7 th Grade CMT review

8.1

Describing Motion

Motion – change in position over time

Reference point – an object against which position or motion is measured

Position – an object’s location in relation to a reference point

Speed – distance an object moves per given unit of time

Instantaneous speed – the speed at any given moment

Average speed – the total distance divided by the total time

Total distanceAverage speed = Total time

= 100 m = 10 m/s 10 s

The graph compares the motion of two cars. Each car is traveling at a constant speed, but the speeds of the two cars are different.

Car one took 60 minutes to travel 80 km. Car 2 it took 60 minutes to travel 50 km. The line for car 1 is steeper because it moved at a greater speed.

The graph shows that the cat walked 20 meters in 40 seconds, rested for 20 seconds, and then ran 60 meters in 40 seconds. The line is horizontal from 40 to 60 seconds. This is when the cat was not moving. The cat moved faster after it rested because the line is steeper. To find the average speed divide the total distance by the total time. (80 meters) divided by (100) seconds. Average speed was .8 meters per second.

Direction – path that a moving object follows (ex. south or east)

Forces and Motion

Force – push or pull

Net force – total of the forces action on an object

Forces can act in the same direction or in different directions. Forces can be balanced or unbalanced

Balanced forces – cancel each other out so the net force is zero. The do not cause motion. Ex. When the rope does not move during a tug of war game.

Unbalanced forces – results in a net force greater than zero. They can set an object in motion. Ex. During a tug of war the rope will move in the direction of the greater force

Newton’s laws of motion

1st law of motion – an object at rest tends to stay at rest and an object in motion will stay in motion moving at the same speed and direction unless acted upon by a net force

o Inertia – tendency of an object to resist any change in motion. Inertia is related to the mass or amount of matter in an object. The greater the mass the greater the inertia

2nd law of motion – force needed to chance an object’s motion depends on two things mass and acceleration.

o Acceleration - rate of change in speed, direction or both (can speed up, slow down, change direction or both)

o Force = mass X acceleration o The greater the force acting on an object the more it will accelerate. The greater the mass

the less it will accelerate. 3rd law of motion – for every action, there is an equal and opposite reaction.

o When one object exerts a force on a second object, the second object exerts and equal and opposite force on the first object

o Ex. Fish pushes on the water and the water pushes the fish forward with equal force

Friction, Gravity and Circular Motion

Friction – is a force that acts between two surfaces that are in contact with each other. Friction opposes motion . Ex. When you slide a book across a table friction slows it down.

o Friction acts between surface that are in contact even when there is no motiono You need to overcome the force of friction to set an object in motion

Air resistance – an object moves through the air and its surface collides with air molecules

Gravity – the force that attracts all objects toward each othero Gravity is a product of mass and because all objects have mass they are all attracted toward each

othero The greater the mass of an object the greater the gravitational forceo Most masses of objects are too small to overcome inertiao Earth has a large enough mass to overcome inertia o Pull of Earth’s gravity keeps you and everything else on the planet from flying off into spaceo Mass of the sun is much greater than Earth but it does not pull on you with as much force because

it is farther awayo Gravitational forces get smaller as the distance between them increaseso The attractive forces between any two objects depends both on their masse and the

distance separating them o IF there was no atmosphere all objects dropped would fall at the same time, however air resistance

acts against the pull of gravityo Effect of air resistance depends on its weight, shape and surface area. Larger surface areas

increase air resistance.

Circular Motion – movement in a circle or circular path

o Basketball spinning on your finger or a figure skater moving in a circle o An object moving in a circle is always accelerating because it is changing directiono A moving objects inertia keeps it moving in a straight path

Centripetal force – net force that acts toward the center of a moving objects’ circular path. Without the acceleration caused by the center-seeking force, an object would not move in a circle

o Gravity can provide a centripetal force; gravity keeps Earth in its orbit around the sun and the moon in its orbit around the Earth

o The moon’s inertia makes it tend to travel in a straight line, at the same time Earth’s gravity pulls the moon toward the center of the planet. The net force keeps the moon moving in a circular path

o Centripetal force is not always gravitational. A ball can be attached to a string and swung in a circular path. The tension on the string provides the centripetal force.

