Register to reply 
Free fall question, where has my reasoning gone wrong? 
Share this thread: 
#55
Oct309, 08:36 AM

P: 218

Could you give me an example? if you say something like "the bolt fell 2 feet into the elevator" then how much will it have fallen from the perspective of the shaft? it would still be a fall of 2 feet wouldn't it? thanks 


#56
Oct309, 08:50 AM

Mentor
P: 41,465




#57
Oct309, 08:57 AM

P: 218

thanks 


#58
Oct309, 09:01 AM

Mentor
P: 41,465




#59
Oct309, 09:05 AM

P: 218

9 + 8t 16t^2 = 8t + 2t^2 and then I got sqrt(9/18) which is 0.71 seconds. then I plugged 0.71 into 9+8t  16t^2 to see the new position it fell to.. then I took that position (6.680 feet) and subtracted it from the original 9 feet to get the distance fallen. I got 2.3 ft, but that led me the question about whether it mattered where I took the distance from and if it even mattered that I took the distance fallen from 9 feet in the air or 25 feet in the air  since I couldn't find the position of the bolt in the elevator shaft. but say, if I found out that the bolt was 15 feet in the air from the elevator shaft, the distance fallen from 9 to 6.680 would be the same as the distance fallen from 15 feet to 12.7 feet. thanks 


#60
Oct309, 09:26 AM

Mentor
P: 41,465

Note that if you assumed that the elevator floor was initially at y0 = 25 ft and thus the bolt was at y0 = 25+9 = 34 ft, that nothing would change. The arbitrary 25ft would just drop out of the equation when you solved for the time or the change in position. But all of your position measurements are with respect to the shaft. 


#61
Oct309, 10:39 AM

P: 218

Just to clear something up though, wouldn't x0 = 9 and x1 = 0 be in respect to the elevator floor? Since I know that the bolt is 9 feet above the elevator floor, so 0 would be the elevator floor. On the other hand, in respect to the elevator shaft would be the distance of the bolt above the ground of the elevator shaft? and how could I have measured it differently in terms of the elevator floor? I'm just not sure how the answer would've been different if I took it with respect to the elevator floor. I know that the position of the bolt after falling is in respect to the elevator floor, since the measurement of 9 feet is in respect to how high the bolt is above the elevator floor. thanks 


#62
Oct309, 11:01 AM

Mentor
P: 41,465

What tells me that you're measuring position with respect to the shaft are the expressions y = 9 +8t 16t^2 (for the bolt) and y = 8t + 2t^2 (for the elevator floor). 


#63
Oct309, 11:19 AM

P: 218

if you don't mind, could you show me an equation like this in respect to the elevator floor, so I can tell the difference in the future? thanks 


#64
Oct309, 11:40 AM

Mentor
P: 41,465

y = 9 18t^2 Of course, the position of the elevator floor with respect to itself is just y = 0. 


#65
Oct309, 03:28 PM

P: 218

thanks a lot! I feel like I can tackle any kind of problem like this now :P 


Register to reply 
Related Discussions  
Free Fall Question  Introductory Physics Homework  3  
AP free fall question  Introductory Physics Homework  3  
Free fall question  Introductory Physics Homework  12  
Free Fall (kinematicswhat I do wrong?)  Introductory Physics Homework  3  
Free fall question  Introductory Physics Homework  6 