# Homework Help: Velocity of a cursor physics

1. Jun 27, 2009

### Apprentice123

In the position shown in Figure, the cursor B moves to the left with a speed of 150 mm/s. Find (a) the velocity of the A (b) the velocity C portion of the cable (c) the velocity of C on B

(a) 300 mm/s
(b) 600 mm/s
(c) 450 mm/s

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2. Jun 27, 2009

### Pengwuino

Re: Velocity

Please show some work first. We are not here to do your homework for you.

3. Jun 27, 2009

### Apprentice123

Re: Velocity

Because of Pulleys, I think

2Vb + 4Va = 0

4. Jun 27, 2009

### kNYsJakE

Re: Velocity

5. Jun 28, 2009

### Apprentice123

Re: Velocity

But I do not know how to solve this problem. As I will show something is not understood the problem ?

6. Jun 28, 2009

### tiny-tim

Hi Apprentice123!
Tell us your reason (in words) for thinking it's 2Vb + 4Va = 0, and then we can see what went wrong, and we'll know how to help.

7. Jun 28, 2009

### Apprentice123

Re: Velocity

I thought the speed at B is 1 pulley (2 * speed) and speed in A has 2 pulleys (4 * speed)

8. Jun 28, 2009

### tiny-tim

?? i've no idea what you're talking about

it's the number of strings that matters

Hint: if A moves distance x to the left, how much longer do the two strings on the right of A get?

9. Jun 28, 2009

### Apprentice123

Re: Velocity

Yes. I think

B (have 2 strings)
A (have 4 strings)

2x = 150
4x = 300 Ok.

But the C portion (have 1 string) x = 75 not the answer

10. Jun 28, 2009

### tiny-tim

No, A has 3 strings (1 on the left, 2 on the right), and only 2 of them are on a pulley

Anyway, answer my question …if A moves distance x to the left, how much longer do the two strings on the right of A get?

11. Jun 28, 2009

### Apprentice123

Re: Velocity

If A have strings (1 on the left and 2 on the right) move x to left and 2x to right. It is ?

12. Jun 28, 2009

### tiny-tim

It is very difficult to understand your English

"If A moves a distance x to the left, then each string on the right must get x longer, so the two strings together get 2x longer"

ok, call the length of the string on the left D, and the length of each string on the right E

then C + D + 2E = constant,

and D + E = … ?

13. Jun 28, 2009

### Apprentice123

Re: Velocity

Thank you.
A)
1 a left
two for right
Va = 2 x 150 = 300 mm/s

B)
1 a left
2x2 for right
Vc = 4 x 150 = 600 mm/s

C)
(2 + CB cable) = 3
Vc/b = 3 x 150 = 450 mm/s

14. Jun 28, 2009

### tiny-tim

Yes, your answers to (a) and b) are correct, but I have no idea how you got them.

In the exam, to get full marks, you will need to explain clearly how you get your answers.
Unfortunately, I do not understand what question (c) means, so I can't say whether your answer is correct … what is "the velocity of C on B"? … can you please explain it?

15. Jun 28, 2009

### Apprentice123

Re: Velocity

The speed of the string C on B