Help Needed: 2D & Projectile Motion Test Prep

[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!

 

[Pengwuino]

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

 

[Let It Be]

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

 

[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?

 

[Let It Be]

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?

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!

 

[Thundagere]

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

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.

 

[Let It Be]

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.

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?

 

[Thundagere]

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?

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.

 

[Let It Be]

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.

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?

 

[Thundagere]

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?

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.

 

[Let It Be]

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.

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

How would Newtons Fnet equation help to find Vi?

 

[jaketodd]

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

 

[Let It Be]

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

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

 

[jaketodd]

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

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.

 

[Let It Be]

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.

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

 

[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.

 

[Let It Be]

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.

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

 

[jaketodd]

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

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

 

[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.

 

[Let It Be]

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

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

 

[Let It Be]

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.

okay...no clue where to even begin

 

[bp_psy]

okay...no clue where to even begin

Me neither.

 

[jaketodd]

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

Here is where you need to go: https://www.physicsforums.com/forumdisplay.php?f=153

 

[JHamm]

To do this with projectile motion equations (which is somewhat ridiculous since it can be solved extremely simply with more advanced physics) would be most easily done by taking your kinematics equation
$$s = v_0t + \frac{1}{2}at^2$$
And add an extra term for the "extra height" you get from the planet curving, you just pretend that instead of the planet curving away it's just a long flat plane and your projectile gets a kick gradually goes UP at the same rate that the planet would curve down.

 

[Thundagere]

I'm not taking AP Physics. Everything I know is from self-studying stuff. That being said, if you're taking AP physics, then you probably know calculus. For the cannon problem, keep in mind that graphing it might make it a lot easier.
Also, since I don't think it states otherwise, even though it says speed and not velocity, you can probably assume they're in roughly the same direction, meaning you can use a Cartesian plane, which makes it easier.
Essentially, just remember that they're going to drop at the same rate, so the main thing you should worry about is the angle. THe way that automatically occurs for ME to do it (Again though, I'm in 9th grade, so there's almost definitely an easier solution) is to write a function from Pythogoreas theorem based off their distance from one another, and find the first derivative to maximize it.