PHYSICS:
Motion, Forces and
Motion,
Forces in Fluids and
Work and Machines
MOTION 11-1
An object is in motion if it changes position
relative to a reference point
 Stationary objects make good reference points

To describe motion accurately and completely, a frame of
reference is necessary.



The answer to “How fast is the butterfly is moving?” depends on which
frame of reference you use to measure its motion.
The answer to “ How fast are the train passengers moving?” depends on the
frame of reference you use measure their motion.
Choosing a meaningful frame of reference allows you to
describe motion in a clear and relevant manner.
RELATIVE MOTION

Whether or not an
object is in motion
depends on the
reference point you
choose.
DISTANCE AND DISPLACEMENT

Distance is the total length of the actual path between
two points. Displacement is the length and direction of
a straight line between starting and ending points.
What is the total distance this
person traveled (in blocks)?
7 Blocks
What is the total displacement
of this person?
5 Blocks Northeast
MORE ON DISPLACEMENT: VECTORS

Vector quantities that have both a magnitude and a direction
CALCULATING SPEED 11-2

If you know the distance an object travels in a
certain amount of time, you can calculate the
speed of the object.
AVERAGE SPEED

The speed of most moving objects is not constant
INSTANTANEOUS SPEED

Rate at which object is moving at a given instant
in time
A few more practice problems…
VELOCITY
Speed in a given direction
 Velocity is a vector because it has both magnitude and
direction
 Changes in velocity may be due to changes in speed,
changes in direction, or both



A cheetah’s speed may be as fast as 90 km/h but to describe its velocity you must know the
direction in which it is moving.
As the sailboat’s direction changes, its velocity also changes, even if its speed stays the same.
GRAPHING MOTION

You can use distance-versus-time graphs to interpret
motion.
REVIEW QUESTIONS
1. Is a moving bus a good reference point from which
to measure your position?
a. No, because it is often late.
b. No, because it is not a stationary object.
c. Yes, because it is very large.
d. Yes, because it can travel very far.
REVIEW QUESTIONS
1. Is a moving bus a good reference point from which
to measure your position?
a. No, because it is often late.
b. No, because it is not a stationary object.
c. Yes, because it is very large.
d. Yes, because it can travel very far.
REVIEW QUESTIONS
2. To describe a friend’s position with respect to you,
you need to know
a. Your friend’s distance from you.
b. The direction your friend is facing.
c. Your friend’s distance and direction from
you.
d. Your friend’s distance from a nearby
object.
REVIEW QUESTIONS
2. To describe a friend’s position with respect to you,
you need to know
a. Your friend’s distance from you.
b. The direction your friend is facing.
c. Your friend’s distance and direction
from you.
d. Your friend’s distance from a nearby
object.
REVIEW QUESTIONS
3. Two cars traveling in the same direction pass you
at exactly the same time. The car that is going
faster
a. moves farther in the same amount of
time.
b. has more mass.
c. has the louder engine.
d. has less momentum.
REVIEW QUESTIONS
3. Two cars traveling in the same direction pass you
at exactly the same time. The car that is going
faster
a. moves farther in the same amount of
time.
b. has more mass.
c. has the louder engine.
d. has less momentum.
REVIEW QUESTIONS
4. To describe an object’s motion, you need to know its
a. position.
b. change in position.
c. distance.
d. change in position over time.
REVIEW QUESTIONS
4. To describe an object’s motion, you need to know its
a. position.
b. change in position.
c. distance.
d. change in position over time.
ACCELERATION 11-3
The rate at which velocity (speed and direction)
changes
 Is a vector quantity
 In science, acceleration refers to changes in speed,
changes in direction or both



Decreasing speed = deceleration
Describe the acceleration
occurring at this instant on the
roller coaster.
CALCULATING ACCELERATION
 To
determine the acceleration of an object,
you must calculate its change in velocity
per unit of time.
LET’S TRY A PROBLEM

Calculate the plane’s acceleration in the first 5 seconds
of motion.
A= Vf – Vi
time
A = 40 m/s – 0 m/s
5s
A = 8 m/s2
CALCULATING ACCELERATION






As a roller-coaster car starts down a slope, its velocity is 4
m/s. But 3 seconds later, its velocity is 22 m/s in the same
direction. What is its acceleration?
Read and Understand
What information have you been given?
Initial velocity = 4 m/s
Final velocity = 22 m/s
Time = 3 s
CALCULATING ACCELERATION










As a roller-coaster car starts down a slope, its velocity is 4
m/s. But 3 seconds later, its velocity is 22 m/s in the same
direction. What is its acceleration?
Plan and Solve
What quantity are you trying to calculate?
The acceleration of the roller-coaster car = __
What formula contains the given quantities and the unknown
quantity?
Acceleration = (Final velocity - Initial velocity)/Time
Perform the calculation.
Acceleration = (22 m/s - 4 m/s)/3 s = 18 m/s/3 s
Acceleration = 6 m/s2
The acceleration is 6 m/s2 down the slope .
CALCULATING ACCELERATION



