# Kinetics of rectilinear

1. Jan 30, 2010

### darkmagic

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

Refer to the diagram.

can someone explain why is the 300lb block moves only one-half the distance of the 200lb then the acceleration and velocity also to one half.

2. Relevant equations

3. The attempt at a solution

#### Attached Files:

• ###### diagram.JPG
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2. Jan 30, 2010

### tiny-tim

Hi darkmagic!

velocity = dx/dt and acceleration = d2x/dt2, so if you halve x, you halve velocity and acceleration also.

(and of course x is halved because the pulley moves only half as far as the string)

3. Jan 30, 2010

### darkmagic

how can you know that the pulley moves only half of the string?

4. Jan 31, 2010

### tiny-tim

When the pulley goes down 1 cm, that makes the string above it 1 cm longer on both sides, which requires 2 cm of string, so the block on the slope moves 2cm.

5. Jan 31, 2010

### darkmagic

ok thanks. another question, how can I know to which the blocks will move and how their acceleration are computed? look at the new attachment.

#### Attached Files:

• ###### multi pulley.JPG
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6. Jan 31, 2010

### tiny-tim

Just ask yourself, does the pulley move? if it does, how many strings are attached? (because each string will have to get longer or shorter by the same distance)

7. Jan 31, 2010

### darkmagic

yes the blocks will move. my problem is in which direction will they move?

8. Jan 31, 2010

### tiny-tim

To find out, write the F = ma equations for each of the three masses.

(and, if the heights of the masses are x y and z, you'll need a linear equation connecting x y and z)