A laptop of mass m m\,m rests in equilibrium on a flat horizontal desk, as shown in the diagram below.
Let:
- F1 F_1\,F1 be the downward gravitational force exerted by the Earth on the laptop.
- F2 F_2\,F2 be the upward contact force exerted by the desk on the laptop.
- F3 F_3\,F3 be the downward contact force exerted by the laptop on the desk.
Which of the following correctly describes the relationship between these forces and Newton's laws of motion?
F1F_1F1 and F2F_2F2 form a Newton's third law action-reaction pair because they are equal in magnitude and opposite in direction.
F1F_1F1 and F2F_2F2 are equal in magnitude because of Newton's first law (due to equilibrium), and the Newton's third law partner to F1F_1F1 is the upward gravitational force exerted by the laptop on the Earth.
F2F_2F2 and F3F_3F3 form a Newton's third law action-reaction pair, and they are equal in magnitude because the laptop is in equilibrium (Newton's first law).
F1F_1F1 and F3F_3F3 form a Newton's third law action-reaction pair, while F2F_2F2 and F3F_3F3 are equal in magnitude due to Newton's second law.