# Uncertainty Principle and speed

#### forty

Suppose Fuzzy, a quantum-mechanical duck, lives in a world in which h = 2pi J.s Fuzzy has a mass of 2.0kg and is initially known to be within a range of 1.0m wide.

(a) What is the minimum uncertainty in his speed.

delta(x)delta(p) = hbar/2

hbar/2 = 1/2
delta(x) = 1
p = mv = 2v

so uncertainty in v is 1/4 ?

Is this in anyway right :S ?

(b) Assuming this uncertainty in speed to prevail for 5.0s, determine the uncertainty in his position after this time.

I'm guessing 2.2m (but really no idea what so ever)

Any help would be greatly appreciated.

#### kreil

Gold Member
$$\Delta x \Delta p >= \frac{\hbar}{2} = \pi$$...(1)

since $${\hbar} = 2 \pi$$...

(a) use (1)

(b) I would guess you just use $$x = vt$$

### The Physics Forums Way

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving