Download Section 2 What Is a Force?

Chapter 19
Section 1 Measuring Motion
Question of the Day
Write a description on your paper
of your position in the classroom using a reference
Point (starting point) and a set of directions.
For example, you might say, “I sit three desks behind
Beth’s desk,” or “I sit 12 floor tiles east of the door and
10 meters north of the Smart Board.”
Then, write a description to your home in your
neighborhood.
Chapter 19
Matter in Motion
Chapter Standard
MOTION - 3
Apply proper equations to solve
basic problems pertaining to
distance, time, speed, and
velocity.
Chapter 19
Matter in Motion
Chapter Preview
Section 1 Measuring Motion
Section 2 What Is a Force?
Section 3 Friction: A Force That Opposes Motion
Section 4 Gravity: A Force of Attraction
Chapter 19
Section 1 Measuring Motion
Today’s Objectives
• Describe the motion of something by the position of
it in relation to a reference point.
• Identify the two factors that determine speed.
• Explain the difference between speed and velocity.
• Analyze the relationship between velocity and
acceleration.
• Demonstrate that changes in motion can be
measured and represented on a graph.
Chapter 19
Section 1 Measuring Motion
Observing Motion by Using a Reference Point
• Motion is an object’s change in position relative to
a reference point.
•A reference point is an object that appears to stay in
place.
•The Earth’s surface is a common reference point.
Chapter 19
Section 1 Measuring Motion
Observing Motion by Using a Reference Point
• The direction of an object’s motion can be described
with a reference direction, such as north, south, east,
west, up, or down.
Chapter 19
Section 1 Measuring Motion
Speed Depends on Distance and Time
• Speed is the distance traveled divided by the time
interval during which the motion occurred.
• The standard SI unit for speed is meters per second
(m/s).
•Other examples of speed units.
•Kilometers per hour (km/h)
•Feet per second (ft/s)
•Miles per hour (mi/h)
Chapter 19
Section 1 Measuring Motion
Speed Depends on Distance and Time, continued
• To determine average speed the total distance is
divided by the total time.
average speed =
total distance
total time
•Speed can be shown on a graph of distance versus
time, as shown on the next slide.
Chapter 19
Section 1 Measuring Motion
Chapter 19
Section 1 Measuring Motion
Chapter 19
Section 1 Measuring Motion
Velocity: Speed with Direction
•You know what speed is now, but what is velocity?
•Velocity is the speed of an object in a particular
direction.
• Speed and velocity are two different things. Velocity
must include a reference direction (example north,
south, left, or right).
Chapter 19
Section 1 Measuring Motion
Velocity: Speed with Direction
• Changing Velocity
•You can think of velocity as how fast an object’s
position changes.
•An object’s velocity is constant only if its speed and
direction don’t change.
• Combining Velocities
•You can combine different velocities to find the
resultant velocity or “net velocity”.
Chapter 19
Section 1 Measuring Motion
Chapter 19
Section 1 Measuring Motion
Acceleration
• Acceleration is the rate at which velocity changes
over time; an object accelerates if its speed, or
direction, or both change.
• Increase velocity, you get a positive acceleration.
• Decrease velocity, you get a negative acceleration, or
deceleration.
Chapter 19
Section 1 Measuring Motion
Acceleration, continued
•You can find average acceleration by using the
equation:
average acceleration =
final velocity  starting velocity
time it takes to change velocity
• Velocity is meters per second (m/s)
•Time is expressed in seconds (s).
•So acceleration is expressed in meters per second
per second, or (m/s)/s, which equals m/s2.
Chapter 19
Section 1 Measuring Motion
Chapter 19
Section 1 Measuring Motion
Acceleration, continued
• Acceleration can be shown on a graph of velocity
versus time.
Chapter 19
Matter in Motion
Chapter Standard
MOTION - 3
Apply proper equations to solve
basic problems pertaining to
distance, time, speed, and
velocity.
Chapter 19
Section 2 What Is a Force?
Question of the Day
Look around the room and think about the objects
you see touching another object.
A force is always exerted by one object on another
object.
Answer the following questions on your paper.
Where do you see a force happening in the room
right now? Which object is exerting the force, and
which is receiving it?
Chapter 19
Section 2 What Is a Force?
Today’s Objectives
• Describe forces, and explain how forces act on
objects.
• Determine the net force (total force) when more than
one force is acting on an object.
• Compare “balanced” and “unbalanced” forces.
• Describe ways that unbalanced forces cause
changes in motion.
Chapter 19
Section 2 What Is a Force?
What is a Force?
•Have you ever wondered what makes things
move?
•A force is a push or a pull exerted on an
object in order to change the motion of the
object; force has size and direction.
• Forces are what make objects move.
• The SI unit of force is the newton (N).
Chapter 19
Section 2 What Is a Force?
What is a Force?, continued
• It is not always easy to tell what is exerting a force or
what is receiving a force. For example, you cannot
see what exerts the force that pulls magnets to
refrigerators.
•What force does that?
•What keeps the air we breath near the Earth’s
surface?
• You cannot see that the air around you is held near
Earth’s surface by a force called gravity.
Chapter 19
Section 2 What Is a Force?
Determining Net Force
• Usually, more than one force is acting on an object at
the same time.
•Net force is the combination all of the forces acting
on an object.
•
Chapter 19
Section 2 What Is a Force?
Determining Net Force, continued
•Finding out the net force on an object depends
on the directions of the forces.
•Forces in the same direction just add the
forces.
• Forces in opposite directions just subtract the
small one from the larger on and keep the
LARGER direction.
Chapter 19
Section 2 What Is a Force?
Chapter 19
Section 2 What Is a Force?
Balanced and Unbalanced Forces
• When an object has a net force of 0 N acting on it, the
forces are called balanced.
• Balanced forces will not cause a change in the motion
of a moving object.
•Balanced forces do not cause a still object to start
moving.
Chapter 19
Section 2 What Is a Force?
Balanced and Unbalanced Forces, continued
•When the net force on an object is NOT equal to 0 N,
the forces are unbalanced.
• Unbalanced forces causes an object to move.