7.1Work and Simple Machines

Work – force moving an object over a distance in the direction of the force

Simple machine – device that makes work easier by changing the size or direction, or both. It does not help you do less work, just makes the work easier for you to accomplish.

Work = force x distance

Unit of force is the newton (N), and unit of work is the joule (J)

Inclined plane – a straight slanted surface. o Ex. Ramp o It is easier to push an object up a ramp than life the same object straight up the same

heighto Increasing the distance decrease the force need to do the same amount of work

Wedges and Screws

Wedge – two inclined planes placed back-to back, transfers force in one direction in two directionso Tools used to split things aparto Knife, blades, axes

Screw – inclined plane that is wrapped around a cylindero You apply a small force over the long distance of the screw’s threads

Levers – simple machine made up of a bar that pivots at a fixed point called a fulcrum. o The force applied to a lever is called the effort, the object moved is the load. o Levers are classified into three groups based upon the location of the fulcrum, effort and loado First class lever - such as a seesaw, the fulcrum is between the effort and loado Second class lever - such as a nutcracker the load is between the fulcrum and the efforto Third class lever – such as your arm the effort is applied between the fulcrum (elbow) and the load

(hand)

Wheel and axle – simple machine that consists of two circular objects of different sizes. o Wheel is always larger than the axleo When effort is applied to move the wheel the axle turns a shorter distance but

moves with more forceo Ex. Door knob, gears in machinery

Pulley – consists of a rope or chain wrapped around a wheel. o Load is attached to one end of the ropeo Force is applied to the other end of the rope

Single fixed pulley – changes the direction of the effort force, when you pull down the load moves up. It changes the direction of the force

Pulley System – changes the direction of the force and increases the distance over which the force is applied

Compound Machines – made up of two or more simple machine

Ex. Scissors: made of wedges and levers attached at a fulcrum Meat grinder – lever that turns a wheel and axle that is attached to a screw

Mechanical advantage – calculation of how much a machine multiplies force

Output force Mechanical advantage = Input force

The input force, usually given in Newtons (N), is the force applied by the user on the machine. The output force is the force applied by the machine to the object being worked on.

Ex. A student uses a force of 30 N to push a 300 N box up a long ramp onto a platform. What is the mechanical advantage of the ramp?

MA = 300 N 30 N MA = 10

When a machine is used to do work some of the input work is lost due to friction. Therefore, the work input is always greater than the work output.

Potential and Kinetic Energy

Energy – the ability to do work

Energy can be classified into two broad categories Potential energy Kinetic energy

Potential energy (PE) is energy an object has because of its position; it is stored energy – it has the potential to do work.

Ex. a rock at the edge of a cliff It is measured in joules (J)’ Near Earth you can calculate an objects gravitational potential energy (GPE) GPE = mgh (m=mass in kilograms, g is acceleration due to gravity (9.8m/s2) and h is the height in

meters

Kinetic energy (KE) – the energy of motion; depends on the object’s mass and its speed KE = ½ mv2

m is mass in kilograms and v is speed in meters per second

Potential energy can change to kinetic energy and vice versaEx. A roller coaster has the most potential energy at the top of a hill as the car rolls down the hill its potential energy changes to kinetic energy. At the bottom of the hill it has mostly kinetic energy. The kinetic energy pushes the car up the next hill and it changes back to potential energy.

Energy Transformations

There are many different forms of energy and each kind can be transformed into other kinds.

Chemical energy inside batteries can make a toy or toothbrush move

When you eat you take in chemical energy (food) which gets changed to mechanical energy to make your muscles move. Some is changed to electrical signals which your nervous system uses to send signals throughout your body.

