A 2.0 kg mass is attached to one end of a spring

ctv1337

I am currently taking physics right now and i had the following question on a homework assignment. I have the right answer but i don't believe its correct. Can anyone help me prove him wrong or give me a better explanation of why his answer is correct?

-------here is the problem-----------
A 2.0 kg mass is attached to one end of a spring with a spring constant of 100 N/m and a 4.0 kg mass is attached to the other end. The masses are placed on a horizontal frictionless surface and set into motion. At one instant the 2.0 kg mass is observed to be traveling to the right with a speed of 0.50 m/s and the 4.0 kg mass is observed to be traveling to the left with a speed of 0.30 m/s. Since the only forces acting are the force of gravity, the normal force of the surface, and the force of the spring, we conclude that:
A) the spring is compressed at the time of the observation
B) the spring is not compressed at the time of observation
C) the motion was started with the masses at rest
D) the motion was started with at least one of masses moving
E) the motion was started by compressing the spring \

he said the answer was D can anyone help?
a picture is attached to help see both ways

https://webspace.utexas.edu/pourkc/PHYS/spring.jpg [Broken]

Attachments

• 3.5 KB Views: 397
Last edited by a moderator:
Related Other Physics Topics News on Phys.org

Tyger

No, he is correct.

Compare the ratios of the speeds and masses of the two weights.

shchr

A)As we don't know the natural length of the spring and the length at the observation, we can't say spring is compressed or extended.
B) same as above.
C)If so, masses stay still.
E)Starting by compressing and stretching both can make the same motion. We can't say so.
Remaining D) is correct.

Guybrush Threepwood

I think it has to do with the conservation of momentum. The momentum of the system is
m1*v1 - m2*v2 (the signs can be changed if you change the reference system) = 2*0.5 - 4*0.3 = -0.2 N/s
So the initial momentum of the system is not 0 => something was initially moving....

Physics Forums Values

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving