Wedge-Spring-Block Problem

  • Thread starter DSM_
  • Start date
  • #1
3
1
rhr09k.png



I can't figure out how the answer is (A). The spring is the problem. I thought the spring would just extend and whole thing would act like a string.
If there were no spring then the answer come out to be (6/5)m. Please help.
 

Answers and Replies

  • #2
293
33
Have you learned about gravitational potential energy and spring energy yet?
 
  • #3
haruspex
Science Advisor
Homework Helper
Insights Author
Gold Member
2020 Award
36,970
7,209
I thought the spring would just extend and whole thing would act like a string.
If there were no spring then the answer come out to be (6/5)m. Please help.
I agree, and since k does not appear in any of the answers, the questioner agrees the spring is irrelevant.
My guess is that the coefficient was supposed to be 1/4.
 
  • #4
293
33
Even when the gravitational force exerted on the block M is equal to the spring force exerted on block M, the block is still moving. Your method presumes that this is not true. If you use conservation of energy to solve for the maximum spring force, answer (A) can be arrived at.
 
Last edited:
  • #5
haruspex
Science Advisor
Homework Helper
Insights Author
Gold Member
2020 Award
36,970
7,209
Even when the gravitational force exerted on the block M is equal to the spring force exerted on block M, the block is still moving. Your method presumes that this is not true. If you use conservation of energy to solve for the maximum spring force, answer (A) can be arrived at.
You are assuming that block M is released with the spring just taut, i.e. at its relaxed length. Yes, that gives answer A, so that is probably what is intended, but I see nothing in the problem statement to justify that assumption.
 
  • #6
3
1
Okay, I have to do it with energy conservation.
If block is released when spring is relaxed, if it moves x downwards,
Work done by grav = Potential energy of Spring.
1/2kx^2 = Mgx
kx = 2Mg
Then tension is kx everywhere in string. Then equating to (friction + mgsin37) gives answer (A).

Is this correct?
 
  • #7
haruspex
Science Advisor
Homework Helper
Insights Author
Gold Member
2020 Award
36,970
7,209
Okay, I have to do it with energy conservation.
If block is released when spring is relaxed, if it moves x downwards,
Work done by grav = Potential energy of Spring.
1/2kx^2 = Mgx
kx = 2Mg
Then tension is kx everywhere in string. Then equating to (friction + mgsin37) gives answer (A).

Is this correct?
Seems so. Still a poorly worded question, and congratulations to AlephNumbers for guessing what was meant.
 
  • #8
3
1
Seems so. Still a poorly worded question, and congratulations to AlephNumbers for guessing what was meant.

I think questioner want us to assume that the spring is relaxed otherwise there wouldn't be any spring at all.

Yup, thanks AlephNumbers :)
 

Related Threads on Wedge-Spring-Block Problem

  • Last Post
Replies
1
Views
797
  • Last Post
Replies
18
Views
3K
  • Last Post
Replies
1
Views
6K
  • Last Post
Replies
10
Views
6K
  • Last Post
Replies
10
Views
915
  • Last Post
Replies
1
Views
2K
  • Last Post
Replies
5
Views
12K
Replies
4
Views
675
  • Last Post
Replies
8
Views
5K
  • Last Post
2
Replies
25
Views
6K
Top