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NASA illustration of LISA, showing three spacecrafts positioned in orbits that form a triangular formation. The triangular formation is positioned to the left of the Sun, Earth, and Moon in the diagram. Figure not to scale.
Space-based future experiments for the measurement of gravitational waves. Shown here is a drawing of LISA’s orbit. Each satellite of LISA will consist of a laser source and a mass. The lasers will transmit a signal to measure the distance between each satellite’s test mass. The relative motion of these masses will provide information about passing gravitational waves. (credit: NASA)

The ideas presented in this section are but a glimpse into topics of modern physics that will be covered in much greater depth in later chapters.

Test prep for ap courses

Which phenomenon correctly describes the direction and magnitude of normal forces?

  1. electromagnetic attraction
  2. electromagnetic repulsion
  3. gravitational attraction
  4. gravitational repulsion

(b)

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Explain which of the four fundamental forces is responsible for a ball bouncing off the ground after it hits, and why this force has this effect.

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Which of the basic forces best explains tension in a rope being pulled between two people? Is the acting force causing attraction or repulsion in this instance?

  1. gravity; attraction
  2. electromagnetic; attraction
  3. weak and strong nuclear; attraction
  4. weak and strong nuclear; repulsion

(b)

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Explain how interatomic electric forces produce the normal force, and why it has the direction it does.

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The gravitational force is the weakest of the four basic forces. In which case can the electromagnetic, strong, and weak forces be ignored because the gravitational force is so strongly dominant?

  1. a person jumping on a trampoline
  2. a rocket blasting off from Earth
  3. a log rolling down a hill
  4. all of the above

(d)

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Describe a situation in which gravitational force is the dominant force. Why can the other three basic forces be ignored in the situation you described?

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Summary

  • The various types of forces that are categorized for use in many applications are all manifestations of the four basic forces in nature.
  • The properties of these forces are summarized in [link] .
  • Everything we experience directly without sensitive instruments is due to either electromagnetic forces or gravitational forces. The nuclear forces are responsible for the submicroscopic structure of matter, but they are not directly sensed because of their short ranges. Attempts are being made to show all four forces are different manifestations of a single unified force.
  • A force field surrounds an object creating a force and is the carrier of that force.

Conceptual questions

Explain, in terms of the properties of the four basic forces, why people notice the gravitational force acting on their bodies if it is such a comparatively weak force.

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What is the dominant force between astronomical objects? Why are the other three basic forces less significant over these very large distances?

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Give a detailed example of how the exchange of a particle can result in an attractive force. (For example, consider one child pulling a toy out of the hands of another.)

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Problem exercises

(a) What is the strength of the weak nuclear force relative to the strong nuclear force? (b) What is the strength of the weak nuclear force relative to the electromagnetic force? Since the weak nuclear force acts at only very short distances, such as inside nuclei, where the strong and electromagnetic forces also act, it might seem surprising that we have any knowledge of it at all. We have such knowledge because the weak nuclear force is responsible for beta decay, a type of nuclear decay not explained by other forces.

(a) 1 × 10 13 size 12{1 times "10" rSup { size 8{ - "13"} } } {}

(b) 1 × 10 11 size 12{1 times "10" rSup { size 8{ - "11"} } } {}

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(a) What is the ratio of the strength of the gravitational force to that of the strong nuclear force? (b) What is the ratio of the strength of the gravitational force to that of the weak nuclear force? (c) What is the ratio of the strength of the gravitational force to that of the electromagnetic force? What do your answers imply about the influence of the gravitational force on atomic nuclei?

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What is the ratio of the strength of the strong nuclear force to that of the electromagnetic force? Based on this ratio, you might expect that the strong force dominates the nucleus, which is true for small nuclei. Large nuclei, however, have sizes greater than the range of the strong nuclear force. At these sizes, the electromagnetic force begins to affect nuclear stability. These facts will be used to explain nuclear fusion and fission later in this text.

10 2 size 12{"10" rSup { size 8{2} } } {}

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Practice Key Terms 2

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Source:  OpenStax, College physics for ap® courses. OpenStax CNX. Nov 04, 2016 Download for free at https://legacy.cnx.org/content/col11844/1.14
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