Chapter 19 Objectives Describe the motion of an object by the - - PowerPoint PPT Presentation

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Chapter 19 Objectives Describe the motion of an object by the - - PowerPoint PPT Presentation

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Section 1 Measuring Motion Chapter 19 Objectives Describe the motion of an object by the position of the object in relation to a reference point. Identify the two factors that determine speed. Explain the difference between speed and

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Chapter 19

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Objectives

  • Describe the motion of an object by the position of

the object 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.

Section 1 Measuring Motion

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Observing Motion by Using a Reference Point

  • 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 _____________.

  • The direction of an object’s motion can be described

with a reference direction, such as north, ______, east, west, up, or _________.

  • The Earth’s surface is a common reference point for

determining motion.

Section 1 Measuring Motion

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Speed Depends on Distance and Time

  • Speed is the distance traveled by an object divided

by the time taken to travel that distance.

  • Average speed equals the total distance divided by

the total time. total ____________ average speed = ---------------------- total __________

Section 1 Measuring Motion

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Section 1 Measuring Motion

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Velocity: Direction Matters

  • The speed of an object in a particular

direction is called ____________.

  • Speed and velocity are two different

terms with two different meanings. Velocity must include a ___________.

Section 1 Measuring Motion

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  • You can think of velocity as the rate
  • f change of an object’s position.
  • An object’s velocity is constant
  • nly if its speed and direction don’t

change.

  • You can combine different velocities

to find the resultant velocity.

Section 1 Measuring Motion

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Section 1 Measuring Motion

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Acceleration

  • The rate at which velocity changes over

time is called ____________________. 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.

Section 1 Measuring Motion

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  • You can find average acceleration by using the

equation:

Section 1 Measuring Motion average acceleration = final velocity − starting velocity time it takes to change velocity

  • Velocity is expressed in meters per second (m/s),

and time is expressed in seconds (s). So acceleration is expressed in meters per second per second, or (m/s)/s, which equals m/s2.

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Section 1 Measuring Motion

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  • Acceleration can be shown on a graph of velocity

versus time.

Section 1 Measuring Motion

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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.

  • The acceleration that occurs in circular

motion is known as ________________ acceleration

Section 1 Measuring Motion

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Objectives

  • Describe forces, and explain how forces act on
  • bjects.
  • Determine the net 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.

Section 2 What Is a Force?

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Forces Acting on Objects

  • In science, a force is simply a push or a pull exerted
  • n an object in order to change the motion of the
  • bject. All forces have both __ __ & ___________.
  • A force can change the acceleration of an object.

This acceleration can be a change in the speed and/or direction of the object.

  • Scientists express force using a unit called the

_______________ (N).

Section 2 What Is a Force?

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Determining Net Force

  • Usually, more than one force is

acting on an object. The net force is the combination of all the forces acting on an object.

  • Determining net force depends on

the directions of the forces.

Section 2 What Is a Force?

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  • 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.

  • If forces are acting in opposite

directions, the net force can be found by subtracting the smaller force from the larger one. The net force will be in the direction of the larger force.

Section 2 What Is a Force?

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Section 2 What Is a Force?

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Balanced and Unbalanced Forces

  • When the forces on an object produce a

net force of 0 N, the forces are ________.

  • 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.

Section 2 What Is a Force?

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  • When the net force on an object is not

0 N, the forces on the object are _____________.

  • 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.

Section 2 What Is a Force?

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Objectives

  • Explain why friction occurs.
  • List the two types of friction,and give examples of

each type.

  • Explain how friction can be both harmful and helpful.

Section 3 Friction: A Force That Opposes Motion

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The Source of Friction

  • ____________ 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.

Section 3 Friction: A Force That Opposes Motion

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  • 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.

Section 3 Friction: A Force That Opposes Motion

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  • 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.

Section 3 Friction: A Force That Opposes Motion

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Section 3 Friction: A Force That Opposes Motion

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  • Rough surfaces have more

microscopic hills and valleys than smooth surfaces do.

  • So, the rougher the

surface is, the ______ the friction is.

