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Transcript
Chapter 5
Matter in Motion
Measuring Motion
• Motion is all around us (the moon is
circling the earth, blood is traveling
through your veins, tiny air particles are
moving through the air)
• Observing motion
– To observe motion you must observe the
object in relation to another object that
appears to stay in place.
– When an object changes position over time
when compared with a reference point the
object is in motion.
Common Reference Points
• The Earth’s surface
is a common
reference point for
determining
position and
motion.
• Non-moving objects
such as buildings,
trees, and
mountains are also
useful reference
points.
Speed
• Speed depends on distance traveled and
time taken to travel that distance.
• The rate at which an object moves is its
speed.
• The SI unit for speed is meters per second
(m/s).
– Other units commonly used are kilometers per
hour, feet per second, and miles per hour.
Speed
• Determining average speed: most of the
time objects do not travel at a constant
speed.
• Average speed = total distance
total time
If a person travels 200 miles, and it takes
them 8 hours to get to their destination.
What was their average speed?
Velocity
• The speed of an object in a particular
direction.
• Velocity must include direction:
– ie) It would not be correct to say the planes
velocity is 600 km/h (that is its speed). It’s velocity
would be 600 km/h south.
– Velocity always includes a reference to direction
The speed of these cars may be similar, but their
velocities are different because they are going in
different directions!
Velocity changes as speed or direction changes
Combining Velocities
• When you combine two
velocities that are in the
same direction, add them
together to find the resultant
velocity.
• When you combine two
velocities that are in
opposite directions, subtract
the smaller velocity from the
larger.
• An example walking on a
moving escalator, or moving
Is the kayaker moving at the same velocity as the
river?
Chapter 5 Quiz 1
1. Give an example of a reference point.
2. What two things must you know to
determine speed?
a. time and direction b. direction and
distance
c. time and distance
d. motion and
direction
3. What is the difference between speed and
velocity?
a. speed uses direction and velocity does not
b. velocity uses direction and speed does not
Acceleration
• Acceleration is the rate at which velocity
changes.
• Velocity changes if speed or direction
changes.
• If speed and or direction changes you
accelerate (even if you are slowing down).
• Acceleration is also how fast velocity
changes.
– The faster velocity changes, the greater the
acceleration.
Acceleration
• Acceleration = final velocity – starting
velocity
time it takes to change
velocity
or Acceleration = change in velocity
time
Calculate Acceleration:
A plane passes over Point A with a velocity
of 8,000 m/s north. Forty seconds later it
Answer:
• Acceleration = 10,000m - 8,000m
40 seconds
Acceleration = 2,000m
40 sec.
= 50 m/s/s north
Circular Motion
• Circular Motion is a continuous
acceleration
– It is always changing direction so is in
constant acceleration
– Standing at the Earths equator is an example
of this (because you are traveling in a circle
as the earth rotates).
– The acceleration that occurs in circular motion
is known as centripetal acceleration.
Chapter 5 Quiz 2
1. What is acceleration?
a. the speed of an object
b. the rate at which an object moves
c. the rate at which time changes
d. the rate at which velocity changes
2. Does a change in direction affect
acceleration?
3. The acceleration that occurs in circular
motion is known as:
a. velocity
acceleration
b. motion
Force
• A force is simply a push or a pull.
• All forces have both size and direction.
• Forces are everywhere! Anytime you see
something moving, be sure that it’s motion
was created by a force!!
• Scientists express force using newtons
(N).
– The more newtons, the greater the force.
Force
• The bulldozer is
exerting a force on
the pile of soil, but did
you know that the pile
of soil is also exerting
a force, even though
it is just sitting on the
ground.
• What objects in our
classroom are
exerting force, and
which are receiving
it?
Forces in Combination
• Often more than one
force is exerted on an
object at the same
time.
• The net force is the
force that results from
combining all the
forces exerted on an
object.
• Example: when you
are moving a piano
Determining Net Force
• When the forces are in
the same direction, you
add the forces together to
determine the net force.
• When the forces are in
different directions, you
subtract the smaller force
from the larger force to
determine the net force.
•
Unbalanced and Balanced
Forces
When the net force on an object is not zero the
forces on the object are unbalanced.
– Unbalanced forces produce a change in motion.
– Unbalanced forces are necessary to cause a nonmoving object to start moving.
• Balanced Forces produce no change in
motion.
– Objects produce a net force of zero when
Are these cards
balanced.
balanced or
unbalanced?
Friction
• Friction is a force that
opposes motion
between two
surfaces that are
touching
• Friction occurs
because the surface
of any object is rough
– Even surfaces that
look smooth are
covered with
microscopic hills and
valleys (see example)
Friction (continued)
• Rougher surfaces create more friction
– They have more microscopic hills and valleys
– Think of sliding on pavement rather than on
grass
Greater Force = Greater Friction
• The amount of friction also
depends on the force
pushing the surfaces
together.