A lamp transforms electrical energy to into light and heat

7.3Earth’s Structure

Crust – outer most layer made of light and brittle rock;

thinnest layer about 5 km to 100 km thick. Only 1% of Earth’s mass Main elements: oxygen, silicon, aluminum Two kinds of crust: continental and oceanic Continental crust: lighter, older and thicker part of

the crust which makes up continents Oceanic crust: rock of the ocean floor; nearly

twice as much iron, magnesium and calcium as continental crust which makes it much denser

Mantle – layer of Earth beneath the crust

Makes up 67% of Earth’s mass Composed of oxygen, silicon, magnesium, iron and aluminum Uppermost part is solid Heat and pressure make the middle section a very thick liquid Lowest section is solid even though it is hotter than the middle layers because the pressure is so great it

keeps the rock from melting Magma – melted rock below Earth’s surface Lava – molten rock or magma that has reached Earth’ surface . (rock rises from mantel and flows onto

surface from active volcanoes

Lithosphere – the crust and rigid upper part of the mantle

It is divided into giant slow-moving chunks of rock called tectonic plates It is like a giant jigsaw puzzle

Asthenosphere – soft layer within the mantle that flow s like a thick liquid in which the tectonic plates float on

Semi molten Dense Convection currents cause the lithosphere to be in constant motion Heat from Earth’s core cause the liquid rock in the mantle to expand, when it expands it becomes less

dense and rises When magma gets closer to the surface it cools and becomes denser and sinks The rising currents push plates apart, and the sinking currents pull tectonic plates together

Core – innermost layer

makes up 33 % of Earth’s mass mostly iron and nickel contains some oxygen and other elements inner core – pressure keeps it solid outer core – liquid

Plate Motion and Earth’s Changing Surface

Folds – occurs when the rocky layers are squeezed together and pushed upward highest mountains are folded mountains

Form where tectonic plates collide

Plate boundary – region where two tectonic plates meet

Continental-continental plate boundary – two plates made of continental crust collide , buckle and thicken and is then pushed up forming folded mountains

Ex. Himalayas, Alps, Ural Mountains

Oceanic –oceanic boundaries – two plates made of oceanic crust collide

One plate is sub ducted or pushed under the other Deep-sea trench – forms under the ocean where plates meet Sub ducted plates sink and melts from hot mantle and forms magma Volcano –opening in Earth’s surface where magma is released Magma will rise and erupt as lava, cool and harden The material that erupt the more visible the volcano; they become volcanic islands; ex. Aleutian Islands in

Alaska

Continental-Oceanic boundary – oceanic plates slide under the continental plates

Chains of volcanic mountains can form on the edge of continental plate

Earthquake – shaking that occurs when rock in Earth’s crust breaks and quickly releases pressure

Pressure can build up as plates press together (compression) or slide past each other (shearing) or stretch and pull away from each other (tension).

Happen along plate boundaries where faults are located

Seismic wave – a wave of energy that travels away from the center of an earthquake in all directions

Fault – break or crack in the Earth’s surface along which movement has occurred

Fault –Block Mountain – can be produced when earthquakes quickly release pressure and create faults.

Ex. Sierra Nevada in California

Mountains and volcanoes can form at plate boundaries where plates push together or pull apart, they do not occur where plates slide past each other sideways

Earthquakes can occur at any plate boundary, but are common where plates slide by each other; they are rare in the middle of plates but can occur when cracks form in the middle of plates.

Ring of Fire – runs along the boundary of the Pacific Plate; it is known as the zone of frequent volcanic eruptions and Earthquakes ; contains more than 75% of all volcanoes on Earth.

Weathering and Erosion

Landforms – geological features on Earth’s surface

Weathering – process in which rock at Earth’s surface is broken down into smaller pieces

Mechanical weather – breaks rocks into smaller pieces by physical means without changing chemical composition,

Water, ice, changing temperatures, wind, gravity, organisms and glaciers Ex. Water can seep into cracks and rocks and freeze, when it freezes it expands and

pushes on the rock. When it melts pressure decreases. The buildup of pressure splits the rock

Plants roots and burrowing animals also exert pressure on rocks and breaks them apart Abrasion – rock, sand and soil particles carried by wind or water that can rub against rock

and slowly wear it away Glaciers – huge masses of slowly moving ice that cause abrasions, as they move across

Earth’s surface they pick up and carry weathered rock material which scratches and wears down rock surfaces over which the glacier flows

Chemical weathering – breaks rocks down into smaller pieces through chemical reactions. Changes rock into one or more new substances.

