# A 9.0 killogram mass and a 3.0 kg mass are at rest on a frictionless

1. a 9.0 killogram mass and a 3.0 kg mass are at rest on a frictionless table. A massless spring is compressed between the masses, which are held together by a thread. The thread breaks and the 9.0 kg mass moves to the left with a velocity of 2.0 m/s. Find the velocity of the 3.0 kg mass.

2, m1v1+m2v2=(m1+m2)v3

3. 9(2)+3
18+0=12v3
v3=1.5m/s
this was my original attempt, but i realize it is not solving for the velocity of the 3.0kg mass but i don't know what else to do

## The Attempt at a Solution

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ehild
Homework Helper

What does v3 mean?

ehild

It stands for the final combined velocity but i am not sure if this equation is right for this problem.

ehild
Homework Helper

No, it is not the final combined velocity. Read the problem: the final velocity of one mass is 2 m/s and you need to find the velocity of the other mass. So there is no combined final velocity. But what do you know about of the initial velocities? Do the masses move?

ehild

The masses are initially at rest and then they move in opposite directions

ehild
Homework Helper

The masses are initially at rest and then they move in opposite directions
What is the momentum of the system then?

ehild

The force that moves the blocks after the thread breaks?

PeterO
Homework Helper

1. a 9.0 killogram mass and a 3.0 kg mass are at rest on a frictionless table. A massless spring is compressed between the masses, which are held together by a thread. The thread breaks and the 9.0 kg mass moves to the left with a velocity of 2.0 m/s. Find the velocity of the 3.0 kg mass.

2, m1v1+m2v2=(m1+m2)v3

3. 9(2)+3
18+0=12v3
v3=1.5m/s
this was my original attempt, but i realize it is not solving for the velocity of the 3.0kg mass but i don't know what else to do

## The Attempt at a Solution

The spring will apply the same sized force on each mass - but opposite in direction.

because the masses are 9kg and 3 kg, the effect of the force on the 3 kg mass will give 3 times the acceleration it causes to the 9kg mass.

That means the 3kg mass will be travelling at 3 times the speed of the 9kg mass at all times.

It sounds like after a while the masses have moved away from the spring. By then the 9 kg mass has accelerated to 2.0 m/s

What do you think that means for the 3 kg mass?

That its speed is 6m/s?

PeterO
Homework Helper

That its speed is 6m/s?
That sounds entirely reasonable!!

Thank you so much!!:)