The two parents shown in (Figure 1) exert upward forces of Fi = 18 N on the left end of the plank and F2 = 65 N
on the right end.

How much does the child weigh?

The weight of the child is equal to 83 Newton.

Given the following data:

To determine the weight of the child:

From the diagram, we can deduce that the system is in static equilibrium. Thus, we would resolve the horizontal component of the forces, in order to determine the downward force (weight of child).

Mathematically, the horizontal component of the forces is given by:

Substituting the given parameters into the formula, we have;

Note: Weight = mg

Weight of child = 83 Newton.

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W = 83 [N] (weight)

Explanation:

We must clarify that the system is in static equilibrium, in such a way that we must perform a sum of forces on the Y-axis equal to zero, in order to determine the downward force corresponding to the weight of the child.

∑Fy = 0

And the mass can be determined as follows:

speed of light in vacuum is 300,000 km/s

speed of light in other mediums is given by the expression v = c/n, where c is the speed of light in vacuum and n is the refractive index of medium.

for the first medium v = 200,000 km/s

  so, refractive index of medium = 300000/200000 = 1.5

    1.5 refractive index is usually for glass. so first medium is glass.

for the second medium v = 250,00 km/s

so, refractive index of medium = 300000/250000 = 1.2

    1.2 refractive index is usually for water. so second medium is water.

for the third medium v = 0 km/s

so, refractive index of medium = 300000/0 = ∞

    ∞ refractive index is usually for opaque mediums. so third medium is opaque medium.