Physics Q

[TawMus]

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Four 100-kg people are holding onto 4 strings attached to a 1200kg balloon. 3 people let go and the balloon accelerates upwards at 2m/s. What is the tension on the rope by the last person still holding.

a. 200 N
b. 400 N
c. 800 N
d. 1200 N

answer was D. I'm not getting this at all, please help.

 

[CdanceNS]

hmmm...so you have forces up and down and need the total force to find the tension in the string...so if it is accelerating upwards you have F=ma=1200kg*2m/s^2. From the guy hanging you have a downward force F=mg=100kg*10m/s^2. So 2400N-1000N = 1400 N which is closest to answer d.....but not right on...maybe I'm missing something too

 

[murdoc9]

to accelerate a 100 kg person 2 m/s^2, f=(100)(2)=200N.
the force due to gravity on the 100 kg person is (100)(9.8)= ~1000N.

these forces are opposing each other, and since we're talking about tension, you can just add 'em up. 1000 + 200.

 

[TawMus]

to accelerate a 100 kg person 2 m/s^2, f=(100)(2)=200N.
the force due to gravity on the 100 kg person is (100)(9.8)= ~1000N.

these forces are opposing each other, and since we're talking about tension, you can just add 'em up. 1000 + 200.

Thats what I got, except I subtracted 200 from 1000 to get 800. I did this because the two forces are opposing each other. So is tension always the total amount of forces causes the string to stretch on both sides of the string?

 

[murdoc9]

this is testing understanding of newton's third law, obviously. if you attach a rope to a wall and pull with 300N of force, the wall pulls back with an equal and opposite force of 300N. however, the tension in the rope is still only 300N. so you have to be careful when adding forces to get tension.

In the case of the balloon, the key is that there is an acceleration. this is another force that can be added to the tension, because it goes beyond just the "equal and opposite" force of 1000N from gravity.

 

[ecoli]

I don't think you subtract them, because, logically, a balloon accelerating upward will increase tension in the rope, and gravity pulling down on the rope will also increase the tension, so they should be added.

But it's an odd question, because I figured the force due to the baloon would be F = ma = 1200kg * 2m/s^2 and the man's force downwards due to gravity would be F = 100 * 10m/s^2

Which is what was posted in #2, but I think you'd have to add the forces. OF course, the result it way too high.

 

[murdoc9]

the mass of the balloon is extra information not needed to solve the problem. only the mass of the person hanging has an effect on the tension.

 

[TieuBachHo]

Four 100-kg people are holding onto 4 strings attached to a 1200kg balloon. 3 people let go and the balloon accelerates upwards at 2m/s. What is the tension on the rope by the last person still holding.

a. 200 N
b. 400 N
c. 800 N
d. 1200 N

answer was D. I'm not getting this at all, please help.

OK ... this problem is very simple if you know the concept of the weight of a person in elevator going up ... Appearently, F(net)= F(tension) - mg = ma. F(tension) = ma + mg. What you did wrong was eliminating the F(tension) that pulls a person upward. And the answer should be D.