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Matter in Motion
Chapter 5
How do you know you
are moving?
Reference Point
Describe your position in the classroom using a reference point and a set of directions
Observing Motion• Motion is an object’s change in position relative to another object, or reference point. The object that appears to stay in place is called a reference point.
•
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Observing Motion• The direction of an object’s motion can be
described with a reference direction, such as north, south, east, west, up, or down.
• Common Reference Points • The Earth’s surface is a common
reference point for determining motion.
Speed
• Speed is the distance traveled by an object divided by the time taken to travel that distance.
Units commonly used for speed are:• Meters per second (m/s). •Kilometers per hour (km/h),•Feet per second (ft/s),•Miles per hour (mi/h)
Speed• Depends on Distance and Time
•Determining Average Speed Average speed equals the total distance divided by the total time.
average speed =total distance
total time
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Speed can be shown on a graph of distance versus time Measuring Motion
Measuring Motion
1. Kira jogs to a store 72 m away in a time of 36 s. What is Kira’s average speed?
Average speed = total distance total time
Measuring Motion
2. If you travel 7.5 km and walk for 1.5 h, what is your average speed?
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Measuring Motion
3. An airplane traveling from San Francisco to Chicago travels 1,260 km in 3.5 h.
What is the airplane’s average speed?
Measuring Motion
4. A bird flies at a speed of 15 m/s, for 10s20 m/s, for 10s25 m/s, for 5s.
What is the bird’s average speed?
1. The distance between Cedar Rapids, Iowa, and Sioux Falls, South Dakota, is about 660 km. How long will it take a car traveling with an average speed of 95 km/h to drive from Cedar Rapids to Sioux Falls?
A less than 1 h
B about 3 h
C about 7 h
D about 10 h
1. The distance between Cedar Rapids, Iowa, and Sioux Falls, South Dakota, is about 660 km. How long will it take a car traveling with an average speed of 95 km/h to drive from Cedar Rapids to Sioux Falls?
A less than 1 h
B about 3 h
C about 7 h
D about 10 h
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Velocity
• The speed of an object in a particular direction is called velocity.
• Speed and velocity are two different terms with two different meanings.
• Velocity must include a reference direction.
Velocity• You can think of velocity as the rate of
change of an object’s position.
An object’s velocity is constant only if its speed and direction don’t change.
Position vs.Time Graph• Motion graphs show the relationships
between position, speed, and time.Position vs. time data
tells you the runner’s position at different points in time.
The runner is at 50 m after 10 sec., 100 m after 20 sec. and 150 m at 30 sec.
• A graph makes it easy to see if changes in one variable cause changes in the other variable (the effect).
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• You can combine different velocities to find the resultant velocity Q & A
1.What is a reference point?A reference point is a point that appears to stay in place in relation to an object being observed and is used to determine if the object is in motion.
Q & A2. What two things must you know
to determine speed?
1. The distanced traveled
2. Time taken to travel that distance
Q & A3. What is the difference between
speed and velocity?
`Speed does not include direction;
Velocity does include direction
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Acceleration• The rate at which velocity changes over
time is called acceleration.•
• An object accelerates if its speed, or direction, or both change.
• An increase in velocity is commonly called positive acceleration.••A decrease in velocity is commonly called negative acceleration, or deceleration.
Calculating Average Acceleration
average acceleration =final velocity - starting velocity (m/s)
time it takes to change velocity (s)
Velocity is expressed in meters per second (m/s)Time is expressed in seconds (s).
Acceleration is expressed in m/s2.
Measuring Motion Examples of Acceleration
• Example How Velocity Changes
• ________________________________________________________________________________________________________
A plane taking off Increase of speed
A car stopping Decrease of Speed
Jogging on a winding trail Change in direction
Driving around a corner Change in direction
Standing on the earths equator
Change in direction
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Circular Motion: Continuous Acceleration
• An object traveling in a circular motion is always changing its direction. Therefore, its velocity is always changing, so it is accelerating.
Earth
F
vv’
• The acceleration that occurs in circular motion is known as centripetal acceleration.
Chapter 5 Section 2
What is a Force?
What Is a Force?
Earth
F
vv’
• A force is simply a push or a pull exerted on an object in order to change the motion of the object. All forces have both size and direction.
. A force can change the acceleration of an object.
Example:•change in the speed or direction of the object
Forces Acting on Objects• Unseen Sources and Receivers of Forces
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.
You cannot see that the air around you is held near Earth’s surface by a force called gravity.
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What is meant by “Net Force”
• “Net Force” is the combination of all the forces acting on an object.
• Determining net force depends on the directions of the forces.
• Scientists express force using a unit called the newton (N).
Forces in the Same Direction
• Two forces are added to determine the net force if the forces act in the same direction.
