Download Chapter 1 Matter in Motion

Chapter Objectives
 Describe the circular motion of a moving object as the
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result of a force acting toward the center.
Given an object in motion, calculate its speed
(distance/time).
Interpret a line graph representing an object’s motion in
terms of distance over time (speed) using metric units.
Explain every object exerts a gravitational force of
attraction on every other object.
Recognize an object’s weight is a measure of the
gravitational force of a planet/moon acting on that object.
Compare the amount of gravitational force acting between
objects (which is dependent upon their masses and the
distance between them).
What is Motion
 Motion occurs when an object changes position over
time compared with an object that appears to stay in
place?????? I know it sounds kind of weird.
 Here’s another way to look at it. When we see
something that appears to be moving our eyes are
actually picking up on a couple of things. One,
obviously the object that is moving, and two an object
that appears to be still. The object that appears to be
still or not in motion is known as a reference point.
(Just because it appears to not be in motion doesn’t
mean it’s not in motion.)
Motion continued
 Motion is usually described with a direction like north,
south, east, or west.
 Sometimes its not easy to determine if an object is in
motion.
Reference Points
 As we have already pointed out a reference point is an
object that appears to stay in place.
 The most common reference point is the earth’s
surface(buildings, trees, and mountains).
 A moving object can also be a reference point.
Speed!!!!
 Speed is the rate at which an object moves. Most of the
time we think of speed as MPH, but the common SI
unit is m/s(meters/second)
 Speed is distance/time.
 Average speed is determined by simply taking the total
distance the object moved and dividing it by the total
time.
Velocity
 Lots of times we get velocity mixed up. It is commonly
associated with speed.
 Velocity is the speed of an object in a particular
direction and is expressed in m/s.
 So how are they different??? Speed is the rate at which
an object moves. It doesn’t have to include direction.
Velocity on the other hand must include direction.
Speed and Direction Affect Velocity
 If either the speed or direction of an object changes
then the velocity changes.
 So if an object is going in a straight line at a constant
speed is velocity changing?
Combining Velocities
 When you combine two velocities that are in the same
direction, add them together to find the actual
velocity.
 When you combine two velocities that are in opposite
directions, subtract the smaller velocity from the
bigger velocity.
Acceleration
 There is a misconception about acceleration. It is
commonly thought of as “speeding up”. That is not so
in science.
 Acceleration is the rate at which velocity changes. So
any time you change your speed or your direction, not
only do you affect your velocity, but you also affect your
acceleration.
Calculating Acceleration
 Acceleration equals Final velocity – starting velocity all
divided by time it takes to change velocity.
 Because acceleration examines the rate(time) at which
velocity(m/s) changes it is expressed in m/s/s.
 So suppose a bicycle’s acceleration is 3m/s/s. that
means that every second your southward velocity
increase by 3m/s.
Acceleration Examples
 Acceleration in which velocity increases is known as
positive acceleration.
 Acceleration in which velocity decreases is known as
negative acceleration or deceleration.
Continuous Acceleration
 The acceleration that occurs due to a circular motion is
known as centripetal acceleration.
 Ex. The blades of a windmill constantly change direction
as they travel in a circle.
Force
 Force is a push or a pull on a object.
 All forces are exerted by one object on another.
 It is expressed in newtons (N).
 The greater the newtons the greater the force.
 Just because a force is exerted on an object doesn’t
mean that motion will occur.
Gravity
 You constantly have an unseen force on you.
 Earth is a massive object in space, and because of this
it exerts gravity on you and every object on Earth.
Net Force
 A force that occurs from combining all the forces
exerted on an object.
 If forces are applied in the same direction then you add
the forces together to get the net force.
 If forces are applied in opposite directions then you
subtract the lower force from the higher force to find
the net force.
Unbalanced vs. Balanced Forces
 Once you have figured out the net force applied to an
object then you can determine whether the forces were
balanced or unbalanced. In other words whether it
moved or not.
 If the net force applied to the object is greater than
zero then the forces are unbalanced. In this case the
object would be in motion.
 If the net force applied to the object is zero then the
forces are balanced. In this case the object wouldn’t be
in motion.
What is Friction?
 Friction is a force that opposes motion between two
surfaces that are touching.
 When a baseball player slides into second base he/she
is experiencing friction. If they weren’t they would just
keep on sliding on past second base.
Why does friction happen?
