PHYSICS: Motion, Forces and Motion, Forces in Fluids and Work and Machines
P HYSICS : Motion, Forces and Motion, Forces in Fluids and Work and Machines.
Dec 30, 2015
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PHYSICS:
Motion, Forces and Motion,Forces in Fluids andWork 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.
Let’s Review!
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 theroller 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
timeA = 40 m/s – 0 m/s
5 sA = 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.
12-1: Forces
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 Laws
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
Area = Length x Width
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, atmospheric 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
BONUS: NOTES 13-2Pascal’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/rgbDyJhBb4c
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
2.9 g/cm3
A piece of metal has a mass of 43.5 g and a volume of 15 cm3. What is its density?
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.
DENSITYChanges 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!
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/auto-parts/brakes/brake-types/air-brake3.htm
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