The net force will be in the same direction as the individual forces.
Forces in Different Directions
• If forces are acting in opposite directions, the net force can be found by subtracting the smaller force from the larger one.
Balanced Forces
• When the forces on an object produce a net force of 0 N, the forces are balanced.
• Balanced forces will not cause a change in the motion of a moving object.
• Balanced forces do not cause a nonmoving object to start moving.
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Unbalanced Forces• When the net force on an object is not
0 N, the forces on the object are unbalanced.
• Unbalanced forces produce a change in motion, such as a change in speed or a change in direction.
• Unbalanced forces are necessary to cause a non-moving object to start moving, or to change the motion of moving objects.
FrictionSection 3
Is a force that opposes motion between two surfaces that are in contact.
• Friction occurs because the surface of any object is rough.
• Even surfaces that feel smooth are covered with microscopic hills and valleys.
• When two surfaces are in contact, the microscopic hills and valleys of one surface stick to the tiny hills and valleys of the other surface. This contact causes friction.
The Effect of Force on Friction
• The amount of friction depends on the force pushing the surfaces together. If this force increases, the hills and valleys of the surfaces can come into closer contact.
• The close contact increases the friction between the surfaces. Objects that weigh less exert less downward force than objects that weigh more.
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•Rough surfaces have more microscopic hills and valleys than smooth surfaces do.• The rougher the surface, the greater the friction .
The Effect of Rougher Surfaces on Friction
• Kinetic Friction The word kineticmeans “moving.” So, kinetic friction is friction between moving surfaces.
• The amount of kinetic friction between two surfaces depends in part on how the surfaces move. Surfaces can slide past each other, or a surface can roll over another surface.
Types of Friction
• Usually, the force of sliding kinetic friction is greater than the force of rolling kinetic friction. It is usually easier to move objects on wheels than to slide the objects along the floor, as shown below.
Static Friction • When a force is applied to an object but
does not cause the object to move, static friction occurs.
• The word static means “not moving.” The object does not move because;
force of static friction = the force applied.
•
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Static friction disappears as soon as an object starts moving, and then kinetic friction immediately begins
• Static friction is friction between two surfaces which are not moving past each other.
• force of static friction = the force applied.
Friction: Harmful and Helpful• Helpful
Without friction -a car’s tires could not push against the
ground to move the car forward -the brakes could not stop the car.
Harmful-Friction between moving engine parts
increases their temperature and causes the parts to wear down.
Some Ways to Reduce Friction
1. Use lubricants. Lubricants are substances that are
applied to surfaces to reduce the friction between the surfaces.
• Some example:• motor oil, • wax,• grease. Lubricants are usually liquids, but they can be
solids or gases.
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• Friction can be reduced by switching from sliding kinetic friction to rolling kinetic friction.
2. Ball bearings can be placed between wheels and axels to make it easier for wheels to turn by reducing friction.
3. Another way to reduce friction is to make surfaces that rub against each other smoother.
Some Ways to Increase Friction
1. Making surfaces rougher. For example, sand scattered on icy roads keeps
cars from skidding.
2. Increase the force pushing the surfaces together.For example,
if you are sanding a piece of wood, you can sand the wood faster by pressing harder on the sandpaper.
Gravity
Is a force of attraction between objects.
Effects of Gravity
• Gravity is a force of attraction between objects that is due to their masses.
• Gravity can change the motion of an object.
Example are;SpeedDirection
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• All matter has mass• Gravity is a result of mass. • All objects experience an attraction
toward all other objects because of there mass. (Not their weight)
• The mass of most objects is too small to cause a force large enough to move objects toward each other.
The Effects of Gravity on Matter
• Earth has a huge mass. • Therefore, Earth’s gravitational force
is very large.
• We must apply forces to overcome the Earth’s gravitational force any time we lift objects or even parts of our body..
Things fall because of gravityNewton and the Study of
Gravity
• Why do objects fall toward the Earth?
• What keeps the planets moving in the sky?
In 1665, British scientist Sir Isaac Newton made the connection between these two questions,
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Isaac Newton (1643 - 1727)
• Newton, (and us) know that unbalanced forces are needed to change the motion of objects.
• And there must be an unbalanced force on the moon that keeps the moon revolving around the Earth.
The Law of Universal Gravitation
• This law describes the relationships between;
- gravitational force, - mass, and - distance.
Why is it considered universal?
b/c it applies to all objects in the universe
The Law of Universal Gravitation
• Part 1: Gravitational Force Increases as Mass Increases
Small mass = small forceLarge mass = large force
Part 2: Gravitational Force Decreases as Distance Increases
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The Differences Between Weight and Mass
• Weight is a measure of the gravitational force on an object. Weight changes whenever gravitational force changes.