 Friction occurs because the surface of any object is
rough. Your desk, even though it may look smooth, if
viewed through a microscope you would notice several
bumps or ridges. These bumps and ridges are what
oppose the motion of your hand across the desk.
 There are many factors that affect the amount of
friction that may occur.
Factors that affect friction
 How rough a surface is.
 This is basically common sense here. Which would you
rather fall on? Ice or Pavement? I would take ice any day.
This is because the rougher the surface the more
friction.
 How much of a force is applied to an object.
 Lets say we had two boxes on the floor. In one box I
filled it with feathers. In the other box I filled it with
bricks. Which one is going to be easier to push across
the floor? With out a doubt the feathers. There is less
friction with the box of feather due to there being lesser
force.
Types of Friction
 Sliding Friction
 Very effective at opposing the movement of an object.
 When you go sledding in the winter you are using
sliding friction. The baseball player sliding into second
is another example.
 Rolling Friction
 Helps reduce the amount of friction.
 Lets take the same box of bricks I talked about earlier
and put them on a cart with wheels. Pushing them
across the floor would become a lot easier. This would
be an example of rolling friction.
Types of Friction
 Fluid Friction
 First of all, a fluid can be water, milk, oil, and even air.
 Where would it be easier to run in a pool or out of a pool?
Fluid friction is the opposition by some fluid like water or
even air. In fact both rolling friction and fluid friction go
against a vehicles gas mileage. Why?
 Static Friction
 This is where a force is applied to an object and that object
doesn’t move. That like you going up to my wall and pushing
it. It doesn’t matter how hare you push. Unless you are
Hercules you are not pushing that wall anywhere.
Friction can be Harmful or Helpful
 Harmful
 Take this example. Your brother has truck with factory
tires on it that gets about 16 MPG. He then decides to
put new mud tires on it. Now that same truck gets about
10 MPG. He will now have to buy gas more often.
 Helpful
 Use the same example, same situation. With the factory
tires your brother was noticing he was getting stuck in
the mud a lot. He gets the new mud tires and now he
can drive out though the muddiest of fields without
getting stuck.
Ways of Reducing Friction
 Lubricants
 Substances that are applied to surfaces to reduce the
friction between them.
 Oil, wax, and grease
 Switch from sliding to rolling friction
 Make the surfaces that are making contact smoother.
Ways of increasing Friction
 Make the surfaces rougher.
 Increase the force applied to the two surfaces.
Gravity
 Gravity is a force of attraction between objects that is
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due to their masses.
Gravity is a result of mass.
All things that have mass exert gravity.
We, for example, have mass so therefore we exert
gravity.
Wait a minute!!!??? If we exert gravity why aren’t object
coming at us???
Earth is the most massive object that is closest to us, so
everything on earth is attracted to earth by gravity.
Earth Large Force
 Earth is so large that everything on it is attracted to its
surface. So in order for something to be moved away
from earths surface that object must have a force
applied to it greater than what Earth is applying to that
object.
Law of Universal Gravitation
 Developed by Sir Isaac Newton due to an apple falling
out of a tree and clocking him on the head (not really).
 The story goes that Newton was observing an apple
falling from a tree. He recognized that an unbalanced
force was drawing it down to Earth. He realized this
force was gravity.
 He developed his observations into a law that 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.
The “LAW” Part 1
 Gravitational Force Increases as Mass Increases.
 If the only two things that existed in outer space were an
elephant and a mouse then the mouse would be pulled
towards the elephant by the elephants gravity because of
its greater mass.
 To put it in more realistic terms. The moon is attracted
to the Earth because the Earth has a greater mass. So
why doesn’t the moon come crashing into the Earth?
The “LAW” Part 2
 Gravitational force decreases as distance increases.
 Here’s a fact for thought. The sun is 300,000 times more
massive than the Earth. So why aren’t we attracted to the
sun? It is because of the distance from the sun. We are so
far that the suns gravity is cancel out by the Earths
gravity.
Weight
 Weight is a measure of the gravitational force exerted
on an object.
 Have you every hopped on that scale at kmart or any
other place that has one that gives you your weight on
earth and on the moon? If you have then you noticed
that your weight on the moon was smaller than that of
Earths. That is because applies a greater gravity due to
its greater mass.
Weight and Mass are Different
 Weight of course is the measure of the gravitational
force exerted on an object.
 Mass is the amount of matter in an object.
 Your weight changes from the Earth to the moon. But
your mass doesn’t unless you lose a leg or an arm while
on your trip to the moon.