Impulse of bungee jumper slowing down

[holly]

Physics class is starting to sound like advertisements for toiletries: Men, try "Momentum!" Ladies, wear "Impulse" or "Joules," enriched with pheremones! Ha ha

Having trouble with this problem, about a bungee jumper jumping off a cliff for 5 seconds total, in free fall for 3 seconds, then slowing and stopping in the last two seconds of his fall. He weighs 100 kg. Okay, I got his momentum for the third second, p=mv, meaning 3000kg m/s...

But, then I'm asked what is the impulse during the 2-s interval of slowing down? The answer is given as 1500N, but I can't get that using the formulas I have. My formula for impulse says F times t = impulse. Okay, I go try to find F by finding acceleration (because F=ma?), which I thought was 10m/s/s but another formula says no, get acc by a=Delta v divided by t interval of change...by using that, I put 30-0 to get the change of velocity, then divide that by the 2 second time interval, giving me 15. Then I find F by using 15 times 100kg (his weight). Then, going back to the other equation, I put 1500 times 2 seconds, and get 3000N. Wrong, it's supposed to be 1500! If I put average velocity in the a=deltav/t equation, and I get 7.5 and then it all works out, but am I supposed to be taking an average velocity? Can someone help me, THERE ARE TOO MANY EQUATIONS and I can't understand which to use!

I don't know any calculus, I don't know the symbols usually used, either, so please dumb it down for me.

Thanking you in advance...

 

[marcus]

impulse is not the same type of quantity as force

force is measured in Newtons
and impulse is a product of force with time

(because a little force applied for a long time can accomplish
the same job as a big force applied briefly----so intuitively you have to multiply the size of the force by the time it is applied)

anyway be that as it may, impulse is a force x time product and it is measured in Newton seconds

So if you apply 1500 Newtons, and you apply that force for 2 seconds, then you give the thing a 3000 Newton second impulse.

that is also true if the force you are applying varies some with time but averages out over time to be 1500 N
if it averages 1500 N and you apply it for 2 seconds, then the
impulse delivered is 3000 Ns

(Ns stands for Newton-second, it does not stand for the plural of the word Newton, sometimes abbreviations throw people off)

thats enough for now, or maybe someone else would like to take over

 

[marcus]

btw holly,

it takes exactly an impulse of 3000 Ns to cancel a momentum
of 3000 kg m/s and bring something with that momentum to a standstill

 

[Doc Al]

marcus explained plenty, but let me add a few notes.

Originally posted by holly
But, then I'm asked what is the impulse during the 2-s interval of slowing down? The answer is given as 1500N, but I can't get that using the formulas I have.

1500N is not the impulse, it's the average force. If that's what the book says is the impulse, then the book is wrong.

My formula for impulse says F times t = impulse. Okay, I go try to find F by finding acceleration (because F=ma?), which I thought was 10m/s/s but another formula says no, get acc by a=Delta v divided by t interval of change...by using that, I put 30-0 to get the change of velocity, then divide that by the 2 second time interval, giving me 15.

What you found is the average acceleration during that 2 second interval = 15m/s/s. Perfectly reasonable.

Then I find F by using 15 times 100kg (his weight).

Right. That's the average force. (But 100kg is the mass, not the weight. Don't mix up those terms.)

Then, going back to the other equation, I put 1500 times 2 seconds, and get 3000N.

The units should be N-s: 1500N x 2s = 3000 N-s. This is the right answer for impulse.

Wrong, it's supposed to be 1500!

1500N is the average force, not the impulse. The book got the answers mixed up.

Another useful fact to know, which would have saved you much work, is that Impulse = F x T = Δp. Impulse equals change in momentum. The momentum went from 3000 kg-m/s to 0; so the impulse is 3000 kg-m/s. (Ignoring direction.) And you're done. (Note that kg-m/s is equivalent to N-s)

 

[holly]

Thank you, thank you, Ye Possessors of Larger Brains!

I am having to teach myself this course. Our professor, and I am not judging him, is very upset he is stuck teaching "dummy physics." He won't teach. He wouldn't even get the air-glide track out for us, said that's too much trouble for us, he'll only do that for his smarter students. That kind of stung, I know I'm not the brightest. Another problem is that the book we are using is pretty bad. It is written by high school science teachers. For use in college! Not that I'm knocking the high school teachers, many are fine.

But, these folks often have wrong answers, which throws us off really bad, because we depend on the book. So that's why I post so many inane questions. He'll never believe me when I show him the book had a wrong answer...he just looks at me and says, "Whatever."

Thank you again for your help, I think I understand better now. Thank you.

 

[marcus]

what you call the "dummy physics" course is potentially one of
the finest and most fun courses in the college curriculum

there was a stand-up showman of a guy at the local university here who used to
request to do "Physics 10" again and again every year. He brought more airtrack and suchlike gear than the physics majors and the engineering students got. He had projections of computer animations on this screen. It was a popular course. He liked teaching it and he made a kind of circus out of it.

the astronomy prof who did the "Astronomy 10" (the corresponding 1 semester general astro for non-scientists or, as you say "dummies") was even better. He won a campus-wide distinguished teaching award-----the physics guy may have too but I don't know about that.

Both put a lot of thought into teaching, brought in toys, show and tell stuff, gave good explanations.
Kids used to come from the high school and just sit in (audit, not for credit). My son did this when he was the right age for it, that's how I know.

You shouldn't call it "dummy". Somepeople call it
"poet's" physics (poets are supposed to be dumb about science but not all are, anyway it is nicer to call it that)

 

[holly]

I am hoping and praying (literally) that my professor does not look at this forum, because then he will truly, truly hate me...and he might also get hurt feelings, he is quite, quite elderly (even older than me, ha ha), some days he looks very pale, and as you say, they push him to teach too much because he is one of the few with a PhD at the college...thank you for the Poet's Physics remark, I will refer to it that way now and stop saying dummy physics, tho' that's what the other people call it. And I'm afraid I will be posting many many more questions, because I am already stuck on another problem concerning work...thanks again, why you Big Brains help people for free (it IS for free, isn't it??!) is beyond me, you are my physics angels anmd I put you in my prayers, for you non-believers, I will put you in my hopes, okay?

 

[marcus]

hello holly, nice to hear from you
I have deleted any remarks of mine that might
be embarrassing. if you see any I have missed tell me.

if you want to delete all or some of a post that you have
written, just press the "edit" button
and you get a screen with the original message that lets
you change anything you want (except the headline of a new thread)

but realistically there seems hardly any chance your prof would
be browsing PF so I would nt worry about hurting feelings

 

[marcus]

quite elderly (even older than me, ha ha), some days he looks very pale, and as you say, they push him to teach too much because he is one of the few with a PhD at the college...

this sounds like I would imagine the end of civilization in a post nuclear world

thank you for the Poet's Physics remark, I will refer to it that way now and stop saying dummy physics, tho' that's what the other people call it.

no harm they don't know any better

And I'm afraid I will be posting many many more questions,

"afraid" heh heh that sounds funny...the people here like to be asked questions and the answers you get are not guaranteed to be right either (AFAIK, as far as I know) but are given purely for the fun of answering.

I did not notice too many people with really big brains yet, only sometimes swelled headsk, but it is hard to tell over the internet.

You had better start asking questions, like the one about work, and not waste time chatting