# Simple misunderstanding.

1. Nov 15, 2007

### Brin

It's 2:30 AM and I'm doing some Physics online, and I can't wrap my brain around the this question:

Specifically, I don't understand what the force as a multiple of the capsule's weight is. I've tried a long list of nonsensical answers, all of which fail.

Some of the ones I tried are:
Note: Fn = Force in Newtons, Fg = G-Force, g = gravity, w = weight
Fn/w
Fn/g
Fn/Fg

None of them work! (which sort of makes sense in my head...)

If you're curious, here is the problem as it is presented:

I've solved all the other problems relating to the g-force upon impacting, the acceleration of the capsule during the crash, and the force in newtons of the crash. Also, the length of time in which the crash occurred. I just have no idea of what solving as a multiple of the capsule's weight means.

Thank you for any help!

2. Nov 15, 2007

### Shooting Star

If weight of the capsule is W, and the force to find is F, you have to write in terrms of W, like 2W or 3.5 W. So, just write as F/W times W.

3. Nov 15, 2007

### Brin

Hm, this confuses me more.

So maintaining that F is force in Newtons that the capsule exerted on the earth (using Newton's 3rd law, I assume the earth is exerting the same force back on the capsule) and that W is the weight of the capsule in newtons:

F/W * W = F?

I still am not understanding (And I'm feeling particularly dense at the moment), could you maybe explain it again? Thanks.

4. Nov 15, 2007

### Shooting Star

Suppose the wt of the capsule is W=100 N and the force F which you have to find is 200 N, then how will you express the force F in terms of W? How many times of W is F? Just think for a second.

5. Nov 15, 2007

### Brin

Okay, so here is what I have
The force in newtons that the capsule exerts on the earth is 9.7 * 10^5 Newtons. - Note that this is the effect of the capsule on the earth, and the question is about the force of the earth on the capsule, using Newton's third law, I assume they are the same. So, as I proceed using your idea that F/W = Answer.

I take 9.7 * 10^5 Newtons divided by the weight of the capsule,( 210kg * 9.8(gravity) = 21.43 newtons) = 45263.65 --- This isn't the answer.

Your example is as the following
W= 100N
F = 200N
Your question is How many times of W is F? 2 Right?!
Then shouldn't the above be the right answer? (it isn't according to this online physics program).

I'm honestly trying to think, but man I'm feeling frustrated! Thank you for your help thus far, I'm sure I'll get it.

6. Nov 15, 2007

### Shooting Star

I got an answer of 470 times the wt of the capsule.

7. Nov 15, 2007

### HallsofIvy

Staff Emeritus
Then you had better check your calculator! 210 * 9.8 is NOT 21.43!

8. Nov 15, 2007

### Brin

Woops - I don't know how I didn't even catch that mistake.

None the less I got the answer of 470 times the weight of the capsule for my answer as well - which is the same answer as the "g force" of the capsule on the earth - which isn't the answer either (according to the physics program).

What the heck!?

9. Nov 15, 2007

### Bill Foster

What acceleration did the object experience that resulted in it going from 311 kph to 0 kph in 81.0 cm?

Use this formula:

$$a\Delta{x}=\frac{1}{2}(v^2-v_0^2)$$

Solve for a.

Then figure out what force it took to decelerate a given mass at that rate.