## Monday, February 1, 2010

### Seatbelt FBD and equations

Here is the FBD for the person on the seat of a car.

sum of the Fy= Fn(bottom)- Fg = 0

sum of the Fx= Fseatbelt - Fn(seatbelt) = ma
Fn(seatbelt) is the seat belt on the person and Fseatbelt is the force of the seat belt.

Fn(seatbelt) would not equal Fseatbelt because the person does actually go forward a little but after a very short distance the person stops.

1. Cool FBD! Is this for a person in a car or for the seatbelt itself?

What's the difference between Fn(seatbelt) and Fseatbelt?

:) Wanted to give you more work!

2. Awesome FBD! AT first, I was confused as to what I was looking at because there wasn't a description to explain what it is. I think a short one sentence will do. Also, for the person to not fly out the windshield, Fn(seatbelt) would have to equal Fseatbelt in order to stop the person from moving forward. I hope this helps!

3. Good job! You might want to check your spelling for your descriptive sentences below the FBD. I think your FBD would be clearer if you showed the seatbelt/person diagram as well. Good job describing your FBD!

4. Good job! You might want to check your spelling for your descriptive sentences below the FBD. I think your FBD would be clearer if you showed the seatbelt/person diagram as well. Good job describing your FBD!

5. Awesome! You added the description! Maybe I am interpreting your problem differently, but I still stick to my statement that the 2 forces are equal to each other. The reason why the person would still move forward slightly is because the seatbelt is accelerated forward by the person before it locks in place. Then, after the seatbelt locks, the forces would be equal so that the person would not fly forward. Based on your equation, which equals ma, the person would just continue to keep accelerating even after the seatbelt locks. However, you're absolutely right if you used that equation for the small amount time that the seatbelt takes to hit the lock.

6. Thank you for suggesting to add the description, I think it helps. But as to the forces being equal, I talked to my teacher and she agreed with me but I can see how you would think that, it is a very complicated situation.

7. The graph was confusing at frist, but the paragraph added was definitely helpful. Btw, in order to stop the person from flying away the seatbelt force and normal force should cancle out.