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What goes up must come down: acceleration and the pull of gravity no

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As you've probably noticed gravity is a constant we reminded of this every time we trip on our shoelaces if it

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wasn't for gravity we would just fly into the air when this happened which [Man flying into the air]

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would actually be pretty cool we'd leave our shoelaces untied all the time if it

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meant we could defy gravity even just a little bit

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well hello people I'm Isaac Newton without the annoying British accent and

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I invented gravity okay okay it was actually just a theory that explains how

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gravity worked and basically I figured out that everything that has mass

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attracts everything else that has mass or to put it another way everything [Man and woman swining around]

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attracts every other thing here on earth the biggest mass we've got is this the

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globe that's why everything falls down toward

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the center of the earth earth is the you know biggest thing around but what if

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you're not standing on flat ground well let's say you're taking a hike up a hill [Woman hiking up a hill]

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gravity is still pulling you toward the center of the Earth's core and since

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you're on an angle on the hill this force isn't perpendicular to the ground

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it's at an angle to the surface you're standing on the steeper the hill the

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more acute that angle is but because we're on an incline that gravity is

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broken into two components the first component is perpendicular to the

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surface 90 degrees relative to the trail so that

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component is pulling us straight down just like the gravity on a flat piece of [Gravity pulling woman towards earth's center]

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ground well another component of gravity is pulling us parallel to the surface

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which you're familiar with if you've ever rolled down a hill our lessons [Woman rolling down a hill]

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lately have been all about acceleration why are we talking about rolling down a

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hill well because gravity is a force of

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acceleration it might take it for granted because it's always around you

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might say it's constant because it is here on earth gravity is a constant

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force of 9.8 meters per second squared now we can overcome that force by [Rocket appears]

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providing a stronger force in the other direction like when a rocket launches

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into space to get off the ground the rocket engines have to provide enough

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force to achieve liftoff and it's the same thing when you throw a ball in the [Man throws ball in the air]

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air your arm creates enough force for the ball to go upwards at least for a

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little while it'll come back to earth eventually though when velocity and

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acceleration are in opposite directions the velocity will eventually change and

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turn in the direction of acceleration of course if velocity is strong an

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acceleration is weak it might take a while to make that turn but if they're

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both working in the same direction well unless another force acts on whatever

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object is in motion the velocity will just keep on getting higher let's take a

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look at how these forces interact well on this chart longer arrows indicate

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greater magnitude than shorter ones and this is all about predicting the future [Velocity, acceleration and future velocity columns appear]

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at least in the short term like we said over time acceleration will always

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overcome velocity but as we can see with this set in the short term if velocity

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is strong and acceleration is weak and they're acting in opposition while

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velocity will be weakened let's look at how this plays out in a realistic

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situation well the situation is that we're going to shoot a guy into the air [Newton puts a man into a cannon]

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with a cannon look you have your reality and I have

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mine the initial velocity will be 50 meters a second and we'll be pointing the

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cannon straight up here's a graph of the guys displacement as he takes flight [Displacement graph appears]

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well what does this graph tell us about the velocity well for one thing we know

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he didn't have a constant velocity otherwise the line would be straight to

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make a velocity time graph we need the slopes of the tangents to the

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displacement graph we've already done that math and come up with this velocity

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versus time graph right there remember if we see a linear graph for velocity it

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means acceleration is constant acceleration is the change in velocity

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divided by the change in time well the final velocity here was negative 48

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meters per second and the initial velocity was 50 meters per second making

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the change in velocity negative 98 metres per second the time elapsed was

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10 seconds so when we divide negative 98 m/s by 10 seconds we find an [Full equations appears]

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acceleration of negative 9.8 meters per second squared magic see the

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acceleration of gravity is constant gravity would never let us down well

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true it actually always lets us down anyway everything so far has been [Man falling to the ground and Newton catches him]

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theoretical I hate to let you down but we didn't actually launch some guy out

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of a cannon all right well why don't we do some actual science that's right it's

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lab time all right let's get our stuff together first we need a ball have a

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ball tennis base golf any of those will do just fine [Newton with a selection of balls]

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we need a way to measure height we can tape some paper to the wall and mark

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measurements here or you can use an app here's one for the iPhone and here's one

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for you Android users and we'll need a stopwatch which you might have on your

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phone we'll be taking notes so you'll need paper on your computer or whatever

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and we'll trust you can figure something out and last we'll also need to make

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graphs well there's software for that and

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there's old-school graph paper on three hit pause and go scavenge one two three [Newton with all of the items for experiment]

