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AP Physics 1 Videos 86 videos

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AP Physics 1: 1.4 Waves. Which of the following is technically true for Max as he stands at the edge of oblivion? 

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AP Physics 1: 2.4 Changes and Conservation Laws
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AP Physics 1: 2.4 Changes and Conservation Laws. Which of the following circuits should the students use?

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AP Physics 1: 3.4 Object Interaction and Forces 194 Views


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Description:

Sure, we can calculate force, as long as we get to stay safely away from the giant death trap elevator. You have fun up there, though.

Language:
English Language

Transcript

00:03

Alright here's your shmoop du jour brought to you by skyscrapers their [Skyscrapers in Chicago city]

00:06

impressive feats of engineering and we love to look at them you know with our

00:09

feet planted firmly on the ground the Willis Tower in Chicago is the tallest

00:13

building in the U.S. at 527 meters the 1,700 feet to reach the top floors [Willis Tower shown to be 1,729 feet]

00:21

tourists take a one-minute trip up an elevator that quickly accelerates to a

00:26

high speed and intrepid schmooper weighs himself before getting on the elevator [Shmoop employee weighing himself before getting on the elevator]

00:30

and finds that the scale reads 500 n Newtons as the elevator begins its ascent

00:36

at constant acceleration he sees that the scale reads 950 Newton if the [Scale reading 950 N]

00:43

elevator descends with a constant acceleration equal to 1/2 the upward

00:49

acceleration what will the scale read on the ride

00:52

down and here are the potential answers... ok well we're dealing with three

00:58

separate forces here we have the normal force before we're even on the elevator [Luke Skywalker fighting Darth Vader]

01:02

the force when we're moving up in the elevator and the force when we're going

01:06

down all right well let's deal with the normal force first when we're on the

01:10

ground gravity pushes us down and the floor pushes us up these two forces [Gravity pushing down and the floor pushing up on a woman]

01:14

balance each other out.. if they didn't we'd float above the floor or crash

01:19

through it'd be kind of cool we're happy to report that the Willis Tower

01:22

has very sturdy floors that push back against gravity and we don't see anyone [Man stood in a glass window looking down from a skyscraper]

01:27

floating in Chicago so it appears the forces balance out so all this means

01:31

that the normal force minus mass times gravity equals zero well we're in luck

01:36

because we know the amount of force pushing down on the schmooper is 500 [White ball lands on 29 black on a roulette wheel]

01:40

Newtons and we can calculate his mass since mass times gravity equals Newtons

01:45

we can rearrange the equation so we have mass equals Newton's divided by gravity

01:50

for our sake here we'll also round gravity up to ten meters a second

01:55

squared when we plug in the numbers we find that his mass is 50 kilograms now [Re-arranged equation to find the mass]

02:00

when the elevator starts going up the force of gravity is still balanced out

02:03

by the force of the elevator floor but this time instead of equaling zero the [Man going up an elevator in the tower]

02:08

forces interact to equal mass times acceleration upwards all right well now

02:13

we can solve for upward acceleration all we have to do is subtract mass times

02:18

gravity from Newtons on the upward ride and divide that by mass okay some quick

02:23

calculations and we'll find that acceleration upward is nine meters per [Equation for acceleration upward shown as 9 meters per second squared]

02:27

second squared now we know that when the elevator starts slowing down the downward

02:32

acceleration is half of the upward acceleration well the downward force

02:36

equals negative mass times acceleration and downward acceleration equals

02:42

one-half of upward acceleration so the downward force minus mass times gravity

02:47

equals negative mass times half of upward acceleration all right we're

02:52

almost there I promise now we solve for the downward force which again equals [Woman waving her finger]

02:56

mass times gravity minus mass times one-half upward acceleration well we

03:01

multiply 50 kilograms by five point five meters per second squared and we get an

03:05

answer of 275 Newtons so the correct answer is option C let's quickly touch

03:11

on the other answer choices we could have knocked a and D out of the running [Boy on his phone gesturing at his laptop screen]

03:14

right away we know that there's a downward force in action here since

03:18

gravity never stops so there's no way a is correct and we know that the force [Young girls jumping up and down]

03:22

during the slowdown will be less than the force standing in place on the

03:27

ground so D would never work either and we also know that next time we go to [Man attempts to float in mid-air and falls down]

03:31

Chicago we'll stick to ground level attractions a super tall building in a

03:35

place called the windy city, yeah we don't even like to climb step ladders [Man looking up at Willis Tower with a step ladder]

03:39

and look out below....whoa boy!

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