| Accelerated motion |
Concept Pages:
 Physical
Science "Key Concepts" Home Page |
Links: Physics For Beginners
Learn Physics with Comics
Section
Review questions
Chapter review questions |
Objectives
|
• Explain how force, mass, and
acceleration are related
• Compare the rates at which objects fall
• Observe the effects of air resistance |
Newton's Second Law of Motion |
|
• A net force acting on an object
causes that object to accelerate in the direction of the force. |
|
More commonly expressed by the
formula:
Force= mass x acceleration
or F = ma |
You may prefer to think of it as
a = F/m :
acceleration directly proportional to force, indirectly proportional to mass. |
• Mass is in kilograms (kg)
• Acceleration is in m/s2 |
So
|
|
|
• Force is expressed as:
kg • m/s2
• How is this related to the Newton?
1N = 1kg • m/s2
|
| Acceleration due to Gravity |
Remember: |
F = ma
|
|
• And weight is a measure of the
force of gravity |
|
Acceleration due to Gravity on Earth
is 9.8 m/s2
|
|
So weight is W = m x 9.8 m/s2
|
|
And so
1kg = 9.8N |
| Air
Resistance |
|
|
• Force exerted by air on a
moving object:
- similar to friction
• varies according to the size/mass ratio of
the object |
| Terminal
Velocity |
|
|
• Refers to the highest speed a
falling object will get to - it will vary as air resistance varies, according to the
size/mass ratio of the object. |
| Projectile and Circular Motion
|
Objectives |
|
|
• Explain why things that are thrown or
shot follow a curved path • Compare motion in a
straight line to motion that is circular
• Define weightlessness |
| Projectiles
|
|
|
Something that's thrown or shot
through the air.
- Both horizontal and vertical velocities
- Curved path results
|
| Horizontal
motion |
|
|
- initially parallel to the pull of
gravity:
e.g. - when you throw a ball |
| Vertical
velocity |
|
|
- due to the pull of gravity |
| Motion around curves |
|
Change in velocity includes change in
direction |
| Centripetal Acceleration |
|
acceleration toward the "center
of a circle" |
| Centripetal
Force |
|
|
The force acting on an object that is
toward the "center of the circle"
- often the result of friction (tires on the road)
|
| Weightlessness |
|
|
Weight is a measure of the pull of
gravity - if object is not feeling the net force of gravity, it is essentially weightless.
|
| Freefall |
|
|
Term applied to a skydiver before
deploying parachute. Imagine that skydiver
with a bathroom-type scale on the bottom of his feet (a little weird, but we're doing an experiment here, okay?!)
- Imagine, also, that the skydiver is vertical, as
if standing.
- Ignore, for the moment, any effect of air
pressure.
Would the scale register any weight?
• The situation is essentially the same with
the astronauts in the Space Shuttle.
- They are in freefall all the time they are in
orbit.
Why don't they fall to the ground?
|
| Isaac Newton |
actually visualized that the moon was
falling, but that the speed of its orbit, with the speed of the Earth in its orbit,
coupled to keep the moon forever falling around the Earth. |
| Effects
of weightlessness |
|
|
Remedies -
|
Satellites |
|
Objectives |
|
|
• Explain how satellites are placed into
orbit • Give examples of uses for satellites
|
| A Satellite |
|
|
• is a body that orbits another |
| An Artificial Satellite |
|
|
• is a man-made device that
orbits the earth or, increasingly, another planet, or the sun. |
| Launching
artificial satellites into orbit takes a rocket that is capable of attaining "escape
velocity" |
|
- once out of the earth's atmosphere,
they are boosted to a high enough speed to stay in orbit |
| Typical
shuttle orbits are 150 to 300 miles above the surface Geostationary orbits are ~ 22,500 miles up
Satellite uses include: |
|
• communications • weather
• military surveillance
• GPS - Global Positioning System |
| Falling Satellites |
|
|
- occasionally satellites fall back
into Earth's atmosphere, and usually burn up. |
| Action/Reaction Forces |
|
Objectives
|
|
|
• Analyze action and reaction forces
• Calculate momentum
• Explain conservation of momentum |
| Newton's Third Law of Motion |
|
• To every action there is an
equal and opposite reaction. >>>> Forces always act in pairs <<<<
|
|
• Many applications -
-rockets, jets
-skateboards
-boat motors
|
| •
Rocket propulsion in the vacuum of space depends entirely on the physical law this
describes, because: |
|
in space there
is no air to push against |
|
Could a
propeller-driven airplane work in space? |
| MOMENTUM |
|
| Momentum= mass x velocity
|
|
(book uses p = m x v ) Does this remind you of Inertia?
- how?
|
| • Expressed by the
unit: kg • m/s (kilogram-meters
per second) |
|
- In what instance could a bullet
and a truck have the same momentum?
|
| The Conservation of Momentum |
| The Law
of Conservation of Momentum
|
|
States that the total momentum does not change unless an
outside force acts on an object or objects. |
|
- e.g. the "break" at the beginning of a game of
billiards:
-why do the back balls move as soon as the front ones do?
|
|
Many sports applications Updated
10/14/07 |
