# Homework Help: Length and Value of KE Question. Please Check Answer

1. Dec 8, 2008

### cbrowne

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

1) Two pieces of wood are glued together to form a long stick. The length of the
first piece is L1 = 0.97 ± 0.02 m, and the length of the second piece is
L2 =1.04 ± 0.01 m. What is the length (including uncertainty) of the stick?

2) An object of mass m = 2.3± 0.1 kg , is moving at a speed of
v =1.25 ± 0.03 m/s . Calculate the kinetic energy ( KE = 1/2 mv2 ) of the object.
What is the uncertainty in the calculated value of KE ?

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L1+L2 = .97 + 1.04 ± Δuncertainty
Δu = sqrt(u12 + u22 + ...) = √ (.02^2+.01^2) =.0224
L1+L2 = 2.01 ± 0.0224m
KE=0.5mv2 ± Δ u= 1.80±Δ

Δu=dX/X = sqrt( (dA/A)2 + (dB/B)2 +...) =
=dX/1.80 = sqrt( (0.1/2.3)2 + (0.03/1.25)2 )
=√.002466*1.80
=.0853
KE=1.80±.0853J

2. Dec 8, 2008

### LowlyPion

Giving your uncertainty with more significant digits than the precision of results is not exactly the best way to handle it in 1).

In 2) you need to add the square of the relative uncertainties of the V twice to account for the V2 and then add the square of the m uncertainty and then take the square root.