# Homework Help: Newton's laws and a carousel

1. Dec 17, 2016

### Eitan Levy

1. The problem statement, all variables and given/known data

A boy with a mass of m1 is sitting in a carousel. The spinning frequency is 0.3355. The boy is also sitting on a weighing scale, will they show a number higher than m1g,lower or the same? (The mass of the weighing scale is 0).

2. Relevant equations
2r=F
ma=F
ω2=4π2f2

3. The attempt at a solution
I am trying to understand by which forces the weighing scale is affected and all I can think of is by the boy and by the normal force (and the centripetal force, but it doesn't matter here I think). Why they will show a value higher than m1g?

2. Dec 17, 2016

### jbriggs444

Always start with a free body diagram. What forces act on the boy?

What force does the scale measure directly?

3. Dec 17, 2016

### Eitan Levy

That's what I always do, the scale will show the normal power it will give to the boy I think, but why the value will be higher? I don't think I need to take the tension into consideration here, do I?

4. Dec 17, 2016

### jbriggs444

See post #2.

5. Dec 17, 2016

### Eitan Levy

Still can't figure it out.

6. Dec 17, 2016

### jbriggs444

You did not answer the question: What forces act on the boy?

7. Dec 17, 2016

### Eitan Levy

Normal, gravity and centripetal.

8. Dec 18, 2016

### haruspex

9. Dec 18, 2016

### Eitan Levy

Okay, so the normal force, gravity and tension. Still why it will shot a value higher than m1g?

10. Dec 18, 2016

### haruspex

Unless the boy is tied to the rope, tension does not act on him.

11. Dec 18, 2016

### Eitan Levy

Thats what I thought. So only gravity and normal? I really have no clue

12. Dec 18, 2016

### haruspex

Right. What is the resultant of those two forces?

13. Dec 18, 2016

### Eitan Levy

Not a native speaker, have no idea what resultant is.
Maybe the chair isn't straight anymore after the carousel starts moving and then the normal must be bigger than gravity? That's the only explanation I can think of.

14. Dec 18, 2016

### jbriggs444

The boy is going around on a carousel. That indicates that he is accelerating. Can you account for that with two forces, both of which are vertical?

15. Dec 18, 2016

### Cutter Ketch

It is hard to tell for sure from what the question says as posted. Is the thin line a cable? Is it free to swing out? However, yes, that is the usual assumption, and that is where you get your nonvertical force.

16. Dec 18, 2016

### Eitan Levy

That's why I asked this question here, I was sure the chair is still straight.

17. Dec 18, 2016

### Cutter Ketch

The problem has an answer either way, and the answer is different depending on whether or not that vertical line is rigid. If you can't be sure from the problem statement, that is definitely something you want to clarify with the teacher.

18. Dec 18, 2016

### Eitan Levy

My teacher unfortunately has a no questions policy, that's why I need to ask it here.

19. Dec 18, 2016

### jbriggs444

To me, the drawing clearly indicates that the [platform supporting the] boy is supported by a rope, chain or cord of some sort.

20. Dec 18, 2016

### Cutter Ketch

If I had to guess that would be my guess, but I'd hate to have to guess.

21. Dec 18, 2016

### Cutter Ketch

Surely when the question is unclear he/she has a responsibility to correct his/her error. Start with "your question is ambiguous" which he can't ignore and then just point and say, "Is that line rigid or free to swing out?" I find it very unlikely he would refuse to answer.

22. Dec 18, 2016

### haruspex

I made the same assumption, but I'm no longer sure. Fairground carousels are of both sorts, those where the the seats tip and those where the seats remain upright, the customer relying on other restraints to provide the horizontal acceleration.
Carousels in factories and airport luggage movement tend to be level, relying on friction.
The diagram has an arrow indicating rotation, yet the support is vertical, suggesting a rigid assembly.
As Cutter Ketch observes, the answer depends critically on this.