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Moving Objects

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					Moving Objects


  Jayne Robinson
   Physical Science
      Biloxi High School
          June 2006
3-1 Describing Motion
When something moves, it changes
position. When changing positions, an
object must have a reference point. The
best reference point is a stationary object.
Motion also often includes speed - how fast
the object moves.
If motion is described as a change in
position, then speed is an expression of how
much time it takes for that change in position
to occur.
Speed - The rate of change in position; a
rate of motion.
 Instantaneous speed - the rate of motion
 at any given instant. A radar gun can
 determine instantaneous speed.
 Constant speed - a speed that does not
 vary; “cruise control” is the closest a car has
 to constant speed.
 Average speed - the total distance traveled
 divided by total time of travel; the most
 common expression for speed.
Calculating & Graphing Speed
The formula to calculate speed:
  v=d/t       speed = distance / time
solve problems page 62
a distance-time graph provides a visual
expression of speed: the slope of the line
equals the speed.
  Look at figure 3-3; What total distance
  did each swimmer cover? (red: ;
  blue:
  What was each swimmer’s average
  speed for the 30 minute period?
3-2 Velocity & Acceleration
What is the difference between speed and
velocity? Velocity describes both speed and
direction.
How can velocity change if speed remains
the same?
Acceleration is the rate of change of
velocity; how quickly speed or direction
changes.
If a car changes direction but remains at
constant speed, velocity & acceleration is
changed.
Calculating acceleration
     a = (vf - vi) / t = Dv/ t
acceleration = final velocity - initial velocity / time =
                    change in velocity / time

 the Greek symbol D stands for “change in”
when calculating, include all units and symbols
the unit for velocity is m/s (meters per second)
the unit for acceleration is m/s2 (meters per
second per second or meters per second squared)
work the problems on page 67
3-3 Crashing to Save Lives
Crash tests with lifelike dummies are
performed on all vehicles to determine what
happens to people during crashes. They
determine what can be done to prevent
injuries and death.
Seat belts and airbags are two of the
devices developed from these crash tests.
The seat belt allows a person to become
“part of the car” and slow down when the
car comes to an abrupt stop.
3-4 Force & Motion
A force is a push or pull one body/object
exerts on another body/object.
Some effects of forces are obvious, such as
a car crash, but others are not obvious (the
force exerted by the floor on your feet)
Some forces cause a change in velocity,
either a change in speed or direction or both.
Balanced forces are equal in size and
opposite in direction: When you stand up the
ground pushes up equal to your weight
pushing down.
A net force changes the velocity of an object;
it either changes direction or speed or both
direction and speed.
Inertia is the tendency of an object to resist
any change in its motion. If an object is moving,
it tends to keep moving unless a force acts
upon it, velocity will remain constant. If at rest,
it tends to remain at rest unless a force acts
upon it, velocity will remain zero.
The more mass an object has, the greater its
inertia
Newton’s First Law
Sir Isaac Newton developed several laws to
describe the effects of forces.
Newton’s First Law of Motion - states that
an object moving at a constant velocity keeps
moving at that velocity unless a net force acts
on it. If an object is at rest, it stays at rest
unless a net force acts upon it. This first law
is also known as the law of inertia.
Friction
Friction is the force that opposes motion
between two surfaces that are touching each
other.
The amount of friction depends on two
factors: the kinds of surfaces and the force
pressing the surfaces together.
If there were no friction you would not be
able to walk, to hold things between fingers,
or keep objects together.
3-5 Effects of Gravity
Gravity is a force that every object exerts on
every other object.
Many forces of gravity are too small to notice:
when you hold a pencil, your hand exerts gravity
on the pencil and the pencil exerts gravity on
your hand.
The amount of gravity depends upon two things:
the masses of the objects and the distance
between the objects.The masses of your hand
and pencil are so small compared to the mass of
Earth, gravity between them isn’t noticeable.
Weight is the measure of the force of gravity
on an object.
Weight is dependent upon mass: the greater
the mass, the greater the gravitational force on it,
the the greater the weight.
Mass is measured in kilograms (kg) & grams (g)
Weight is measured in Newtons (N)
On Earth, a kilogram weighs 9.8 N.
Since the mass of the moon is about 1/6 that of
Earth, a person’s weight on the moon would be
about 1/6 their weight on Earth, even though their
mass has not changed.
Measuring Forces
Scales use the principle of balanced forces to
measure weight.
Weight is most often measured on a spring
scale. The force of gravity is taken in the
calculation of weight on the scale.
Answer questions 1 and 4 on page 77.

				
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