Homework Help: Two blocks with different mass are attached to either end of a light rope that passes

1. Feb 18, 2008

kdizzle711

1. The problem statement, all variables and given/known data
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?

2. Relevant equations

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

3. 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

2. Feb 18, 2008

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.

3. Feb 18, 2008

kdizzle711

What does u^2 stand for?

4. Feb 18, 2008

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.

5. Feb 18, 2008

kdizzle711

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

6. Feb 18, 2008

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.

7. Feb 18, 2008

kdizzle711

Thanks, I got it. You guys are amazing

8. Feb 18, 2008

lavalamp

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

Happy to help.

9. Apr 2, 2010

00iCon

Re: Two blocks with different mass are attached to either end of a light rope that pa

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