# 1 string, 2 balls, motion problem

1. Oct 13, 2012

### thoradicus

1. The problem statement, all variables and given/known data
http://www.xtremepapers.com/papers/...S Level/Mathematics (9709)/9709_w09_qp_41.pdf

number 6III
http://www.xtremepapers.com/papers/...S Level/Mathematics (9709)/9709_w09_ms_41.pdf

2. Relevant equations
s=ut+0.5at^2

3. The attempt at a solution
Okay, the distance of P from ground is 3m and the distance of Q from ground is 7m, already calculated. However, im confused when the marking scheme says that the initial velocity for Q is -2m/s? why is that?

3 = 2tP + 5tP^2 initially i wrote this
7=2t+5t^2 for Q, the problem is why is the intial velocity is -2?

2. Oct 13, 2012

### mishek

P & Q are moving in different directions? If one has a "positive" velocity, what will the other have?

3. Oct 13, 2012

### thoradicus

But since the string breaks, the balls should all be moving downwards, right?

4. Oct 13, 2012

### HallsofIvy

Staff Emeritus
It doesn't. I will admit that the answer as shown is ambiguous. The problem asked for two speeds and only the speed of P is given. The speed of P (not "initial" but at the time the string broke) is -2 m/s (downward) so the speed of Q is 2 m/s (upward).

5. Oct 13, 2012

### thoradicus

Ok, but why is speed of Q taken as -2, though? A bit confused. Can elaborate more?

6. Oct 14, 2012

### mishek

Yes, but you are asked to find the speed of the particles when the string brakes.

Before string brakes, the particles are moving. P goes down (it's heavier) and Q goes up. When the string breaks, then they both go down.

I've attached a photo. Position 1 is at rest, position 2 before string breaking and position 3 after breaking.

#### Attached Files:

• ###### velocities.JPG
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7. Oct 14, 2012

### mishek

If you mean why did they put -2 at (iii), it's because the initial velocity has different direction than the path s. The direction of the path goes from up to down. For particle P it's +2 because the initial velocity (2 m/s) has the same direction as s.

8. Oct 14, 2012

### thoradicus

Ah that explains it!. thanks all much appreciated

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