# How Smart Am I?

1. Dec 30, 2011

### Let It Be

Can someone give me some two dimensional and projectile motion questions in this thread to work on? I've got a test coming up, and I'm still not 100% with the topics. If you can help-please do so! We all know how it feels to be stressed, cmon people!

2. Dec 30, 2011

### Pengwuino

If you are in a course, don't you have a whole textbook full of problems?

3. Dec 31, 2011

### Let It Be

We do you're right-however they're not the same equations and it's only about a page of concepts :(

4. Dec 31, 2011

### jaketodd

Look up how much force is imparted to a bullet when fired. You fire the bullet 21 degrees above parallel to the ground. The atmosphere provides as much friction as there is on the moon. The planet you are on is spherical but only 30 km in diameter. The mass of the planet is 1 million kg. How far around the planet does the bullet go before hitting the ground?

5. Dec 31, 2011

### Let It Be

Oh boy...okay let me try to figure this out....we're not doing AS difficult stuff.
So the sin/cos θ would be 21 degrees
I have no clue what the friction is on the moon...
and besides that I have no idea!

6. Dec 31, 2011

### Thundagere

Concepts are important. Memorizing a bunch of formulae gets you nowhere fast, you won't do well in applying them. I'd suggest you go through the book's problems, and get a full grasp on the concepts, then start working on mathematics.
For instance, the above mentioned problem is pretty simple. It merely requires the application of several concepts, which are definitely worth learning.

7. Jan 1, 2012

### Let It Be

No you're right-two dimensional motion & projectile motion just don't com easily to me and it's hard studying.

I think I want to use R=2Vi^2sinθcosθ/g

but I don't know if that right because I don't know how to derive Vi, and then would g be 9.8?

8. Jan 1, 2012

### Thundagere

It's always going to be hard :), but the fact is that once you master the concepts, the problems come a lot more easily.
This problem requires you to combine kinematics, forces, gravitation, etc, it's quite interesting. g is -9.8 on EARTH. Remember, read the full problem. THis is, essentially, a different planet.

Vi you would have to use forces, but I'm not quite sure on that, since different guns=different bullet mass and different forces. I think finding this is the hardest part of this problem.

9. Jan 1, 2012

### Let It Be

Right, that's where I'm getting stuck is trying to find Vi.
Do you have any good website suggestions that are helpful for trying to understand this a bit better?

10. Jan 1, 2012

### Thundagere

I'm in 9th grade, so I'm presuming that you're somewhere in high school too. Hyper physics is always pretty useful. The formula you're looking for is one of Newton's laws, F=m*a.

11. Jan 1, 2012

### Let It Be

A freshman and you're taking AP Physics? Wow, I'm a junior.

How would newtons Fnet equation help to find Vi?

12. Jan 1, 2012

### jaketodd

I didn't think to define the bullet mass; choose an arbitrary mass.

13. Jan 1, 2012

### Let It Be

If you could tell me how to combine the horizontal and vertical componenets with an angle involved, it would be so helpful for me to try the problem

14. Jan 1, 2012

### jaketodd

It's been a long time since I've solved a problem like this. One way that might be a valid way to solve it is to "weight" it with ratios. For example, if it's 21 degrees above horizontal, that's 21° / 90° % into the vertical component (if you fired it 90° above horizontal it would be 100 % into the vertical component). Use the ratio with other information you have or can calculate.

15. Jan 1, 2012

### Let It Be

Okay...sorry but you completely lost me again? Any way you could dumb this down?

16. Jan 1, 2012

### jaketodd

Here's a tidbit that might be helpful: If you fire a gun parallel to the ground, and drop a bullet from the same height as the gun at the same time, both bullets will hit the ground at the same time, neglecting friction.

17. Jan 1, 2012

### Let It Be

Hmm..how does this relate to the angles? Like where is the 90° coming from?

18. Jan 2, 2012

### jaketodd

Maybe I'm not the best person to help you. Perhaps try the homework forum; you'll probably get a lot better help there; the homework helpers don't usually look in the General Physics part of Physics Forums.

I wish you the best,

Jake

19. Jan 2, 2012

### bp_psy

Two canons fire two identical projectiles with the same speed v=10m/s. One has an incline of 25° the other or 40°. Find the distance in the x direction where the trajectories of the projectiles have the maximal distance from each other in the y direction.

20. Jan 2, 2012

### Let It Be

How do I get to the homework forum? I thought I was in it already haha