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By extremely careful measurement, you determine the \(x\) -coordinate of a car's center of mass with an uncertainty of only 1.00\(\mu \mathrm{m} .\) The car has a mass of 1200 \(\mathrm{kg}\) . (a) What is the minimum uncertainty in the \(x\) -component of the velocity of the car's center of mass as prescribed by the Heisenberg uncertainty principle? (b) Does the uncertainty principle impose a practical limit on our ability to make simultaneous measurements of the positions and velocities of ordinary objects like cars, books, and people? Explain.

Short Answer

Expert verified
The minimum velocity uncertainty is negligible; thus, the principle does not limit practical measurements of macroscopic objects like cars.

Step by step solution

01

Understand the Heisenberg Uncertainty Principle

The Heisenberg Uncertainty Principle states that it is not possible to simultaneously know with high precision both the position and the momentum of a particle. Mathematically, it is given by the inequality:\[\Delta x \cdot \Delta p \geq \frac{\hbar}{2}\]where \(\Delta x\) is the uncertainty in position, \(\Delta p\) is the uncertainty in momentum, and \(\hbar = \frac{h}{2\pi}\) is the reduced Planck's constant with \(h = 6.626 \times 10^{-34}\ \text{Js}.\)

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Uncertainty in Position
The Heisenberg Uncertainty Principle introduces a fascinating limitation in our measurements. When it comes to measuring the position of an object, there's always a tiny bit of uncertainty. For example, if you measure a car's position to extremely fine detail, say within a micron (1.00\ \mu \text{m}), this represents the uncertainty in position, denoted as \(\Delta x\).

This means you won't know the car's exact position; instead, you'll be aware of its likely location within a tiny range. It's essential when the measurements reach exceedingly small scales, such as particles or ultra-sensitive tasks.

The smaller \(\Delta x\) becomes, the fuzzier other physical quantities get, due to the intrinsic nature of quantum mechanics. This uncertainty forms the basis for other measurements, such as momentum.
Uncertainty in Momentum
Momentum is the product of an object's mass and velocity, and in quantum mechanics, knowing this precisely has its challenges. When the position measurement is incredibly accurate, the Heisenberg Uncertainty Principle tells us that there's more uncertainty in measuring the momentum accurately.

For the car example, while its position is measured with high precision, uncertainty in momentum \(\Delta p\) could be calculated using the principle's inequality: \(\Delta x \cdot \Delta p \geq \frac{\hbar}{2}\). This shows how tightly position and momentum are intertwined—decreasing the uncertainty in one increases it in the other.

In practical terms, the momentum uncertainty impacts the accuracy of other motion-related measurements, like velocity, and it shows why certain details cannot be measured with infinite accuracy at the same time in the realm of very fine measurements.
Velocity of Center of Mass
The velocity of the center of mass is the speed at which the entire mass of an object, like a car, is moving. In the realm of classical mechanics, calculating this velocity is straightforward using the formula \(v = \frac{p}{m}\), where \(p\) is the momentum and \(m\) is the mass.

However, diving into quantum mechanics complicates things. As we saw, with a small uncertainty in position, \(\Delta v\), the uncertainty in the car's velocity, becomes more pronounced because \(\Delta p\) becomes larger.

Even with precise utilities, this sets a limit on how accurately we can determine it. Nonetheless, for everyday macroscopic objects like cars, these quantum uncertainties are often negligible compared to the scales we usually work with. This makes Heisenberg's principle primarily impactful at the atomic or subatomic scale, rather than in typical everyday scenarios.

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Most popular questions from this chapter

Removing Birthmarks. Pulsed dye lasers emit light of wavelength 585 nm in 0.45 -ms pulses to remove skin blemishes such as birthmarks. The beam is usually focused onto a circular spot 5.0 \(\mathrm{mm}\) in diameter. Suppose that the output of one such laser is 20.0 \(\mathrm{W}\) . (a) What is the energy of each photon, in eV? (b) How many photons per square millimeter are delivered to the blemish during each pulse?

An electron has a de Broglie wavelength of \(2.80 \times 10^{-10} \mathrm{m}\) . Determine (a) the magnitude of its momentum and (b) its kinetic energy (in joules and in electron volts).

Calculate the energy in electron volts of (a) an electron that has de Broglie wavelength 400 \(\mathrm{nm}\) and (b) a photon that has wavelength 400 \(\mathrm{nm} .\)

A sample of hydrogen atoms is irradiated with light with wavelength \(85.5 \mathrm{nm},\) and electrons are observed leaving the gas. (a) If each hydrogen atom were initially in its ground level, what would be the maximum kinetic energy in electron volts of these photoelectrons? (b) A few electrons are detected with energies as much as 10.2 eV greater than the maximum kinetic energy calculated in part (a). How can this be?

In a TV picture tube the accelerating voltage is 15.0 \(\mathrm{kV}\) , and the electron beam passes through an aperture 0.50 \(\mathrm{mm}\) in diameter to a screen 0.300 \(\mathrm{m}\) away. (a) Calculate the uncertainty in the component of the electron's velocity perpendicular to the line between aperture and screen. (b) What is the uncertainty in position of the point where the electrons strike the screen? (c) Does this uncertainty affect the clarity of the picture significantly? (Use nonrelativistic expressions for the motion of the electrons. This is fairly accurate and is certainly adequate for obtaining an estimate of uncertainty effects.)

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