Motion, Forces, & Machines

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# Motion, Forces, & Machines - PowerPoint PPT Presentation

Motion, Forces, &amp; Machines. Describing Motion. Motion : the state in which one object’s distance from another is changing . Are You Moving? Reference Point : place or object used for comparison to determine if something’s in motion Good Stationary RPs: Trees , Signs, Buildings

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### Motion, Forces, & Machines

Describing Motion
• Motion: the state in which one object’s distance from another is changing.
• Are You Moving?
• Reference Point: place or object used for comparison to determine if something’s in motion
• Good Stationary RPs: Trees, Signs, Buildings
• The Backwards Moving Bus
• An Object Is In Motion If It Changes Position Relative To A Reference Point
Relative Motion
• Wait… Are you sure you aren’t moving?
• Movement Depends on your Reference Point
• Chair- NO SUN- YES
• We are actually moving at 30 k/sec
System of Measurement
• Really Important Experiment…. but different units
• It’s important scientists can communicate together
• They must have a “universal language”
• Metric System- International System of Units or S.I
• Base 10
• Length (meter)
• 1 meter = 39.4 inchesor .91 yards
• Centimeters (cm) are used to measures distances less than 1 meter
• 100 centimeters in 1 meter
Easy Conversions!
• Kicking (Kilo)
• Her (Hecto)
• Down (Deka)
• May (meter, gram, liter)
• Damage (Deci)
• Carol’s (Centi)
• Mind (Milli)
• UP to the LEFT, DOWN to the RIGHT
Calculating Speed
• If you know the distance an object travels in a certain amount of time, you can calculate the speed
• Speed = Distance

Time

• Speed: distance object travels per unit of time
• Various ways to express speed: m/s or km/h
• Average Speed: total distance & total time
• Instantaneous Speed: rate at which an object is moving at a given instant time
Velocity & Graphing Motion
• A storm is coming at a speed of 25 km/h!!!
• Velocity: Speed AND Direction of an object’s motion
• What if you want to show somebody motion?
• Line Graph: plotting distance (y) vs. time (x)
• Slope: steepness of the line
• Slope = Rise / Run
• Rise: vertical difference between two points
• Run: horizontal difference between two points
Motion of Earth’s Plates
• Plates: major pieces of Earth’s rocky outer layer
• Fit together like puzzle pieces- Pangaea
• Theory of Plate Tectonics: Earth’s landmasses have changed position over time because they are part of plates that are moving slowly
• Why they movin’?
• Heat from below the Earth pushes rock up
• The cooler rock gets pushed aside and sinks down
• Slow moving action of rock causes plate movement
Plate Movement
• OMG! The Plates are gonna collide!
• Plates move at a rate of only a few mm-cm each year
• Distance = Speed X Time
• Distance = 5 cm / year X 1,000 yrs = 5,000 cm
Acceleration
• The Crazy Life of a Baseball
• Acceleration = Speeding Up….. NOT!!!
• Acceleration: rate at which velocity changes
• Increase speed, decrease speed, change direction- examples??
• Can an object at a constant speed accelerate?
• Yes!! changing lanes, running a curve, ferris wheel
Calculating Acceleration
• Acceleration = Final Speed – Initial Speed

Time

• Units: meters/sec per second m/s2
• Let’s Practice!
• The Black Eyed Peas private plane is about to take off. It reaches a final speed on the runway of 40 m/s after 5 seconds. What is the acceleration of the plane?
Graphing Acceleration

What can we tell from this graph?

Increasing Speed

Constant Acceleration

Graphing Acceleration

What can we tell from this graph?

Curve= a

Each second traveled a greater distance & speed than the second before

Speed Increasing

Forces
• Force: a push or pull described by its strength and direction
• Newton (N): SI Unit used for measuring the strength of a force
• Exert about 1 N when lifting up a lemon
• We represent forces using arrows
• Arrows point in the direction of the force
• Length of arrow tells strength- Longer = Bigger F
Combining Forces
• Net Force: combination of all forces acting on an object
• Determines if an object moves
• Determines which direction an object moves

5 N

5 N

=

10 N

5 N

10 N

5 N

=

5 N

5 N

=

0

Unbalanced & Balanced Forces
• UnbalancedForce: a net force acting on an object causing it to start or stopmoving or change direction
• Causes a change in the object’s motion
• Balanced Forces: equal forces acting on one object in opposite directions
• Causes no change in the object’s motion