• Less massive objects exert
less force on surfaces than
more massive objects do
• For example a harder push
is needed in order to
overcome friction to move a
more massive object.
Types of Friction
• Sliding Friction: You use sliding
friction when you go sledding,
apply brakes on a bicycle, or
when you write with a piece of
chalk. The force of sliding friction
is large.
• Rolling Friction: usually requires
less force than sliding friction, so
is easier to move. Examples are
anything with wheels.
• Example: Sliding Friction-pushing
furniture across floor. Rolling
Friction-put wheels on furniture to
Types of Friction (cont)
• Fluid Friction: Have you ever wondered
why it is more difficult to walk on a freshly
mopped floor vs. a dry one? This is
because of Fluid Friction.
• This is why swimming is such a good work
out. You must exert force to overcome fluid
friction.
Types of Friction (cont)
• Static Friction: When a force is applied to
an object but does not cause the object to
move.
• The force of static friction balances the
force applied.
Ways to reduce Friction
• Lubricants: such as
motor oil, wax, and
grease.
• Switching from sliding
friction to rolling
friction.
• Make surfaces
smoother. For
example sanding a
surface.
Ways to Increase Friction:
• Make Surfaces Rougher
– For example adding salt or gravel to icy roads
• Increase force pushing the surfaces
together.
– For example putting a heavy rock on your
magazine so that it doesn’t blow away while
you are at the beach.
Nerdy Science Joke Break
• Why did the teacher insist that her
students wear rain slickers to class?
Answer: She wanted to reduce the friction
between them!
Friction Poetry
•
•
•
•
•
An impatient young girl named Lenore
Tried to run on a freshly waxed floor.
Since the friction was less,
She made quite a mess
As she slid right under the door.
Chapter 5 Quiz 3
1. Friction is a force that:
a. opposes an object’s motion
b. does not exist when surfaces are very
smooth
c. decreases with larger mass.
d. All of the above
2. Rolling friction
a. is usually less than sliding friction
b. makes it difficult to move objects on
wheels
c. is usually greater than sliding friction
Chapter 5 Quiz 3 (continued)
3. A force
a. is expressed in newtons
b. can cause an object to speed up, slow down, or
change directions
c. is a push or a pull
d. All of the above
4. What opposes motion between surfaces that are
touching?
a. Gravity
b. Friction
c. net force
d. mass
Gravity
• Gravity is a force of attraction between
objects that is due to their masses.
• All matter is affected by gravity
– All matter has mass, and gravity is a result of
mass.
– All objects experience an attraction toward all
other objects.
– The greater the mass the stronger the
attraction.
Gravity on Earth
• Earth’s gravitational force is large.
– Compared to all other objects around you, Earth
has the largest mass.
– Earth’s gravitational force pulls everything toward
the center of the Earth.
• That is why objects in this room stay in place, and why
dropped objects fall to the Earth rather than moving
together or towards you.
The Law of Universal Gravitation
• Newton made the connection, as legend has it
while sitting under an apple tree.
• He knew unbalanced forces are necessary to
move or change the motion of objects- he
called this force the force of gravity.
• This became known as the law of universal
gravitation.
– This law describes the relationships between
gravitational force, mass, and distance.
– It is called universal because it applies to all objects
in the universe.
Law of Universal Gravitation
• The Law states:
– All objects in the universe attract each other
through gravitational force.
– The size of the force depends on the masses
of the objects and the distance between them.
Gravitational Memory Lane
• Remember these key points about Gravity
– Gravitational force increases as mass increases
• The amount of gravity between an elephant and Earth is
going to be greater than the amount of gravity between a
cat and Earth.
– Gravitational force decreases as distance increases
• Venus and Earth have the same mass, however Venus is
much closer to the Sun than Earth, thus the gravity
between Venus and the Sun is greater than between
Earth and the Sun.
– Weight and mass are different!
Chapter 5 Quiz 4
1. If Earth’s mass doubled, your weight would:
a. increase because gravity increases
b. decrease because gravity increases
c. increase because gravity decreases
d. not change because you are still on Earth
2. The amount of gravity between 1kg of lead
and Earth is _______ the amount of gravity
between 1kg of marshmallows and Earth
a. greater than b. less than
c. the same as d. None of the above
Chapter 5 Quiz 4 (cont)
3. As distance is increased between two
objects, gravity is ____________.
a. increase b. decreased
c. doubled d. not changed at all
4. _________ is affected by gravity, while
_______ is not
a. mass, weight b. weight, mass
c. force, weight c. mass, force