# Blocks connected by rope, Newton's Third Law

1. Mar 14, 2008

### StephenDoty

The figure shows two 1.0 kg blocks connected by a rope. A second rope hangs beneath the lower block. Both ropes have a mass of 250 g. The entire assembly is accelerated upward at by force. What is F? What is the tension at the top end of rope 1? What is the tension at the bottom end of rope 1? What is the tension at the top end of rope 2?

The force on block B:
T-(m of B)g=(m of B)a

The force on block A:
F-[(m of a + m of b)g] = (m of b)a

What do I do now?
Thank you.
Stephen

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2. Mar 14, 2008

### Staff: Mentor

To find F, consider the masses and ropes as a single system. What external forces act on the system? What's the acceleration?

3. Mar 14, 2008

### StephenDoty

The figure shows two 1.0 kg blocks connected by a rope. A second rope hangs beneath the lower block. Both ropes have a mass of 250 g. The entire assembly is accelerated upward at by force. What is F? What is the tension at the top end of rope 1? What is the tension at the bottom end of rope 1? What is the tension at the top end of rope 2?

If we make it one system the external forces are: F and weight of the blocks
So F= (m of a + m of b + 2*m of rope)a
and w=(m of a + m of b + 2*m of rope)g

since F-w=(m of a + m of b +2*m of rope)a, right??
Now what??

Thank you.

4. Mar 14, 2008

### Staff: Mentor

Right.
No. (The net force will equal M*a, but that comes later.)
Right.
Right!
What's the acceleration? That must be given. Use it to solve for F.