Achild lifts a box from the floor. the child then carries the box with constant speed to the other side of the room and puts the box down. how much work does he do on the box while walking across the floor at a constant speed?

Zero

Explanation:

Given child moves the box with constant velocity over his head .

Force acting on box due to gravity is equal to his weight

Displacement of the box is Perpendicular to the direction of Applied force

Work done is given by

here is the angle between Force and displacement which is

so work done is zero

can be answered using momentum.

momentum = p

p1= mass1 * velocity1

p1 = p2

therefore

p1 = 6kg * 4m/s

p1 = 24kgm/s

p2=m2 * v2

24 kgm/s = 7kg * v2

v2 = 24 kgm/s / 7kg

v2= 3.42 m/s