Which of the following is an example of a force described by Newton's law of universal gravitation?

• A. The Moon is attracted toward Earth due to their masses, producing an attractive force between them.
• B. A charged balloon sticking to a wall due to electrostatic attraction.
• C. A swimmer moving forward because of the force of water pushed backward.
• D. A book resting on a table is supported by the table’s normal force.

Answer: (A)

Verified step by step guidance

1. Recall Newton's law of universal gravitation, which states that every two masses attract each other with a force given by the formula: \(F = G \frac{m_1 m_2}{r^2}\), where \(F\) is the gravitational force, \(G\) is the gravitational constant, \(m_1\) and \(m_2\) are the masses, and \(r\) is the distance between their centers.
2. Identify the nature of the forces described in each option: gravitational force involves attraction between masses; electrostatic force involves attraction or repulsion between charges; forces from pushing or normal forces are contact forces, not gravitational.
3. Analyze the option 'The Moon is attracted toward Earth due to their masses, producing an attractive force between them' — this directly involves two masses attracting each other over a distance, matching Newton's law of universal gravitation.
4. Compare this with other options: electrostatic attraction involves charges, not masses; swimmer's movement involves reaction forces in fluid; normal force is a contact force from a surface supporting an object.
5. Conclude that the example involving the Moon and Earth is the correct illustration of Newton's law of universal gravitation because it describes the gravitational attraction between two masses.