Chapter 13Chapter 13Chapter 13Chapter 13

Forces in MotionForces in Motion

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You Will Discover:• How motion is measured.

• How force affect motion, work, power.

• What Newton’s laws of motion are.

• How simple machines make work easier.

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Chapter 13 Vocabulary• Velocity: the speed and direction of an object’s motion• Force: a push or pull that acts on an object• Work: energy used when a force moves on an object• Power: the rate at which work is done• Equilibrium: the state in which the net force is zero• Inertia: the tendency of an object to resist a change in

motion• Acceleration: the rate at which the velocity of an object

changes over time.• Machine: a device that changes the direction or the amount

of effort needed to do work.

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Lesson 1: How can you describe motion?Types of Motion

• Steady or Constant - the Earth has steady motion as it moves around the Sun.

• Variable - cars, trucks, and buses because they move in many directions and at many speeds.

• Periodic - a pendulum, swings back and forth• Circular - wheels • Vibrational - a rubber band when you pluck it

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Examples of MotionQuickTime™ and aTIFF (Uncompressed) decompressorare needed to see this picture. QuickTime™ and a

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Steady or Constant

Periodic

Variable

Vibrational Circular

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Speed • Average Speed: describes how far an object

moves during a certain amount of time.

Average Speed = Distance

Time• The motion of an object cannot be measured by

just looking at the object.• Motion is always measured in relationship with

some location called a point of reference.• The speed at which a person is moving can vary

according to which point of reference you use.

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Speed FormulaPoint of Distance Time SpeedReference

Train Car 10 m 5 s 10m = 2 m/s 5 s

Signal 150 m 5 s 150m = 30 m/s

Light 5 s

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Velocity• Speed and Velocity are not the same!• Velocity describes the speed and the

direction of an object’s motion.• Example: the speed of the train might be

described as 30 meters per second, but its velocity is 30 meters per second North.

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Questions to Discuss• What are five types of motion? Give an

example for each one.• What do you need to know to find an objects

velocity?• What is a point of reference?• If you were riding a bicycle, is your motion

constant, variable, or periodic? Explain.

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Lesson 2 - What are forces?

Pushes and Pulls• A Force is a push or pull that acts on an object.

– When one object pushes or pulls another object, the first object is exerting a force on the second one.

• Forces can make a moving object speed up, slow down, or change direction.

• Forces have both magnitude and direction.– Magnitude is measured in newtons (N).– The direction of a force can be described by telling which

way the force is acting.

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Gravity• Every object in the universe exerts a

gravitational pull on every other object.• An object’s weight is the amount of

gravitational force between it and Earth.-This depends on the object’s mass and Earth’s mass.

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Magnetism• Magnetism is a force that pushes or pulls on

other objects.• Magnets will strongly pull on objects made of

iron, cobalt, nickel, and gadolinium.• The north and south poles of

two magnets will push away from each other. QuickTime™ and a

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Electricity• Electric forces act between objects that are

electrically charged.• Objects get electrically charged when they gain or lose

electrons.

• All atoms have negative electrons and positive protons.

• If an object gains electrons, the object will be negatively charged.

• If an object loses electrons, it will be positively charged.

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Electricity• Electrons move from one object to another

when:• The objects are rubbed together.

• Objects that are electrically charged will exert forces on each other.

• Oppositely charged objects are attracted to each other.

• Objects with the same charge are repelled from each other.

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Friction• Friction is the force that results when two

materials rub against each other.• Friction acts to slow down the motion of an

object or keep it from starting to move.• Friction depends on the qualities of the

object involved.• The shape, speed, or texture of one object can affect

the amount of friction.

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Work• Work is done when a force moves an

object.

• To calculate work:Force x Distance = Work

• Work is measured in joules.Force Distance Work

10 N 1 m 10 N x 1 m = 10 J

10 N 2 m 10 N x 2 m = 20 J

20 N 2 m 20 N x 2 m = 40 J

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Work• If the force applied to an object, does not

make the object move, then no work has been done.

• Work is only done when the object moves.• Holding an object in place can require a

force, but since the object doesn’t move no work is done.

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Power• Power is the rate at which work is done.• The faster work is done, the power is

increased.• To calculate the amount of power:

Work = PowerTime

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Questions to Discuss• What are the causes of electrical and

magnetic forces?• How does friction affect movement?• What are two examples of forces?• What kind of force can make a paper clip

cling to a comb?• In what unit is work measured? In what unit

is power measured?

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Lesson 3 - What are Newton’s laws of motion?

NET FORCES• Different forces can act on an object at the same

time.• They may act in the same direction, different

directions, and some may be stronger than others.• The combination of all these forces is the NET

FORCE!• The net force determines whether the object start

or stop moving or change direction.

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Net Forces• Equal forces working in opposite directions,

balance those forces. The net force is zero.• This is called equilibrium.

• A stationary object will remain motionless.• A moving object in will continue to move at a constant

speed in a straight line.

• Unbalanced forces acting on an object causes it to change motion.

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Newton’s First Law• Unless a net force acts on an object, the

object will remain in constant motion.• An object at rest stays at rest until a net force

acts upon it.• An object moving at a constant speed will

continue to move in a straight line and at a constant speed.

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Newton’s Second Law• Acceleration is the rate at which the velocity of an

object changes over time.• The net forces acting on an object can change an

object’s velocity by causing it to speed up, slow down, or change directions.

• The formula that describes the relationship between force, mass, and acceleration is:

Force = Mass x Acceleration• The stronger the force the more that object will

accelerate.

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Newton’s Third Law• When one object exerts a force on a second object, the second object exerts

a force on the first object.• This is sometimes called the action-

reaction law of motion.• Action-reaction forces are always equal

and opposite, and they occur in pairs.

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Questions to Discuss• What is the net force on an object?• What does an object look like that is in

equilibrium?• What does Newton’s first law of motion

state? The second law? The third law?• How does force affect acceleration?• Why do action-reaction forces always occur

in pairs?

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Lesson 4 - What are simple machines?

Machines and Work:• Work is done when a force causes an object to move.• A machine is a device that changes the direction or the

amount of force needed to do work.• Machines do not reduce the amount of work that needs

to be done, it just makes it easier.• There are four types of simple machines: pulley, wheel

and axle, lever, inclined plane

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Pulley• A pulley consists of a rope or cable that runs

through a grooved wheel.

*Examples:

flag pole

drapery rods

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Wheel and Axle• A wheel and axle is made up of a circular

object and a shaft.

*Examples:

steering wheeldoor knob

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Lever• A lever is made up of a stiff bar that rotates

around a fixed point called a fulcrum.

*Examples:

tongsclothes pinspliers

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Inclined Planes• An inclined plane consists of a flat

surface with one end higher than the other.

*Examples:

screws, wedge, doorstop

ramps,

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Complex Machines• A complex machine uses two or more simple

machines put together.• Many complex machines use electricity,

gravity, burning fuel, human force, or magnetism to operate

• Examples: washing machine go cart car sailboat QuickTime™ and a

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Questions to Discuss• Why is a screw considered a simple machine?• What is a complex machine?• How is the movement of rolling a weight up an

inclined plane different from climbing stairs?• What is a machine?• List the four simple machines. What is one

example of each simple machine?