Practice Problem
A falling raindrop accelerates from 10 m/s to 30 m/s
in 2 seconds. What is the raindrop’s acceleration?
(30 m/s - 10 m/s) ÷ 2 seconds = 10 m/s2
CALCULATING ACCELERATION



Practice Problem
A certain car can accelerate from rest to 27 m/s in 9
seconds. Find the car’s acceleration.
(27 m/s - 0 m/s) ÷ 9 s = 27 m/s ÷ 9 s = 3 m/s2
GRAPHING ACCELERATION

You can use both a speed-versus-time graph and
a distance-versus-time graph to analyze the
motion of an accelerating object.
MR. EDMONDS!!

http://youtu.be/4CWlNoNpXCc
WHAT IS A FORCE?
A push or pull that acts on an object
 Is a vector quantity
 Described by its magnitude and by the direction in which it
acts
 Arrow represents direction
 Magnitude unit = Newton (N)


1N = 1 kg*m/s²
NET FORCE

Often there is more than one force acting on an
object at the same time




Pushing a car that’s run out of gas
The result is net force, a combination of all the
forces
Net force determines whether an object moves
and in which direction
Sometimes the net force is zero
COMBINING FORCE VECTORS

The strength and direction of the individual
forces determine the net force.
UNBALANCED FORCES

Unbalanced forces acting on an object result in a
net force and cause a change in the object’s velocity.
BALANCED FORCES

Balanced forces acting on an object do not change
the object’s velocity.
FRICTION
A force 2 surfaces exert on each other when they
rub against each other
 Acts as an unbalanced force to slow motion down
 The strength of the force of friction depends on
the types of surfaces involved and how hard the
surfaces push together
 4 types of friction

1) STATIC FRICTION
Acts on objects that are not moving
 You must exert a force greater than the force of
static friction to make the object move

2) SLIDING FRICTION

Occurs when two solid surfaces slide over each
other
3) ROLLING FRICTION
Occurs when an object rolls across a surface
 Rolling friction is less than sliding friction for
similar surfaces

4) FLUID FRICTION
Occurs when a solid object moves through a fluid,
such as air, water, oil, etc.
 Fluid friction is usually less than sliding friction

GRAVITY
Force that pulls towards the center of the earth
 Newton realized that gravity acts everywhere,
not just on earth
 Called the Law of Universal Gravitation
 Any 2 objects in the universe attract each other

GRAVITY BETWEEN OBJECTS

The force of gravity between objects increases with
greater mass and decreases with greater distance.
MASS AND WEIGHT
Mass is how much matter is in an object
 The gravitational force exerted on a person or object at
the surface of a planet is known as weight.
 Weight = Mass x Acceleration due to gravity

Acceleration due to
gravity at Earth’s
surface
= 9.8 m/s2
FREE FALL



If the only force acting on the object is gravity, it
is said to be in free fall
An object in free fall is accelerating because of
the force of gravity at a rate of 9.8 m/s/s
This means that every second an object is free
falling, it increases its velocity 9.8 m/s


Is this affected by mass?
If dropped from the same height at the same time,
will a heavier object fall faster?
FREE FALL

No! If there are no other forces to consider, then the
objects will fall at the same rate, regardless of mass.
AIR RESISTANCE
A
type of fluid friction that acts on
objects falling through the air
 An
upward force acting on a falling
object
AIR RESISTANCE

Falling objects with a greater surface area
experience more air resistance.
PROJECTILE MOTION
Occurs when an object is thrown horizontally
 Gravity will act on the object in the same way as
it does when an object is dropped vertically

NOTES 12-2 AND 12-3
NEWTON’S FIRST LAW OF MOTION



An object will remain at rest or
moving at a constant velocity
unless it is acted upon by an
unbalanced force
Inertia is the tendency of an object
to resist a change in motion
Newton’s First Law of Motion =
The Law of Inertia
NEWTON’S 2ND LAW


Acceleration depends on the net force acting on the
object and on the objects mass
Acceleration = Net Force
Mass
Or
Net Force = Mass *
Acceleration
ELASTIC

Matter is considered elastic if it returns to its
original shape after being squeezed or stretched.
2 TYPES OF ELASTIC FORCES:
Compression
 Tension

COMPRESSION

Elastic force that squeezes or pushes matter
together


Example: sitting on a coach
Balanced force
TENSION
An elastic force that stretches or pulls matter
 Example: Swinging on a tire swing
 Balanced forces

BRIDGES
NEWTON’S 3RD LAW
If one object exerts a force on another object, then
the 2nd object exerts a force of equal strength in
the opposite direction on the first object.
 WHAT????
 = for every action there
is an equal but opposite
reaction.

