When a planet moves under a central gravitational force, the torque about the center (the sun) is zero because the force points along the radius. That makes the angular momentum about the sun constant. Angular momentum for the planet is L = m r × v, and in planar motion its magnitude is L = m r v_t, with v_t the tangential component of velocity. Since dL/dt = r × F = 0 for a central force, L does not change as the planet orbits. This conservation leads to Kepler’s second law: equal areas are swept in equal times, meaning the planet adjusts its speed so that the product of its radius and its tangential speed stays the same as it moves. The other possibilities don’t stay constant in general. Linear momentum of the planet isn’t constant because gravity (an external force) acts on it. Kinetic energy isn’t constant because gravitational potential energy is exchanged with kinetic energy as the planet moves closer to or farther from the sun. Only the total mechanical energy of the planet-sun system is conserved. Speed itself isn’t constant either, since the planet speeds up when closer to the sun and slows down when farther away.

A couple is a pair of equal, opposite and parallel forces whose lines of action are distinct. The forces cancel in translation, giving zero resultant force, but because they act apart from each other they produce a pure rotation. The turning effect, or moment, is equal to the force magnitude times the perpendicular distance between the lines of action (M = F × d). Moment (or torque) describes that rotational effect, but the term that names the two forces together is a couple.

Rolling resistance is the small opposing force that a tire experiences as it deforms and recovers while rolling on a surface. For ordinary car tires on concrete, this resistance is relatively modest because the tire’s deformation losses are moderate on a hard, smooth surface. The coefficient describing this rolling resistance typically lies around 0.01 to 0.02; the range 0.01 to 0.015 fits right in this practical band and reflects common tires and concrete conditions. In practice, the resisting force is F = Crr × N, where N is the normal load; for a typical car, this yields a few hundred newtons of resistance, which aligns with real-world drivetrain requirements and fuel efficiency. Values much higher would imply unrealistically large losses for smooth concrete, while values far lower would underestimate the energy lost to tire deformation.

Gravity on the Moon is about 1.6 m/s^2. This value is roughly one-sixth of Earth’s gravity, since Earth’s surface gravity is about 9.8 m/s^2. The Moon’s smaller mass and size mean its surface gravity is weaker, and the standard rough figure is g ≈ 1.6 m/s^2. So the option around 1.6 m/s^2 is the best match. The other numbers are farther from the actual lunar gravity. This also explains why objects weigh about six times less on the Moon, even though their mass remains the same.