Section 3 Friction: A Force That Opposes Motion

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Types of Friction

  • The word kinetic means “moving.”

So, kinetic friction is friction between ___________ surfaces.

  • When a force is applied to an object

but does not cause the object to move, ___________ friction occurs.

Section 3 Friction: A Force That Opposes Motion

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  • The word static means “not moving.”

The object does not move because the force of static friction balances the force applied.

  • Static friction disappears as soon as

an object starts moving, and then becomes kinetic friction immediately!

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Section 3 Friction: A Force That Opposes Motion

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Friction: Harmful and Helpful

  • Without friction, a car’s tires could not push

against the ground to move the car forward, and the brakes could not stop the car. Without friction, a car is useless.

  • However, friction can also cause problems in

a car. Friction between moving engine parts increases their temperature and causes the parts to wear down.

Section 3 Friction: A Force That Opposes Motion

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Friction can be both harmful and helpful, so it may be necessary to decrease or increase friction.

  • Some Ways to Reduce Friction One way

to reduce friction is to use lubricants. _______________ are substances that are applied to surfaces to reduce the friction between the surfaces.

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  • Some examples of common lubricants

are motor oil, wax, and grease. Lubricants are usually liquids, but they can be solids or gases.

  • Another way to reduce friction is to

make surfaces that rub against each

  • ther smoother.

Section 3 Friction: A Force That Opposes Motion

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  • Making surfaces rougher is one way to

__________ friction. For example, sand scattered on icy roads keeps cars from skidding.

  • Another way to increase friction is to

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.

Section 3 Friction: A Force That Opposes Motion

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Objectives

  • Describe gravity and its effect on matter.
  • Explain the law of universal gravitation.
  • Describe the difference between mass and weight.

Section 4 Gravity: A Force of Attraction

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The Effects of Gravity on Matter

  • __________ is a force of attraction between
  • bjects that is due to their masses. Gravity can

change the motion of an object by changing its speed, direction, or both.

  • All matter has mass, and gravity is a result of
  • mass. Therefore, all matter is affected by gravity

and all objects experience an attraction toward all other objects.

Section 4 Gravity: A Force of Attraction

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  • The mass of most objects is too small to

cause a force large enough to move objects toward each other.

  • Compared with all objects around you, Earth

has a huge mass. Therefore, Earth’s gravitational force is very large.

  • You must apply forces to overcome the

Earth’s gravitational force any time you lift

  • bjects or even parts of your body.

Section 4 Gravity: A Force of Attraction

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Newton and the Study of Gravity

  • Why do objects fall toward the Earth? What keeps the

planets moving in the sky?

Section 4 Gravity: A Force of Attraction

  • In 1665, British scientist

Sir Isaac Newton made the connection between these two questions when, as legend has it, he saw an apple falling from a tree.

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  • Newton knew that unbalanced forces are

needed to change the motion of objects. He concluded that an unbalanced force on the apple made the apple fall.

  • He also reasoned that an unbalanced force
  • n the moon kept the moon moving around the

Earth.

  • He proposed that these two forces are

actually the same force––gravity.

Section 4 Gravity: A Force of Attraction

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  • Newton summarized his ideas about

gravity in a law known as the law of universal gravitation. This law describes the relationships between gravitational force, mass, and distance.

  • The law is called universal because it

applies to all objects in the universe.

Section 4 Gravity: A Force of Attraction

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The Law of Universal Gravitation

  • Part 1: Gravitational Force

___________ as Mass Increases

  • Part 2: Gravitational Force

______________ as Distance Increases

Section 4 Gravity: A Force of Attraction

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Section 4 Gravity: A Force of Attraction

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Weight as a Measure of Gravitational Force

  • The Differences Between Weight and

Mass Weight is related to mass, but they are not the same.

  • __________ is a measure of the gravitational

force on an object. Weight changes whenever gravitational force changes.

  • __________ is the amount of matter in an
  • bject. An object’s mass does not change if

gravitational force changes.

Section 4 Gravity: A Force of Attraction

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Section 4 Gravity: A Force of Attraction

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  • 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.

Section 4 Gravity: A Force of Attraction