# Homework Help: Did I do this dynamics test question correctly?

1. Oct 19, 2011

### TB123

After getting my engineering dynamics test back today, I really don't agree with what my professor says. To me, the first sentence implies that there will be no force when the cylinder is at rest, and therefore the spring will NOT be stretched initially. To me, the word subjectED implies that the 100N force will not be present until the cylinder has fallen 250mm. If my professor wished to imply that the spring has a 100N force at rest he should have used "subject", not "subjected." Or even better yet, he could have said something along the lines of, "the spring is initially subject to a tension of 100N at rest."

I worked the problem assuming that the spring was unstretched at rest. My questions are:

1. Am I correct to assume that there is no initial stretch? Do I have a valid argument?
2. Assuming that there is no initial stretch, did I solve the problem correctly?

Thanks

2. Oct 19, 2011

### cepheid

Staff Emeritus
What he meant was:

At the instant that the cylinder is released, the spring is BEING subjected to a tension force of 100 N. It's fairly clear that he means to say that the force is present at the time of release, grammatical issues aside. Besides, YOUR interpretation of the problem does not follow from the wording at all. There is nothing in the problem to suggest that the 100 N force occurs when the cylinder has dropped by 250 mm.

Although his wording may have been somewhat problematic, I think you're still out of luck here.

Since he meant to just describe the state of affairs, rather than referring to specific event or change of state, he would have been better off saying "At the instant the cylinder is released, the spring HAS a 100 N tension force on it." The idea is that to HAVE a force acting on you and to BE subjected to a force mean the same thing.

3. Oct 19, 2011

### TB123

Alright, thanks for your reply. I see that it can be interpreted the way he intended but I think that both interpretations are correct according to the English language (Although I still feel that my way would be "more correct" haha). The problem is really the use of the word "when." I think that "when" implies "durring AND after the event of dropping the cylinder." Like you suggested, he should have replaced "when" with something like "at the instant."

My real concern is whether or not I computed the problem correctly for how I interpreted it. I spoke with my professor and he said that I did it wrong regardless of the interpretation of the first sentence, but I looked at it again and don't understand why it is wrong.

Thanks again

Last edited: Oct 20, 2011