ACTION- REACTION PAIRS
UNIVERSAL FORCES 12-4

Electromagnetic
Electric
 Magnetic


Nuclear Forces
Strong Nuclear Force
 Weak Nuclear Force


Gravitational Force
Gravity Acts Over Large Distances
 The Earth, Moon and Tide

FLUID PRESSURE 13-1
What Is Pressure?
 The amount of pressure you exert depends on the
area over which you exert a force.
CALCULATING PRESSURE
Width
Area = Length x
Units:
Force- Newton (N)
Area-square meters (m2)
Pressure- Pascal (Pa)
AREA

The area of a surface is the number of square units that
it covers. To find the area of a rectangle, multiply its
length by its width. The area of the rectangle below is 2
cm X 3 cm, or 6 cm2.
AREA





Practice Problem
Which has a greater area: a rectangle that is 4 cm X 20
cm or a square that is 10 cm X 10 cm?
The square has the greater area.
4 cm X 20 cm = 80 cm2
10 cm X 10 cm = 100 cm2
FLUIDS

A material that can easily flow

Examples?
Liquids
 Gases


Tiny particles are constantly moving and
colliding with surfaces, which exerts forces on the
surfaces.
FLUID PRESSURE

All of the forces exerted by the individual particles in a
fluid combine to make up the pressure exerted by the
fluid.
AIR PRESSURE



Right now, there is approximately 100 km of fluid on top
of you… AIR!
The weight of the air exerts a force which causes air
pressure or atmospheric pressure.
Why are you not crushed by these fluids?

The forces are exerted from all directions so they are
balanced.
VARIATIONS IN FLUID PRESSURE

As your
elevation
increases,
atmospheri
c pressure
decreases.
VARIATIONS IN FLUID PRESSURE

Water pressure increases as depth increases.
MEASURING PRESSURE



You can measure atmospheric pressure with a
barometer
Decrease in pressure = storm
Meteorologists use barometers to measure pressure to
help forecast the weather
AIR PRESSURE AND ALTITUDE

http://youtu.be/7_yf-iRf8Vc
BONUS: NOTES 13-2
Pascal’s and Bernoulli’s Principles
PASCAL’S PRINCIPLE
Pressure increases by the same amount
throughout an enclosed or confined fluid*
 When force is applied to a confined fluid,
the change in pressure is transmitted
equally to all parts of the fluid.

*Keep in mind
that a fluid in
science means
liquid or gas so
Pascal’s
principle
pertains to
gases as well.
HYDRAULIC DEVICES


A hydraulic device is a
device that uses liquids to
transmit pressure equally
from one point to another.
They consist of a column of
confined fluid with a piston
at each end.
In a hydraulic device, a
force applied to one piston
increases the fluid
pressure equally
throughout the fluid.
HYDRAULIC DEVICES


Pascal stated when the pressure
is changed in one part of the
confined fluid (such as a small
piston pushing down on the
fluid), it will equal the change in
pressure on the other end of the
fluid and this pressure will be
undiminished. The hydraulic
device multiplies the small force
put into the system yielding a
large force exerted on the other
end (the large piston).
In other words, a small force can
be exerted which will yield a
large force that can do work.
WHAT ARE HYDRAULIC SYSTEMS?



Use liquids to transmit pressure
and multiply force in a confined
fluid
Multiplies force by applying the
force to a small surface area. The
increase in pressure is then
transmitted to another part of the
confined fluid, which pushes on a
larger surface area.
Hydraulic systems are used to
crush garbage, lift a car, move a
bulldozer blade, lift a wheel chair,
and even to raise the backdoor to
an SUV.
EXAMPLES OF
HYDRAULIC LIFTS
HYDRAULIC BRAKES

The hydraulic brake
system of a car multiplies
the force exerted on the
brake pedal.
• http://youtu.be/rgb
DyJhBb4c
EXTENSION

Hydraulic systems with air
(remember, air is a fluid)

In some cases you can't or
wouldn’t want to use a liquid
hydraulic system



The brakes on a big truck are
ALL gas--specifically, air-powered, and they work very
well
On a big printing press, there
are a lot of compressed-air roller
lifters because no one wants a
book that has hydraulic fluid
spattered on the pages.
Advantages to hydraulic
systems with liquid
Fluids are incompressible
(almost all of the pressure
applied through the system is
directly transmitted to the
object which you want to lift)
 A complete lack of lag

BERNOULLI’S PRINCIPLE
Bernoulli’s principle states that as the speed of a
moving fluid increases, the pressure exerted by
the fluid decreases.
 http://youtu.be/olVJzVadiFs

