What Is the Mass of the More Massive Block?

[kdizzle711]

Homework Statement

Two blocks with different mass are attached to either end of a light rope that passes over a light, frictionless pulley that is suspended from the ceiling. The masses are released from rest, and the more massive one starts to descend. After this block has descended a distance 1.30 m, its speed is 3.50m/s .

If the total mass of the two blocks is 14.0 kg, what is the mass of the more massive block?

Homework Equations

(1/2)mv1^2+mgy1=(1/2)mv2^2+mgy2

The Attempt at a Solution

Can someone help me get started with this problem? I'm not sure I am using the right equation or approaching it correctly

 

[lavalamp]

Since you know that a 14 Kg mass has been accelerated from rest to 3.50 m/s over a distance of 1.30 m, you can detemine the acceleration from the equation:
v^{2} = u^{2} + 2as

From there you can calculate the force required to accelerate the mass at that rate with:
F = ma

That will give you the difference in the weights of the two masses.

 

[kdizzle711]

What does u^2 stand for?

 

[lavalamp]

v is the final velocity, u is the initial velocity. Perhaps you use a different type of notation. The same formula is listed last here in a different notation.

 

[kdizzle711]

I found that the F=ma is 65.94N, but where do I go from there?

 

[lavalamp]

Like I said, that's the difference in their weights (though I actually get 65.96 N). You can divide that by g to find the difference in their mass since weight = mass * gravitational field strength.

If you call the two weights a and b, that will give you:
a - b = 65.96/g

You also have
a + b = 14

Now you have to solve the simultaneous equations.

 

[kdizzle711]

Thanks, I got it. You guys are amazing

 

[lavalamp]

Nah, not amazing just ... yeah OK, amazing.

Happy to help.

 

[00iCon]

This appears to be a common question. I got the same thing with different values, but the formulae here worked!