• Support PF! Buy your school textbooks, materials and every day products Here!

Implementing angular momentum approach in problem

  • #1
410
11

Homework Statement


An object is in uniform circular horizontal motion at the end of a chord of length L. Its tangential speed is v. The chord is pulled in to length 0.5L in such a way that the tension in the chord remains constant. As a result, the tangential speed:

a) remains constant
b) increases to 2v
c) decreases to 0.5v
d) increases to 1.4v
e) decreases to 0.7v

Homework Equations


The correct answer is E.
This can be solved using T = m * v^2/r approach. I get this, and I know how this works.

The Attempt at a Solution


Here's what I though initially: we do not have any net torques on the system [using an analogy of planetary motion around the sun, where angular momentum is conserved, substituting Tension force for gravitational force], so angular momentum has to be conserved.

Thus:

Li = Lf
[m*v*r]i = [m*v*r]f

I eliminate the m's, and then plug:
v*L = x * 0.5 L
thus x = 2v, which is b, an incorrect answer.

Could anyone please explain why this approach is incorrect?

Thanks in advance for the assistance!
 

Answers and Replies

  • #2
PeroK
Science Advisor
Homework Helper
Insights Author
Gold Member
12,326
5,121
Could anyone please explain why this approach is incorrect?

Thanks in advance for the assistance!
If the tension in the chord doesn't change, why would the object move in?

Try to imagine actually carrying out this experiment.
 
  • #3
410
11
If the tension in the chord doesn't change, why would the object move in?
Object moves in because it slows down?
 
  • #4
PeroK
Science Advisor
Homework Helper
Insights Author
Gold Member
12,326
5,121
Object moves in because it slows down?
So, someone would have to slow it down in this experiment. If they didn't, the tension would necessarily increase.

This is another slighly disengenuous question, where you are being caught out by what is not said in the question!
 
  • #5
410
11
This is another slighly disengenuous question, where you are being caught out by what is not said in the question!
The authors write really crummy questions. I have been dealing with them for the past 1.5 days now :-/
 
  • #6
PeroK
Science Advisor
Homework Helper
Insights Author
Gold Member
12,326
5,121
The authors write really crummy questions. I have been dealing with them for the past 1.5 days now :-/
Personally, I don't like this sort of question. But, perhaps it's good not to assume that everything is in the question. Or, at least to analyse the question more. My initial thought was: that's not right, the tension must increase to pull the object in. Then I realised what was intended.
 

Related Threads on Implementing angular momentum approach in problem

Replies
1
Views
7K
Replies
9
Views
2K
Replies
3
Views
473
Replies
2
Views
691
  • Last Post
Replies
10
Views
47K
Replies
30
Views
5K
Replies
2
Views
3K
Replies
6
Views
874
Replies
7
Views
10K
Replies
2
Views
5K
Top