# AP Physics 1: 3.4 Object Interaction and Forces

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.

AP Physics 1 | Object Interaction and Forces |

Language | English Language |

### Transcript

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

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

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

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

acceleration what will the scale read on the ride

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

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

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

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

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]

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

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

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]

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

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

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

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

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

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

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

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

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

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

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

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

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

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

acceleration is half of the upward acceleration well the downward force

equals negative mass times acceleration and downward acceleration equals

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

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

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

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

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

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

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]

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

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

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

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]

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

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]

and look out below....whoa boy!