(one more) Physics Question

[johnjohn320]

I know, some of you are laughing at me, I'm pathetic, stupid, whatever, I don't give a fsck but please help me. I have a test tomorrow and I've done nothing but study today, and it's just going nowhere-I keep getting the wrong answers. I can't even string sentences together right now.

A 0.50kg ball that is tied to the end of a 1.5m light cord is revolved in a horizontal plane with the cord making a 30 degree angle with the vertical.

a) what is the ball's speed.

Now, I drew my little diagram, solved for the force of tension (got 5.66N). I plug that in for my Centripetal Force in

Centripetal Force = mv^2/r, solve for v, and got 4.1 . (see work below) The book gets 2.1 .

cos(30) = (4.9) / FT
FT = 5.66 N

F = mv^2/r
5.66 = 0.50v^2/1.5
v = 4.12

Let me know if you can help. Thanks

 

[TuxDave]

Originally posted by: johnjohn320
I know, some of you are laughing at me, I'm pathetic, stupid, whatever, I don't give a fsck but please help me. I have a test tomorrow and I've done nothing but study today, and it's just going nowhere-I keep getting the wrong answers. I can't even string sentences together right now.

A 0.50kg ball that is tied to the end of a 1.5m light cord is revolved in a horizontal plane with the cord making a 30 degree angle with the vertical.

a) what is the ball's speed.

Now, I drew my little diagram, solved for the force of tension (got 5.66N). I plug that in for my Centripetal Force in

Centripetal Force = mv^2/r, solve for v, and got 4.1 . (see work below) The book gets 2.1 .

cos(30) = (4.9) / FT
FT = 5.66 N

F = mv^2/r
5.66 = 0.50v^2/1.5
v = 4.12

Let me know if you can help. Thanks

One error. Your radius that you used is 1.5 That's not the radius of the circular motion. You need to use 1.5meters * sin(30deg).

 

[TuxDave]

Oh... and don't look so down on yourself. Physics can be hard,

 

[PowerMacG5]

Originally posted by: TuxDave
Oh... and don't look so down on yourself. Physics can be hard,

Physics is fun though. I love my AP Physics C class (Calculus based). It is my favorite class, with Calculus coming in second.

 

[kaizersose]

Originally posted by: KraziKid

Originally posted by: TuxDave
Oh... and don't look so down on yourself. Physics can be hard,

Physics is fun though. I love my AP Physics C class (Calculus based). It is my favorite class, with Calculus coming in second.

my favorite class was photography, my hs physics sucked. i would just get the hall pass and walk around during class.

 

[johnjohn320]

Originally posted by: TuxDave

Originally posted by: johnjohn320
I know, some of you are laughing at me, I'm pathetic, stupid, whatever, I don't give a fsck but please help me. I have a test tomorrow and I've done nothing but study today, and it's just going nowhere-I keep getting the wrong answers. I can't even string sentences together right now.

A 0.50kg ball that is tied to the end of a 1.5m light cord is revolved in a horizontal plane with the cord making a 30 degree angle with the vertical.

a) what is the ball's speed.

Now, I drew my little diagram, solved for the force of tension (got 5.66N). I plug that in for my Centripetal Force in

Centripetal Force = mv^2/r, solve for v, and got 4.1 . (see work below) The book gets 2.1 .

cos(30) = (4.9) / FT
FT = 5.66 N

F = mv^2/r
5.66 = 0.50v^2/1.5
v = 4.12

Let me know if you can help. Thanks

One error. Your radius that you used is 1.5 That's not the radius of the circular motion. You need to use 1.5meters * sin(30deg).

Hm...I don't get it. The cord's 1.5 m long the whole time, so that's the radius, isn't it?

 

[PowerMacG5]

Originally posted by: kaizersose

Originally posted by: KraziKid

Originally posted by: TuxDave Oh... and don't look so down on yourself. Physics can be hard,

Physics is fun though. I love my AP Physics C class (Calculus based). It is my favorite class, with Calculus coming in second.

my favorite class was photography, my hs physics sucked. i would just get the hall pass and walk around during class.

LOL. I took Photo, enjoyed it, but had a bitch for a teacher that gave me a sh|tty grade for my second semester. I enjoy physics not because of school, but because the topic itself has always fascinated me.

 

[MegaloManiaK]

Originally posted by: johnjohn320

Originally posted by: TuxDave

Originally posted by: johnjohn320
I know, some of you are laughing at me, I'm pathetic, stupid, whatever, I don't give a fsck but please help me. I have a test tomorrow and I've done nothing but study today, and it's just going nowhere-I keep getting the wrong answers. I can't even string sentences together right now.

A 0.50kg ball that is tied to the end of a 1.5m light cord is revolved in a horizontal plane with the cord making a 30 degree angle with the vertical.

a) what is the ball's speed.

Now, I drew my little diagram, solved for the force of tension (got 5.66N). I plug that in for my Centripetal Force in

Centripetal Force = mv^2/r, solve for v, and got 4.1 . (see work below) The book gets 2.1 .

cos(30) = (4.9) / FT
FT = 5.66 N

F = mv^2/r
5.66 = 0.50v^2/1.5
v = 4.12

Let me know if you can help. Thanks

One error. Your radius that you used is 1.5 That's not the radius of the circular motion. You need to use 1.5meters * sin(30deg).

Hm...I don't get it. The cord's 1.5 m long the whole time, so that's the radius, isn't it?

You measured the length of the cord, the radius is not from the point where it is anchored. r should be measured from the center of its motion. Thus a/sin(30)=1.5 or 1.5 sin(30) (law of sines to solve for the base of the right triangle).

Edit: My little diagram doesn't translate at all


Basicly your radius should be the base of the right triangle it is making and Not the hypotenous which is your chord length.