To determine the child's overall velocity relative to the ground, you need to account for both the child's speed and the speed of the escalator.
Since the child is running up the escalator and the escalator is moving down, their relative velocities are in opposite directions.
Here's how to calculate the overall velocity of the child:
Child's speed (upwards): +2.8 m/s
Escalator's speed (downwards): -1.2 m/s (the negative sign indicates the opposite direction)
The overall velocity of the child relative to the ground:
= Child's speed + Escalator's speed
= 2.8 m/s - 1.2 m/s
= 1.6 m/s (upwards)
So, the child's overall velocity relative to the ground is 1.6 m/s upwards.