# Algebra Question

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1. Jul 22, 2015

### Ellen W.

1. The problem statement, all variables and given/known data
Find Fg if G=6.67*10-11m3 kg-1s-2, M=2.6*1023 kg, m=1200kg, and r = 2000m

2. Relevant equations
Fg= -GMm/r2

3. The attempt at a solution
Fg=-GMm/r2

Fg=- (6.67*10-11m3kg-1s-2)(2.6*1023 kg)(1200kg)/(2000m)2

Fg=- (6.67*10-11m3kg-1s-2)(2.6*1023 kg)(1200kg)/4000000 m2

Fg= - 2.08*1016 kg m3 s-2/4000000m2

Fg= - 5,202,600,000 mkg/s2

Fg= - 5.21*109 mkg/s2

First, I'm not sure how to deal with the units here. I added the exponents across and subtracted if they had the same unit below them but I'm not sure I ended up with the right units. Also, I don't know if you can have a negative number in scientific notation. Lastly, I don't even know if the problem was solved correctly.

Thank you for any help, I'm beyond rusty at math like this.

2. Jul 22, 2015

### Bystander

Look like a unit of force to you?
Can you think of any reason there shouldn't be negative numbers?

3. Jul 22, 2015

### Qwertywerty

I would think it best to leave the units out of the calculations . The formula will eventually work out to give you the Unit of force - Newton(N) .

Yes you can . Scientific notation only helps to make working with large numbers easier - It doesn't have any relation with the sign of a number . -120000 = -1.2*105 .

Also , you seem to have worked out your answer probably .

Hope this helps .

4. Jul 25, 2015

### micromass

Staff Emeritus
No! Never leave out the units! Aside from needing units to be formally correct, the units provide a very neat way of checking your answers.

5. Jul 25, 2015

### Qwertywerty

Just to be clear , I was talking about using units during steps .

However , if this is what you intended to reply to , point well taken .

6. Jul 25, 2015

### HallsofIvy

Staff Emeritus
I am puzzled as to why this was titled "Algebra Question". Other than the "physics" involved in the given formula, this is purely arithmetic.

7. Jul 26, 2015

### FallenLeibniz

Your units are correct. The unit for Force is a Newton, which is defined as one kilogram times one meter per second squared (mass times acceleration).

8. Jul 26, 2015

### insightful

An interesting problem. Realistically though, what could that massive object be?

9. Jul 26, 2015

### haruspex

It's not the mass in itself that's remarkable, it's less massive than the earth. The difficulty is combining that with a distance of 2000m.
Strictly speaking, there is not enough information. We have to assume that M is a sphere composed of concentric uniform spherical shells, with an outer radius less than 2000m.
Maybe there was more text in the original question.

10. Jul 26, 2015

### insightful

My first thought was "neutron star" but, although the density is right, it doesn't have enough total mass to be one.