APPLYING BERNOULLI’S PRINCIPLE

Bernoulli’s principle helps explain how planes fly.
APPLYING BERNOULLI’S PRINCIPLE

An atomizer is an application of Bernoulli’s
principle.
APPLYING BERNOULLI’S PRINCIPLE

Thanks in part to
Bernoulli's principle, you
can enjoy an evening by a
warm fireplace without
the room filling up with
smoke.
APPLYING BERNOULLI’S PRINCIPLE

Like an airplane wing, a flying disk uses a curved
upper surface to create lift.
DENSITY 13-3

Density is a measure of how closely packed the
atoms in a substance are

Density is a physical property

All matter has measurable density

Density =
Mass
Volume
MASS

Measured in grams using a balance scale
VOLUME OF LIQUIDS

Measured in mL using a graduated cylinder
VOLUME OF REGULAR SOLID


Measured in cm3 using
math
Volume = L x W x H
VOLUME OF IRREGULAR SOLID

Measured in mL by using the displacement
method
DENSITY UNITS

g/mL
Or
g/cm3
CALCULATING DENSITY



The density of a substance is its mass per unit of volume.
For example, a sample of liquid has a mass of 24 g and
a volume of 16 mL. What is its density?
CALCULATING DENSITY

Practice Problem
A piece of metal has a mass of 43.5 g and a volume of 15 cm3.
What is its density?
2.9 g/cm3
SINK OR FLOAT?




By comparing densities, you
can predict whether an object
will sink or float in a fluid.
An object that is more dense
than the fluid it is in sinks.
An object that is less dense
than the fluid it is in floats on
the surface
An object with a density equal
to that of the fluid floats at a
constant depth.
DENSITY
 Changes
in density cause a submarine to
dive, rise, or float.
DENSITY

Changes in density cause a submarine to dive, rise,
or float.
DENSITY

Changes in density cause a submarine to dive, rise,
or float.
BUOYANCY
Ability to float
 Ships are designed to have buoyancy

BUOYANCY

The pressure on the
bottom of a submerged
object is greater than the
pressure on the top. The
result is a net force in
the upward direction.
BUOYANCY

The buoyant force works opposite the weight of
an object.
BUOYANCY

Archimedes’ principle
states that the buoyant
force acting on a
submerged object is equal
to the weight of the fluid
the object displaces.
BUOYANCY

A solid block of steel sinks in water. A steel ship
with the same weight floats on the surface.
LET’S REVIEW!


What is the volume of a box with the dimensions
8cm by 5cm by 5cm?
8cm x 5cm x 5cm = 200 cm3
LET’S REVIEW!


What is the volume of
the dinosaur?
5.6mL-4.8mL
= 0.8mL
LET’S REVIEW DENSITY WITH MR. EDMONDS!

http://youtu.be/TRkCz3zG7w0
LET’S REVIEW!




If an object has a mass of 20 g and a volume of 40
cm3, what is its density?
0.5 g/cm3
Water has a density of 1.0 g/cm3. If the object
described above were placed in water, will it sink
or float?
Float!
WORK AND POWER 14-1

Work requires motion and direction

Power is the rate of doing work
CALCULATING FORCE





A speedboat pulls a 55-kg water-skier. The skier to
accelerates at 2.0 m/s2. Calculate the net force that causes
this acceleration.
Read and Understand
What information have you been given?
Mass of the water-skier (m) = 55 kg
Acceleration of the water-skier (a) = 2.0 m/s2
CALCULATING FORCE

A speedboat pulls a 55-kg water-skier. The skier accelerates at 2.0
m/s2. Calculate the net force that causes this acceleration.

Plan and Solve

What quantity are you trying to calculate?

The net force (Fnet) = __

What formula contains the given quantities and the unknown
quantity?

a = Fnet/m or Fnet = m x a

Perform the calculation.

Fnet = m x a = 55 kg x 2.0 m/s2

F = 110 kg • m/s2

F = 110 N
CALCULATING FORCE




A speedboat pulls a 55-kg water-skier. The skier accelerates at 2.0
m/s2. Calculate the net force that causes this acceleration.
Look Back and Check
Does your answer make sense?
A net force of 110 N is required. This does not include the force
that overcomes friction.
CALCULATING FORCE


Practice Problem
What is the net force on a 1,000-kg object
accelerating at 3 m/s2?

3,000 N (1,000 kg x 3 m/s2)
SOURCES OUTSIDE OUR TEXTBOOK
http://wiki.answers.com/Q/Why_is_a_fluid_in_a_
hydraulic_machine_a_liquid_rather_than_a_gas
 http://auto.howstuffworks.com/autoparts/brakes/brake-types/air-brake3.htm
