Circular Motion: Calculate Velocity & Period

[runningirl]

Homework Statement

A person is twirling a .7 kg mass over their head. The mass is attached to a string that is 1.5 m long. The mass spins in a HORIZONTAL circle. The string makes a 60 degree angle. Calculate the velocity of the mass, and then the period, that is necessary in order to make this work.

Homework Equations

The Attempt at a Solution

1.5*((sqrt3)/2) = 1.3 m=r
9.8(.7) sqrt3=11.88
F=mv^2/r
11.88=.7(v^2)/1.3
v=4.697 m/s

T=2*pi*r/V
=2*pi*1.3/4.697
=1.738 s?

i don't know if my method is correct.

 

[rock.freak667]

The Attempt at a Solution

1.5*((sqrt3)/2) = 1.3 m=r

This part is correct.

There is a tension acting in the string. The vertical component of the tension is balancing out the weight of the mass while the horizontal component is providing the centripetal force. You need to get the value of this tension (which is easily done from the vertical component part)

 

[Sourabh N]

Your method looks correct. What problem do you have?

 

[BobG]

The mass spins in a HORIZONTAL circle. The string makes a 60 degree angle.

A 60 degree angle to relative to what? The person spinning the mass or to the horizontal circle.

If your angle is 60 degrees relative to the person, then your answer is correct.

You might notice that the mass is going to cancel out and you could do this problem using just the acceleration, but, other than that, your approach is correct.

 

[runningirl]

it makes a degree relative to the person spinning.

thanks!