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 45° with the horizontal direction?
Ignore air resistance.
According to the Newton’s second law of motion:
F = m × a
Where,
‘F’ is the net force acting on an object.
‘m’ is the mass of the object.
‘a’ is the acceleration of the object.
Given,
Mass of the pebble ‘m’= 0.05 kg
a = acceleration due to gravity = 10 m/s2
In all the three cases A, B and C the only force acting on the pebble is that of gravitational force in the downward direction. Irrespective of the motion of an object, the acceleration due to gravity always acts downwards.
The magnitude of the force due to gravity will be,
F = 0.05 kg × 10 m/s2
= 0.5 N
Thus, the net force acting on the pebble ignoring the air resistance in upward or downward motion is 0.5 N and this gravitational force always acts downwards towards the centre of the earth.
The only difference if a pebble is thrown at an angle of 45° with the horizontal is the change in its velocity as it travels all the way up where the vertical velocity becomes zero at the highest peak but the horizontal velocity remains. The acceleration due to gravity is still acting constantly downwards in this case too and ignoring the air resistance. Thus, the net force still remains the same.