Physics
Physics
Problem
4.2 A pebble of mass 0.05 kg is thrown vertically upwards. Give the direction and magnitude of the net force on the pebble, (a) during its upward motion, (b) during its downward motion, (c) at the highest point where it is momentarily at rest. Do your answers change if the pebble was thrown at an angle of with the horizontal direction? Ignore air resistance.Intuitive explanation
The key insight is that gravity is the only force acting on the pebble once it leaves your hand—it's constant in magnitude and direction regardless of the pebble's motion. Don't confuse the direction of velocity with the direction of force; the net force depends only on what's pulling on the object, not how fast or which way it's moving. Whether the pebble goes up, down, or pauses momentarily, the gravitational force points downward and never changes—the angle of launch only affects the trajectory, not the forces at play.
Intuitive explanation
The key insight is that gravity is the only force acting on the pebble once it leaves your hand—it's constant in magnitude and direction regardless of the pebble's motion. Don't confuse the direction of velocity with the direction of force; the net force depends only on what's pulling on the object, not how fast or which way it's moving. Whether the pebble goes up, down, or pauses momentarily, the gravitational force points downward and never changes—the angle of launch only affects the trajectory, not the forces at play.