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 inches or .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 timeVarious 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 upThe cooler rock gets pushed aside and sinks downSlow 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

Speed vs. Time

0

2

4

6

8

10

12

0 1 2 3 4 5 6

Time (s)

Spee

d (m

/s)

What can we tell from this

graph?

Increasing Speed

Constant Acceleration

Graphing Acceleration

Distance vs. Time

0

5

10

15

20

25

30

0 1 2 3 4 5 6

Time (s)

Dist

ance

(m)

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 arrowsArrows point in the direction of the forceLength of arrow tells strength- Longer = Bigger F

Combining Forces Net Force: combination of all forces acting

on an object Determines if an object movesDetermines which direction an object moves

=5 N5 N

=5 N 5 N10 N

=5 N 5 N

0

10 N

Unbalanced & Balanced Forces Unbalanced Force: a net force acting on an object

causing it to start or stop moving or change directionCauses 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

=

10 N

=5 N5 N

=5 N 5 N10 N

5 N 5 N0=

5 N10 N

5 N 5 N0

5 N

=

5 N10 N

5 N 5 N0

5 N

=

5 N10 N

5 N 5 N0

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 restMoving a Desk & Body Builders moving cars

Sliding: two solid surfaces slide over each otherSand 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 SAMEWeight- 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 unbalancedObjects 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

Momentum & Collisions

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 forceCarrying 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 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 Mechanical advantage of 3

Increase Distance : M.A less than 1 You input 20 N on a stress ball Stress ball outputs 10 N on your hand Mechanical advantage of 0.5

Changing Direction: M.A always equal to 1 Input force is the same as output force

Efficiency of Machines 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?

X 100%

Simple Machines: Inclined Plane A flat, sloped surface… aka ramp Exert input force over a longer distance

Input force- pushing or pulling objectOutput 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 – WheelObject with smaller radius - Axle

Greater the ratio between the radius of the wheel and the radius of the axel- Greater M.A

Mechanical Advantage = Radius of Wheel

Radius of Axel 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