 mi/s or
670, 616, 629 mi/hr
• Speed of sound:
.3 km/s or 761.6 mi/hr
Velocity
• Speed in a certain direction (always includes a reference
direction)
• Rate of change of position
• Is constant only if speed and direction do not change
• Velocity can be combined to get resultant velocity
Acceleration
• Rate at which velocity changes (speed up,
slow down, change direction)
• Tells you how fast velocity changes
Graphically, acceleration appears different than speed.
Constant speed assumes a straight line and average speed
may take any shape on a line graph, depending on the actual motion of the object.
Acceleration appears as an upward or downward curved line:
Chapter 9 Motion and Energy
Both graphs show acceleration!!!
acceleration
• Final velocity- initial velocity
time it takes to change velocity
Vf
t
Vi
m
s
m
s
s
m
= s =
s
m m
1
m
s = s x s = s2
s
1
Unit for acceleration
A car starts from a stoplight and is traveling with a velocity of
10 m/sec east in 20 seconds. What is the acceleration of the car?
• Circular motion is acceleration due to change
of direction
• Centripetal acceleration
We have, so far, concentrated acceleration caused by changes in
an object's speed. However, acceleration is also produced by changes
in the direction of an object's travel. A good example of this is objects
engaged in circular motion. Consider the motion of a ferris wheel.
Once you have gotten up to the normal operating speed, the
ferris wheel's speed will remain constant. However, because it is
moving in a circle, every movement of the device produces a change of direction.
Therefore, throughout the entire ride the passengers on the ferris wheel
are accelerating, even though they are traveling with constant speed.
• All objects accelerate towards Earth at a rate
of 9.8 m/s2
• Air resistance slows down acceleration
• Fluid friction also known as air resistance
•
Depends on size and shape of object
• Acceleration stops at terminal velocity
• As objects fall, air resistance increases until air
resistance = downward force of gravity
• Object continues to fall, but at a constant
velocity
• A GREATER FORCE MUST BE APPLIED TO A
GREATER MASS TO PRODUCE THE SAME
ACCELERATION
• F=M*A
• A=F/M
• Free fall means that object is falling with no
other force (such as air resistance) acting on it
except gravity
• Astronauts appear to be weightless due to
“freefall” in orbiting spacecraft
Work
• Is done when an object is caused to move a
certain distance
• The ability to do work is called energy
• SI unit for both work and energy is called the
Joule (J)
Energy
Main “forms’ of energy
Mechanical
Heat
Chemical
Nuclear
Electromagnetic
Electrical
Mechanical energy can be either
“potential” or “kinetic”
Kinetic Energy
• The energy an object has due to motion is
called kinetic energy
• “kinetos” means moving (Greek)
• Kinetic energy depends on mass and speed
• Speed, kinetic energy
Mass,
kinetic energy
Chapter 9 Motion and Energy
•Kinetic energy = ½ x mass x speed 2
Potential energy
• Potential energy is stored, resulting from
position or shape and has “potential” to do
work
• Potential energy related to height is
“gravitational potential energy”
• Depends on weight and height relative to
reference point
• Potential energy = weight x height
Chapter 9 Motion and Energy
• Potential energy of objects that can be
compressed or stretched is called “elastic
potential energy”
• Mechanical Energy = Potential Energy +
Kinetic Energy
• One common transformation of energy is
between potential and kinetic energy
• Any object that rises or falls experiences a
change in kinetic and gravitational potential
energy
Chapter 9 Motion and Energy
• Law of conservation of energy:
• Energy cannot be created or destroyed, only
transformed from one type to another