ShmoopTube

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All right well here's your shmoop du jour brought to you by hoops. Back in

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the day kids used to play a game by making a hoop roll along the ground by hitting [Child hitting a hoop along the ground]

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it with a stick and we're really happy we didn't live back in the day.. it was really boring... [Man uses his mobile phone]

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A 60kg ball is dropped from a platform, passes through a hoop and then

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it hits the ground. If the platform is 30 meters above the ground and the hoop is

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10 meters above the ground about how fast is the ball falling when it passes

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through the hoop? Our choices are 10,15, 20, 25 meters a second. All right well

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there are different ways to figure this question out, we could use kinematics for

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example or we could actually build this whole set up and break out a radar gun [Picture of a tower that has a hoop attached to the front]

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any policeman around?.. Well that sounds like a lot of work all that including

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the cops. But just having a ball through a hoop isn't that exciting, [Speed gun measures how fast the ball falls through the hoop]

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instead of just a plain old ball let's make this about a high diver and let's

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set that hoop on fire, all right now our diver has to fall through the flaming [Diver appears at the top of the tower and the hoop is now on fire]

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hoop of doom and jump into this tiny pool of water. There you go that's better...

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We'll model this divers motion using conservation of energy, while she's up on

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the platform preparing for her death-defying leap the diver only has [A person bungee jumps from a bridge]

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potential energy, with that good old equation mass times gravity times height.

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When we plug in the numbers and do the math we find the potential energy to be

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18,000 joules. Now once she's actually falling and

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reaches the height of the hoop we can look at a combination of kinetic and [Diver jumps at stops at the flaming hoop]

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potential energy, well the total energy aka mechanical energy still has to

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equal 18 thousand joules. Figure out the potential energy slice of this pie

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first. We're using the same equation and the same numbers except the height has [People take slices of pizza]

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changed so now the potential energy is 6,000 joules, we subtract that number

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from the mechanical energy to find the kinetic energy and we get 12,000 joules.

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Well kinetic energy equals one-half mass times the square of velocity, we know the

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mass and we know the kinetic energy so we can just solve for velocity. Multiply

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each side by 2 and then divide each side by 60 and find that V squared equals 400

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meters a second and when we take the square root of that 400 meters a second

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we end up with a velocity of 20 meters a second also known as

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answer C. When we're dealing with falling objects like this we can use the

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potential energy as a starting point to solve all sorts of variables. We just [Man dodges big boulders falling from above him]

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have to remember how to calculate it and how it relates to mechanical energy and

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kinetic energy. Now we're going to go play with our new favorite app it's

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called 'hoopster', you control a kid from way back who's rolling a hoop along the

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ground with a stick, yeah isn't modern technology great... [Some playing the 'hoopster' game on their phone]

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