Unbalanced

Unbalanced

Balanced

5 N

5 N

5 N

5 N

=

10 N

5 N

10 N

10 N

10 N

10 N

=

5 N

5 N

5 N

5 N

5 N

5 N

5 N

5 N

5 N

5 N

5 N

5 N

=

=

=

=

0

0

0

0

Friction
• Friction: force that 2 objects exert on each other when they rub together
• Strength of the force of friction depends on:
• 1.) How hard the surfaces push together
• 2.) The types of surfaces involved
• Let’s try it! Rub your hands together
• Friction always acts in the opposite direction to the direction of the objects motion
• Metal Slides… Yikes!
• Without friction, moving objects might not stop until it hits another object
Types of Friction
• Static: acts on objects that aren’t moving
• requires extra force to start motion of objects at rest
• Moving a Desk & Body Builders moving cars
• Sliding: two solid surfaces slide over each other
• Sand on ice, chalk on hands, brakes of bike
Types of Friction
• Rolling: objects roll across a surface
• Easier to overcome than sliding friction
• Skateboards & Bikes use ball bearings
• Fluid: solid objects move through a fluid
• Easier to overcome than sliding friction
• Use of water, oil, or air
• WD40 (oil), streamlined helmet (air), hairy legs (Water) eek!
Gravity
• Gravity: force that pulls objects toward each other
• Issac Newton- Law of Universal Gravitation
• Gravity acts everywhere in the universe!
• A force acts to pull objects straight down toward the center of Earth
• The Famous Apple!
Factors of Gravity, Weight, & Mass
• 2 Things Affect Gravitational Attraction
• Mass- amount of matter in an object (gram)
• More mass = Great Gravitational Force
• Distance
• Farther apart = Less Gravitational Force
• Mass & Weight are NOT the SAME
• Weight- measure of gravitational force exerted
• Force of gravity on person/object at surface of a planet
• Weight varies w/ strength of gravities force, mass doesn’t
Gravity & Motion
• Free Fall: motion of a falling object when the only force acting on it is gravity
• Force of gravity is unbalanced
• Objects in free fall are accelerating
• Acceleration due to gravity on Earth = 9.8 m/s2
• All objects in free fall accelerate at the same rate regardless of mass
Gravity & Motion
• Air Resistance: fluid friction experienced by objects falling through the air
• Upward force exerted on all falling objects in air
• Objects with more surface area = more resistance
• Air resistance increases with velocity
• As object speeds up, resistance gets greater & greater
• Eventually force of air resistance & gravity are equal
• Force is balanced, no acceleration, constant velocity
• Terminal Velocity: greatest velocity a falling object reaches when force of air resistance equals weight of object
• An object that is thrown vertically will land at the same time as an object that was dropped
First Law of Motion- Inertia
• An object at rest will remain at rest, and an object moving at a constant velocity will remain moving at a constant velocity unless it is acted upon by an unbalanced (net) force
• Tennis Game- Ball moves until gravity or friction change objects motion
• If an object is not moving, it will not move until a force acts on it
• Clothes on your bedroom floor!
Inertia
• Inertia: tendency of object to resist change in motion
• Greater the mass of object = Greater Inertia =

Greater force needed to change its motion

Second Law of Motion
• Acceleration depends on the object’s mass and on the force acting on the object
• Force = Mass X Acceleration F = M x A
• Unit of Force = Newtons (N)
• Increase Acceleration = Increase Force
• Increase Mass = Decrease Acceleration
• Boo Yah! Practice Problem!
• You are cruzin’ the streets of Mattoon with an acceleration of 20 m/s in your Lamborghini that has a mass of 1250 kg. What is the net force?
Third Law of Motion
• If one object exerts a force on another object, then the second object exerts a force of equal strength in the opposite direction of the first object…. What????
• For every action there is an equal but opposite reaction
• Action Reaction Pairs: Examples…?
Action Reaction Forces Cancel?
• “Ms. Genta, you said before, forces with equal and opposite direction cancel out and cause no movement…??? You must be trippin!”
• Don’t Cancel If Acting on DIFFERENT objects!
Momentum
• Momentum: quantity of motion
• Momentum = Mass x Velocity (kg m/s)
• Momentum of an object is in the same direction as its velocity
• More Momentum = Harder to Stop
• What same velocity, different mass?
• Car & Baseball both moving at 20 m/s
• Law of Conservation of Momentum: in the absence of outside forces, it can be transferred from one object to another, but none is lost
Rocket Motion
• Rockets rise into the air because it expels gases with a downward force, then the gases exert an equal but opposite reaction force on the rocket
• Upward thrust is greater than downward gravity
• Centripetal force: causes an object to move in a circle
• Force on satellites that are accelerating & revolving around Earth
• Satellites in orbit around Earth continuously fall towards Earth, but because Earth is curved they travel around it
What is Work?
• Work: force exerted on an object causing it to move in the same direction as the force
• Pushing a swing, lifting bags up, pulling blinds down
• It is not work unless the object moves!
• Pushing a car, lifting an enormous boulder
• It is not work unless the motion is in the same direction as the force
• Carrying your books to class
• The good news: Homework is not work!! 
Calculating Work
• The amount of work you do depends on both the amount of force you exert and the distance the object moves
• Work = force X distance
• Measured in Joules (J)
• Work done to exert a force of 1 Newton/ 1 Meter
• Heavier Object = Greater Work
• Greater Distance = Greater Work
• Let’s Practice!
• An old, precious lady asks you to move her 95 N sewing kit a distance of 12 m. How much work are you going to have to exert?
Power
• If 1 person sprints up the stairs with a box and 1 person creeps up the stairs with the same box, you are doing the same amount of work but….
• Power: the amount of work done on an object in a unit of time
• Power = Work or Power = Force X Distance

