Question

A pebble of mass 0.05 kg is thrown vertically upwards. Give the direction and magnitude of the net force on the pebble, 

1. during its upward motion. 

2. during its downward motion. 

3. at the highest point where it is momentarily at rest. Do your answers change if the pebble was thrown at an angle of 45° with the horizontal direction? Ignore air resistance.

Answer 1

1. When the pebble is moving upward the force acting on it is gravitational force in downward direction. F = mg = 0.05 × 10 = 0.5 N 

2. Even in this case F = mg = 0.5 N in downward direction. 

3. Since there is no force other than gravitational force acting on pebble, during the whole process F = mg = 0.5 N. Note that pebble moves in opposite direction because of its initial velocity. The situation remains same for pebble thrown at an angle.