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Answers for “当质量和速度都被测量时,如何传播动能的不确定性?”

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When both mass and velocity are measured, the uncertainty in kinetic energy is found by adding the two uncertainty sources in quadrature. You compute the partial derivative of kinetic energy with respect to each variable, multiply each derivative by the corresponding absolute uncertainty, square those terms, sum them, and take the square root.

Conditions: The kinetic energy formula is K=12mv2K = \frac{1}{2}mv^2.; Both mass mm and velocity vv carry independent absolute uncertainties δm\delta m and δv\delta v.; The uncertainties are small enough for a first-order linear approximation.

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Independent uncertainties are added in quadrature because they represent random effects that can push a measurement up or down. If the effects are random and independent, their combined influence is neither always cancelling nor always additive.

Conditions: The two uncertainty effects are random.; The two effects are independent from one another.; The combination is modeled by a perpendicular geometric construction.

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On a two-variable surface, partial derivatives represent the slopes of the tangent plane in the coordinate directions. The partial derivative with respect to xx is the slope along the xx-direction, and the partial derivative with respect to yy is the slope along the yy-direction.

Conditions: The surface is locally smooth enough to have a tangent plane.; The variables are distinguished as xx and yy.

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The curly d symbol (∂\partial) represents a partial derivative. It means taking the derivative of a multivariable function with respect to one specific variable at a time, while treating all other variables as constants.

Conditions: The function has more than one independent variable.; All variables except the differentiation variable are treated as constants during the partial derivative.