# Homework Help: Physics Gravity Problem and Kinetic Friction

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1. Jan 22, 2015

### yomo710

1. Two astronauts are trying to figure out the mass of Mercury and the mass of Neptune. The astronauts decide to hit a 142 g ball with a bat on each planet and see how far it goes. They know the radius of mercury is 2,440 km and the radius of Neptune is 24,622 km. On mercury one astronaut hits the ball with a force of 52n over .1 seconds at a 40 degrees and it travels 700m. The second astronaut hits the ball with the same force at the same angle on Neptune and it travels 100m. What is the mass of the two planets?

For the planet problem, I attempted to use the formula to find the gravity but need the mass of the planet. Then I attempted to find the mass but need the gravity. I don't even know where to start.

Last edited: Jan 22, 2015
2. Jan 22, 2015

### Simon Bridge

Welcome to PF;
You've shown us a general outline of how you are approaching the problem, but we need to see your best attempt in more detail.
You clearly know where to start, and you appear to starting in the right place: using some sort of gravity formula.
But I cannot advise you because I don't know which gravity equation nor how you have applied it.

3. Jan 22, 2015

### yomo710

Sure! As far as the planet question, I decided to use the M = ar^2/G formula. I have every part beside the mass (which I am trying to solve for) and the acceleration. I feel as if the acceleration is what is holding me back of figuring out both problems. But I do not know which formula to use for acceleration because I do not have the final velocity. When I do use d = vi(t) + 1/2 at^2 I get that the acceleration is 140,000 but I do not think that that is correct.

I also used the F=MA formula to get the acceleration that way but I get 366.197 as my acceleration and that does not seem correct.

4. Jan 22, 2015

### BvU

d and a are perpendicular. Did you make use of the 40 degrees ? Ever do projectile trajectories ?

5. Jan 22, 2015

### yomo710

I did not. I faintly understand projectile trajectories. How could I use and apply it?

6. Jan 22, 2015

### BvU

By doing the exercise :) It's the only way. Want to know where to start ? Read your textbook or google around. Some of the formulas are here on PF.

PF principle is: help all you can, but don't take the exercise away from the student. He/she needs it.

"When I do use d = vi(t) + 1/2 at^2 I get that the acceleration is 140,000 but I do not think that that is correct." Show your work in detail and help is on the way.

7. Jan 22, 2015

### yomo710

That puts me at my original problem. I do not know the final velocity or acceleration.

8. Jan 22, 2015

### BvU

You know the initial velocity magnitude and angle, so speed in vertical and in horizontal direction.
No force in the horizontal direction, so you can recover the time of flight.
Vertically the trajectory goes from initial height to final height according to your formula, where the only unknown is now a.
Just for the fun of it you can calculate final velocity magnitude and angle. You' ll be surprised !

9. Jan 22, 2015

### yomo710

Would using the range formula instead be better?

10. Jan 22, 2015

### yomo710

Well you're right. I guess I could use it but it would be more work for me. The question is asking about the canon being pushed back, will projectiles help?

11. Jan 22, 2015

### BvU

Home, home on the range ? The word has many meanings. That's why we use formulas. What range formula ? (And it's probably exactly the same, so no "better" and no "worse" either. -- To me better would mean more insightful )

12. Jan 23, 2015

### BvU

By now we have two threads in shambles ! Totally incomprehensible. What happened to the cannon recoil exercise ? Did you figure out it was a matter of momentum conservation, followed by linear decelerated motion ?