# Homework Help: Gravitational forces

1. Jun 29, 2008

### ally1h

A solar airplane is going from Earth to Mars. Its sail is oriented to give a solar radiation force of 8.00x10^2 N. The gravitational force due to the Sun is 173 N and the gravitational force due to the Earth is 1.00x10^2 N. All forces are in the plane formed by Earth, Sun, and sailplane. The mass of the sailplane is 14,500 kg. What is the net force (magnitude and direction) acting on the sailplane?

We NEVER covered this in class and I am thoroughly stumped. I've been searching everywhere for a potential solution. I REALLY hope someone can help me here. I honestly have no clue. All I can think of is adding all the forces and multiplying it by the mass. Please help!

2. Jun 29, 2008

### Staff: Mentor

Just add up the forces to find the net force. (But don't multiply by the mass. :yuck:)

Draw yourself a diagram showing all the forces acting on the sailplane. Then add them up. (Are you expected to ignore the gravitational force due to Mars?)

3. Jun 29, 2008

### ally1h

Okay. So I have 1073 N. I understand that this is the magnitude. All forces are the same direction, which is why I am adding them up. What I really don't understand is how I find the direction the sailplane is going in. I feel like I'm missing some vital piece of information, like the position of the planets or something, haha

4. Jun 29, 2008

### ally1h

The gravitational force due to Mars was not given so I am assuming we're supposed to ignore it.

5. Jun 29, 2008

### Staff: Mentor

Are you sure about that?

6. Jun 29, 2008

### ally1h

Uh... I thought I was, now I'm not so sure. I was under the impression that the solar radiation due to the sail was in the direction of the Sun, but I suppose it has to be in the opposite direction (towards Mars), to make the sailplane move? If that's true then I subtract that force from the rest?

7. Jun 29, 2008

### Staff: Mentor

Right!

8. Jun 29, 2008

### ally1h

Awesome! As for direction, should I just say it's along the -x axis? The direction is the 2nd part that I am missing and without angles or anything, I have no idea if the answer is supposed to be along a coordinate line or at an angle or what....

Thanks for your help, by the way.

9. Jun 29, 2008

### gamesguru

Call the direction towards Mars positive. So the earth's and sun's gravitational force is pulling you away from Mars, right? The solar wind always radiates outward from the sun, pushing things away from the sun, I'd expect that that's not something you'd be required to know in an introductory physics class. You should be able to figure it out on your own now. It always helps to draw a diagram with all the forces, keep that in mind.

10. Jun 29, 2008

### Staff: Mentor

As far as direction, I would specify it as being towards or away from the Sun (or from Mars). (In the absence of data to the contrary, assume that Sun, Earth, sailplane, and Mars are all in a straight line.)

11. Jun 29, 2008

### ally1h

Okay, thank you both for your help! :)

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