Multiplying Uncertainty of Speed by two to find deltaV

$\Delta p \Delta x\geq \frac{h}{4\pi }$

Simer_Shera_2J
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Multiplying Uncertainty of Speed by two to find deltaV

In one of the audio-visual lectures, Prof Lavelle mentions that if we are given a speed with an uncertainty of plus/minus some number, we should multiply that number by two to get delta v. For example, if given 100m +/- 2m, then delta v=4m. In the examples in the textbook, however, like in 1D #27, we're given the speed of a bowling ball at 5sec +/- 5 seconds and in the textbook solution, delta v is 5 seconds, not 10 seconds. Do we only multiply the uncertainty in speed by two in some situations?

Vivian Leung 2D
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Joined: Wed Sep 30, 2020 10:09 pm

Re: Multiplying Uncertainty of Speed by two to find deltaV

Hello Simer,

I also had a similar question as you regarding that homework problem. The correct value of delta v should be 10 seconds. On Dr. Lavelle's website, there should be a link titled
"Solution Manual Errors 7th Edition" and that gives the correct answers to the hw problems that the textbook does not explain correctly. I believe that the problem you are asking about here is also explained on there if you want to take a look.

Posts: 46
Joined: Wed Sep 30, 2020 9:36 pm

Re: Multiplying Uncertainty of Speed by two to find deltaV

I believe that you multiply by two when there’s a ± because that represents the range of values that the speed could be. In this case, the uncertainty in velocity should be 10 seconds.

aashmi_agrawal_3j
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Re: Multiplying Uncertainty of Speed by two to find deltaV

You do multiply by 2 because the uncertainty is double the amount when it could either be plus or minus. There was actually a problem in the solution in the textbook for that problem.

Ryan_Kien_1L
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Re: Multiplying Uncertainty of Speed by two to find deltaV

The total uncertainty is the range of numbers that the answer could be, which is why you multiply it by 2.