# AP Physics 1: 2.2 Changes and Conservation Laws

And now, the moment you've all been waiting for: a stuntwoman refuses to do a stunt and does math problems instead! (The crowd goes wild).

AP Physics 1 | Changes and Conservation Laws |

Language | English Language |

Science Practice 2 | Using math appropriately |

### Transcript

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

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

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

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]

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

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]

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

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]

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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