Calculating Speed of m3 Using Energy Concepts in a Frictionless System

[jman1211]

I am not sure how to do this problem without using netF=ma.

Three objects with masses, m1 = 5 kg, m2 = 10 kg, and m3 = 15kg, are attached by strings over frictionless pulleys. The horizontal surface is frictionless, and the system is released from rest. Using energy concepts, find the speed of m3 after it moves down 4m.

And the picture has a table with m1 hanging off of one side, m2 on the table, and m3 hanging off of the right side.

I have no idea how to start this problem, it is probably easy

 

[quantumdude]

Draw the system in its initial configuration and write down an expression for the total energy. What does the fact that it is being released "from rest" tell you about the kinetic energies of each block?

Then draw the system after Block 3 has moved 4 m, and write down the total energy in that case.

Then, what physical law can you use that pertains to energy?

 

[jman1211]

work=change in kinetic energy?

 

[quantumdude]

The principle I am getting at is conservation of energy. You don't need to consider work here because the only force at play here is gravity, which is conservative. That means you can write down an expression for potential energy for the gravitational force.

 

[jman1211]

Oh, of course, so the whole problem only depends on the initial and final positions of m3 because gravitiy is a conservative force.

That makes it easier, thanks a lot. I forgot about that section.

 

[quantumdude]

Looks like you've got it.