Time Time

• Unit of Power: Watts (W) = 1 J/s
• So… more power to sprint up the stairs!
• Mr. Smith exerts a force of 900 N to push a cart of ice cream down to Ms. Genta’s amazing science students! Oh Ya! The cart moves 250 meters in 40 seconds. What is the power of Mr. Smith?
What is a Machine?
• Machine: device that allows work to be easier
• Hands, shovel, wheelbarrow, crane
• Machines make work easier by changing either the force, distance, or direction
• Input Force: force exerted on the machine
• Input force moves machine- input distance
• Output Force: force machine exerts on object
• Machine exerts a force- output distance
• Input Work = Input Force X Input Distance
• Output work is never greater than Input work
• Mechanical Advantage = Output Force

Input Force

• Mechanical Advantage the number of times a machines increases a force exerted on it
• Increase Force: M.A greater than 1
• You input 10 N on a can opener
• Can opener outputs 30 N on the can
• Increase Distance : M.A less than 1
• You input 20 N on a stress ball
• Stress ball outputs 10 N on your hand
• Changing Direction: M.A always equal to 1
• Input force is the same as output force
Efficiency of Machines

X 100%

• Efficiency: compares output & input
• Efficiency = Output Work

Input Work

• Friction decreases the efficiency of machines
• Think about old rusty scissors!
• Efficiency of machines, always less than 100%
• Practice Time!
• Your sweet dad asks you to mow the lawn and pulls the worst lookin’ mower out the garage. I mean this thing was made in 1875. Your input is 250,000 J and the work done by the mower is 100,000 J. How efficient is this machine?
Simple Machines: Inclined Plane
• A flat, sloped surface… aka ramp
• Exert input force over a longer distance
• Input force- pushing or pulling object
• Output force- lifting object without inclined plane
• Input far less than output
• Ideal Mechanical Advantage = Length of incline

Height of incline

Simple Machines: Wedge
• Thick at one end, gradually goes to a thin edge
• Literally moving the inclined plane
• Ideal Mechanical Advantage = Length of wedge

Width of wedge

• Longer, thinner the wedge, greater M.A
• Input Force splits into two output forces
• Examples: knife, zipper, axe, sharpener, mouth
Simple Machines: Screws
• Inclined plane wrapped around cylinder -“spiral”
• Threads on a screw act like an incline plane to increase distance over which force is exerted
• Screw exerts an outward force on the wood
• Closer the threads, greater M.A
• Calculating M.A = Length around threads

Length around screw

• Examples: screws, jar lids, light bulb
Simple Machines: Levers
• Bar that is free to pivot or rotate

on a fixed point

• Fixed point that a lever pivots around: Fulcrum
• Three Classes of Levers:
• 1.) 1st Class- always change direction of input force
• Scissors, pliers, seesaws, paint can opener, lifting neck
• 2.) 2nd Class- increase force, no change direction
• Wheelbarrow, doors, nutcrackers, bottle openers, walking
• 3.) 3rd Class- increase distance, no change force
• hockey stick, fishing pole, shovel, baseball bat, flexing
Simple Machines: Wheel & Axle
• Two circular objects fastened together

that rotate around a common axis

• Object with larger radius – Wheel
• Object with smaller radius - Axle
• Greater the ratio between the radius of the wheel and the radius of the axel- Greater M.A

• Examples: screwdriver, doorknob, fairy boat
Simple Machines: Pulleys
• Grooved wheel with a rope or cable wrapped around it
• Fixed Pulley: attached to a structure
• Top of flagpole
• Moveable Pulley: attached to moving object
• Construction Cranes
• Block & Tackle: combination of fixed & moveable
• Mechanical advantage is equal to the number of sections of rope that supports the object
Compound Machines
• Compound machines: use two or more simple machines
• Handle- Wheel & Axel
• Screw- also part of axel
• Wedge- Peels Skin
• Lever- suction cup