Eats away at rocks Oxygen combines with iron in rock and soil to form iron oxide or rust which weakens the

rock and makes it break down Rain mixes with carbon dioxide in the air to form carbonic acid in rain which eats away at

soft rocks like limestone. Organisms can break down rocks; ex. Lichens grow on rocks and produce acids which

break down the rocks Decaying organisms make acids that eat away at rock

Erosion – process that picks up and moves pieces of rock and soil

Together weathering and erosion wear down Earth’s surface

Wind, moving water and glaciers are agents of erosion Gravity also moves sediments Ocean waves pound against shorelines which can remove sand and wear away at beaches or cliffs Rivers remove rock and soil from their channels and carve out river valleys which are V-shaped, some can

form deep canyons. Ex. Grand Canyon - carved by the Colorado river Winds erodes by lifting sand and blowing it away Glaciers erode rock and soil as they scrape Earth’s surface; will deepen valleys into a U shape

Deposition –drop or deposition of eroded sediment

constructive process that when wind or moving water slows down or stop and when the glacial ice melts they drop the sediment

water in rivers deposits huge amounts of sediment, rain washes sediment off the land into rivers which carry it down stream, when the current slows down it can’t carry as much sediment and drops their sediment

Rivers deposit sediment outside their banks to form flood plains Floodplains are good farmland CT’s coastline is a result of glaciers

8.4Bridges

Bridges must hold themselves up, bear the weight of moving vehicles and withstand wind and other weather. There are destructive forces that act on bridges

Gravity is the most important – bridges are suspended in the air so they must resist this pull Compressions – force that pushes together or shortens the thing acting on it. If a bridge was not designed to withstand compression, it would shortened until it buckled inwards Tension – expands or lengthens the thing it is acting on it; a bridge that could not withstand the tension

acting on it would eventually snap If a bridge is designed properly it balances the opposing forces of compression and tension so the

net force is zero

Beam Bridge – horizontal structure resting on columns or piers

Simplest type of bridge When cars, trucks or other loads cross the bridge, they drive on top of the structure; the load compresses

and shortens the top part of the beam, this causes the bottom part of the structure to lengthen and undergo tension. The vertical columns that support the bridge also are compressed

Easy and inexpensive to build If the load is too heavy the bridge may sink or bend in the middle Cannot be very long (no more than 60 meters) The longer a beam bridge, the weaker its supports become If the beam bridge was too long weak columns or piers could cause it to collapse

Truss Bridge – a bridge that consists of a horizontal structure with a lattice of ridged supports

Removes compression from the horizontal structure that carries traffic across the bridge Compression force caused by loads is applied to the top of the truss system instead Can be made longer than beam bridge; but are still limited in length by their design Truss system above the bridge can be heavy; the longer the bridge the larger and heavier the truss needs to

be If the truss was too long its weight would be too great for the trusses to support

Suspension Bridge – consists of a series of cables hung from a vertical towers

Cables support the horizontal platform that carriers traffic across the bridge Roadway is not affected by compression or tension Towers of a suspension bridge absorb all the compression acting on the bridge Since they are firmly placed on the ground most of the compression transfers to the ground Cables and endpoints anchor the bridge to land which absorbs all the tension acting on the bridge Major advantage is how well they balance and resolve the forces of compression and tension Suspension bridges can be made longer than any other kind of bridge; as much as 2000 meters Disadvantage is the roadways lack of stability; horizontal platform tends to sway and ripple – to solve this

most suspension bridges have truss systems below the horizontal platforms this stiffens the roadway and prevents most of the unwanted motion

Designing Bridges

Many engineers use computer-aided design (CAD) to develop bridges CAD allows engineers to create, test, and revise designs before starting expensive construction. Testing helps engineer design bridges that fit the needs of each project, including cost, construction time,

safety and capacity Computers help engineers simulate how a bridge will react to different stresses so they can make the bridge

strong and safe