• Mass is the amount of matter in an object. An object’s mass does not change if gravitational force changes
Weight is related to mass, but they are not the same.
Units of Weight and Mass • The SI unit of force is a newton (N).
Gravity is a force, and weight is a measure of gravity. So, weight is also measured in newtons.
• The SI unit of mass is the kilogram (kg). Mass is often measured in grams (g) and milligrams (mg) as well.
• On Earth, a 100 g object weighs about 1 N.
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Position, Speed and Time Graphs
• What kind of motion happens when an object rolls down a hill?
The position vs. time graph• Motion graphs are an important tool
used to show the relationships between position, speed, and time.
A runner can learn more about performance by studying data and graphs.
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The position vs. time graph
• Position vs. time data tells you the runner’s position at different points in time.
• The runner is at 50 meters after 10 sec., 100 meters after 20 sec. and 150 meters at 30 sec.
Graphs show relationships• A good way to show a
relationship between two variables is to use a graph.
• A graph makes it easy to see if changes in one variable cause changes in the other variable (the effect).
The position vs. time graph
• To graph data, you put “position” on the vertical (y) axis .
• “Time” goes on the horizontal (x) axis.
.• Data are plotted between x and y axis
The position vs. time graph
• An object moving at a constant speedalways creates a
position vs. time graph that is a straight line.
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Graphs show relationships
• This table shows how quickly the car gets from A to B as the angle of the track changes.
Graphs show relationships
If we plot the data on a graph, what kind of relationship does the graph show?
As the volume of water increases / the mass increases
As the mass of water in the pot increases / the temp. after 10 min decreases
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Four steps to make a graphStep 1: CHOOSE WHICH WILL BE THE DEPENDENT ANDINDEPENDENT VARIABLES. THE DEPENDENT VARIABLE GOESON THE Y-AXIS AND THE INDEPENDENT VARIABLE GOES ONTHE X-AXIS.
Step 2: MAKE A SCALE FOR EACH AXIS BY COUNTING BOXES TOFIT YOUR LARGEST VALUE. COUNT BY MULTIPLES OF 1, 2, 5, OR10.
Step 3: PloT EACH POINT BY FINDING THE X-VALUE ANDDRAWING A LINE UPWARD UNTIL YOU GET TO THE RIGHT Y-VALUE.
Step 4: DRAW A SMOOTH CURVE THAT SHOWS THE PATTERN OFTHE POINTS. DO NOT JUST CONNECT THE DOTS.
Reading a graph Slope
• You can use position vs. time graphs to quickly compare the speeds of different objects.
A steeper line on a position vs. time graph means a faster speed.
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Slope• The “steepness” of
a line is called its slope.
• Visualize a triangle with the slope as the hypotenuse.
• The rise is equal to the height of the triangle.
• The run is equal to the length along the base of the triangle.
Slope
• The slope is the ratio of the “rise”(vertical change) to the “run” (horizontal change).
• The slope is therefore a distance divided by a time, which equals speed.
Speed vs. time graphsThe position vs. time graph has position on the y-axis and time on the x-axis.
Which runner has the fastest constant speed?
Speed vs. time graphs
These graphs each show the same event.What differences do you notice?
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Calculating distance• A speed vs. time graph can also be used
to find the distance the object has traveled.
Interpreting Graphics
• The graph below shows the data collected by a student as she watched a squirrel running on the ground. Use the graph below to answer the questions that follow.
1. Which of the following best describes the motion of the squirrel
between 5 s and 8 s?• A The squirrel’s
speed increased.
• B The squirrel’s speed decreased.
• C The squirrel stopped
• D The squirrel moved backward.
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1. Which of the following best describes the motion of the squirrel
between 5 s and 8 s?• A The squirrel’s
speed increased.
• B The squirrel’s speed decreased.
• C The squirrel stopped
• D The squirrel moved backward.
2. Which of the following statements about the motion of the
squirrel is true?
F The squirrel moved with the greatest speed between 0 s and 5 s.G The squirrel moved with the greatest speed between 8s & 10 s.H The squirrel moved with a constant speed between 0 s and 8 s.I The squirrel moved with a constant speed between 5 s and 10 s.
2. Which of the following statements about the motion of the
squirrel is true?
F The squirrel moved with the greatest speed between 0 s and 5 s.G The squirrel moved with the greatest speed between 8s & 10 s.H The squirrel moved with a constant speed between 0 s and 8 s.I The squirrel moved with a constant speed between 5 s and 10 s.
3. What is the average speed of the squirrel between 8 s and 10 s?
A 0.4 m/s
B 1 m/s
C 2 m/s
D 4 m/s
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3. What is the average speed of the squirrel between 8 s and 10 s?
A 0.4 m/s
B 1 m/s
C 2 m/s
D 4 m/s
