# Homework Help: Constrained motion problem

1. Apr 8, 2013

### Clever_name

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

See attached picture.

2. Relevant equations

3. The attempt at a solution

I have the solution, but I'm puzzled by the first step in their solution, which reads

Xa^(2) + Xb^(2) = L^(2)

The solution provider then takes the time derivative of this expression which then produces

2Xa(Xa(dot))+2Xb(Xb(dot)) = 0

where Xa(dot) and Xb(dot) is the Xa or Xb symbol with a dot over it.

Thanks for you're assistance!

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Last edited by a moderator: Apr 10, 2013
2. Apr 8, 2013

### Clever_name

Here is the solution if my explanation was confusing in any way.

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3. Apr 8, 2013

### Clever_name

Any help would be greatly appreciated.

4. Apr 9, 2013

### haruspex

I don't understand what's puzzling you. The first line is just Pythagoras' theorem applied to the triangle of which AB is the hypotenuse.

5. Apr 9, 2013

### Clever_name

The line below it, when taking the time derivative of the first line how do they end up with that expression?

6. Apr 9, 2013

### haruspex

If x = x(t), can you write down the expression for (d/dt)x2?

7. Apr 10, 2013

### Clever_name

2x? or x(t)dx/dt = x(t)*x'(t)

8. Apr 24, 2013

### Clever_name

How did you know that x was a function of time?

9. Apr 24, 2013

### haruspex

No, but each answer you offer is half right. Do you know the chain rule?

10. Apr 25, 2013

### Clever_name

yep, i understand now haruspex, thanks!