# Homework Help: Circular motion-what is the radius of the loop de loop in meters

1. May 5, 2012

### dani123

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

Snoopy is flying his vintage warplane in a "loop de loop" path being chased by the Red Baron. His instruments tell him the plane is level (at the bottom of the loop) and travelling at 180 km/hr. He is sitting on a set of bathroom scales that him he weighs four times what he normally does. What is the radius of the loop in meters?

2. Relevant equations

Fc=mv2/R
ac=v2/R

3. The attempt at a solution

I am completely lost with this problem but this is what I attempted...

I started by converting 180km/hr into m/s and came up with 648 000 000m/s.

Then I used this value and plugged it into the centripetal acceleration equation and used ac=9.8m/s2 and then solved for R= 4.28*1016m

I know this must be the wrong answer but I am very lost as to what I should do or even how I should be looking at this problem... If anyone could help me out that would be greatly appreciated! Thank you so much in advance!

2. May 5, 2012

### tiny-tim

hi dani123!
:rofl: :rofl: :rofl:
hint: what is the equation for the reaction force between snoopy and the scales?

3. May 7, 2012

### dani123

im confused as to how thats gonna help me find the radius..

4. May 7, 2012

### tiny-tim

but the only information you have is the magnitude of that reaction force

5. May 7, 2012

### dani123

so do i just convert 180km into meters?

6. May 7, 2012

### dani123

nvm! scratch that last post ...

7. May 7, 2012

### tiny-tim

yes, and hours into seconds

8. May 7, 2012

### dani123

i tend to over think the problems that have the simplest answers! haha

9. May 9, 2012

### dani123

after i converted it into m/s... thats the answer? really?

10. May 9, 2012

### Staff: Mentor

That's the answer to his speed in m/s. It doesn't answer the question about the radius of the loop he's following.

Consider what his effective acceleration must be if he weighs 4x normal weight (when the usual acceleration due to gravity is just g). What additional acceleration is operating when he moves in circular motion?

11. May 28, 2012

### dani123

im lost

12. May 28, 2012

### tiny-tim

what is the equation for the reaction force between snoopy and the scales?

13. May 28, 2012

### dani123

i dont know anymore,, im really confused

14. May 28, 2012

### truesearch

edit...delete

15. May 28, 2012

### Staff: Mentor

If the radius were infinite, there would be a 1g upward force exerted by the scale on him. How many additional g's of force are required to keep him moving in a circle? That should be v2/R.

Chet

16. May 28, 2012

### tiny-tim

well, how many forces are there, and what is the acceleration?

17. May 28, 2012

### Staff: Mentor

The net force is 3g's.

18. May 28, 2012

### dani123

how did you come up with these g's ?

19. May 28, 2012

### azizlwl

What is the radius of the loop in meters?

You draw a free body diagram. From this you can calculate the radius needed.
There are 3 forces exerted on the man.
1. Gravitional force.
2. Centripetal force.
3. Normal force. This force shown by the scale.

20. May 29, 2012

### vela

Staff Emeritus
There are only two forces on Snoopy: the gravitational force and normal force. The net force on Snoopy results in his centripetal acceleration.

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