Discussion in 'Community Discussion' started by touchher, Nov 9, 2008.

1. ### touchher macrumors regular

Joined:
Nov 23, 2007
#1
you are swinging a .25 kg ball on a string above your head in a horizontal circle (at an angle) the length of the string is 1.5 m. It takes 1.2 s for the ball to make one revolution. Determine the angle the string makes relative to the horizontal.

2. ### gauchogolfer macrumors 603

Joined:
Jan 28, 2005
Location:
American Riviera
#2
Which part of this are you having trouble with?

3. ### richard.mac macrumors 603

Joined:
Feb 2, 2007
Location:
51.50024, -0.12662
#3
you need to find out the radius which im not sure how to do and then use Sin^-1 to find the angle.

4. ### gusious macrumors 65816

Joined:
Dec 2, 2007
Location:
Greece
#4

Of finding the angle....

5. ### gauchogolfer macrumors 603

Joined:
Jan 28, 2005
Location:
American Riviera
#5
Well, if I'm going to do his homework, I thought I'd at least make him think for a moment.

In order to solve this problem you need to make all of the forces on the weight balance out. The only two forces acting on the weight are gravity and the tension in the string. You know the direction that gravity is acting, and the force of the string is along the string itself.
By knowing the period of revolution, and the length of the string, you can determine the angular acceleration. Since the angular acceleration is proportional to the force acting inwards due to the string, you'll be able to sort out the angle necessary to apply such a force.

Looking at it again, it looks like there's no way to keep the weight above your head, since the string can't hold it up. What's to keep gravity from pulling it down?

Strange question actually.

6. ### Marc Fraser macrumors newbie

Joined:
Nov 1, 2008
#6
Assuming you are neglecting gravity, I get the angle to be 49.84°.

7. ### chrmjenkins macrumors 603

Joined:
Oct 29, 2007
Location:
CA
#7
You can't neglect gravity, it is responsible for the angle.

8. ### Marc Fraser macrumors newbie

Joined:
Nov 1, 2008
#8
Yes you can if the question says it.

If you dont neglect gravity on this question, its not a question - there's nothing to keep the weight up! Unless there is data that the opening poster hasn't told us.

Totally depends what level the OP is doing physics at.

9. ### atszyman macrumors 68020

Joined:
Sep 16, 2003
Location:
The Dallas 'burbs
#9
The way I understand it is that the angle of the string is caused by gravity and therefore it cannot be neglected.

You have the ball at the end of the 1.5 m string with a certain angular acceleration which causes an outward force, adding to it the acceleration caused by gravity the 1.5 m string is the hypotenuse of a right triangle with sides set proportional to the outward force from the angular acceleration and gravity.

Flip richthomas's picture and r would be proportional to the angular force out, and the vertical side would be proportional to the gravitational acceleration, and then it's a trigonometry problem.