Why Is dr/dt=-V in Polar Coordinates?

[Andrax]

In the solution , it says we have dr/dt= -V (polar coordinates)
How? i can't see how this can be possible , we know that r(t)=V/w(t), and that's it .

 

[A.T.]

In the solution , it says we have dr/dt= -V (polar coordinates) How? i can't see how this can be possible ,

r gets smaller as the vertical string gets longer.

we know that r(t)=V/w(t), and that's it .

Why?

 

[Dale]

You have to be careful when using equations in physics. You cannot just blindly plug in variables, you need to know what each variable means.

In this problem V is NOT the tangential velocity so V is not equal to rw as it is in many circular motion problems. Here V is the velocity of the downward pull on the string.

 

[Andrax]

You have to be careful when using equations in physics. You cannot just blindly plug in variables, you need to know what each variable means.

In this problem V is NOT the tangential velocity so V is not equal to rw as it is in many circular motion problems. Here V is the velocity of the downward pull on the string.

thank you , the differential equations gives me w(t)=2 if i use V=rw , i just presumed that since it's alays used in these kind of problems