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go.... all right got everything? awesome ish ideally this will be done as a team

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we'll need someone who's throwing the ball and someone who's taking and [Girl with the ball and boy with stop watch]

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recording the data so we need to get organized we need a grid that lets us

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track the height of each toss and the time between letting the ball go and

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catching it again and we'll eventually be figuring out the velocities for each

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toss so include a column for that too and take a few practice throws make sure [Girl throwing ball into the air]

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you can get a good read on how high the ball is going and you want to be as

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consistent as possible in these tosses we don't want one throw to go two meters

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high and another to go 12 high if for no other reason and when our ceiling isn't

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that high and we want to throw the ball as straight up as we can don't worry

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we're just human so we know there's gonna be some degree of error here...

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Once you've gotten that toss down we can start doing this thing for real

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we need a minimum of three good tosses and if you've got a big group together [Girl tossing ball into air and group watches]

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for this lab well take some more tries at it let everyone collect some of their

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own data you can never have too much data ideally you'll be catching the ball

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at the same height it left your hand otherwise whoever is working the clock

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should try and stop timing when the ball returns to that same height if you make

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a bad toss don't worry about it just don't include the data in that toss in

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your chart we're gonna be doing some graphing now you know we're pretty

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stoked too, can you tell? any graph we make needs a starting point well for a

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displacement graph we can have a starting point at zero since the ball is [Displacement graph appears]

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coming back to the same position it started from but what about velocity

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well when we've seen velocity graphs before

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they've usually started with an initial velocity of zero but the ball is

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already moving when you let it go so we got to figure out what the initial

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velocity is all right good news we've got an equation for that here it is in [Initial velocity equation appears]

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this equation the change in displacement which is that Delta X thing right there

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equals the initial velocity times the elapsed time that's the V sub zero thing

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and the T there plus one half acceleration aka A multiplied by the

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square of the time period that's the equation we know our change in

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displacement equals zero so we can put that into our equation and now we can

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solve for initial velocity let's start by subtracting the last part of our

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equation one half A T squared from each side leaving us with negative one half A

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T squared equals V sub 0 times T now we can divide both sides by T to isolate

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initial velocity so the initial velocity equals the negative of one half

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acceleration times T our acceleration is negative 9.8 meters per second squared

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and cutting that in half gives us negative four point nine meters per

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second squared but we need the negative of that number so now we've got a [Acceleration formula appears]

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positive number of 4.9 and since time will be in units of seconds we know that

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will cancel out one of those seconds in meters per second squared and we'll have

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the right unit for velocity so our starting velocity equals 4.9 times T

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well it's important to remember that once the ball leaves our hand gravity is [Girl throws ball into the air]

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the only force that's being applied to it even though it's moving up and

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doesn't last very long so when we do our graphs for the data we collected we can

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be confident that the acceleration versus time graph will be flat which

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means the velocity versus time graph will be linear

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now let's do the graphs for each throw we made oh but before we do let's think

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of one more thing if we have our velocity correct and we have our time

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period correct we can calculate the max displacement let's take another [Displacement graph appears]

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look at our graph for cannon guy well we can see that this looks pretty

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symmetrical right and the highest point with the most displacement is halfway

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through the journey there since velocity equals the change in displacement over

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the change in time we can rearrange the equation to solve for displacement the

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change in displacement equals the velocity times the time halfway through

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the ball's whole journey but this is all in theory go ahead and find the

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theoretical max height for some of your throws and compare that to what you

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actually measure is the max height which do you think is more accurate do you [Recorded height and theoretical max height readings appear]

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think you measured the height wrong did you not get the time quite right

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throwing off the velocity again don't expect yourself to get any of this

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perfect it's more important to be able to think through any errors we've made

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okay for reals now put that pause button to use and get graphing compare your

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graphs for each throw do they look like what you expected if you worked as a

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team do your graphs look like those your colleagues drew and while we're thinking [Colleagues holding graphs]

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about this stuff let's think about some other stuff too what determines how long

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the ball is in the air what's the acceleration when the ball is stopped at

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the peak of its arc and what do we have to eat around here [Tennis ball transforms into an apple]

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okay well that last ones just means we're getting hungry so let's wrap this

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thing up but yeah it's important to exert some

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brainpower on these questions there's no point in doing an experiment and taking

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down data if we don't understand what it means and if you're feeling super

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ambitious write up a whole report but for now it's snack time, oh an apple for

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Isaac Newton yeah very funny [Boy gives Newton an apple]

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