(a) 392 N/m
Hook's law states that:
(1)
where
F is the force exerted on the spring
k is the spring constant
is the stretching/compression of the spring
In this problem:
- The force exerted on the spring is equal to the weight of the block attached to the spring:

- The stretching of the spring is

Solving eq.(1) for k, we find the spring constant:

(b) 17.5 cm
If a block of m = 3.0 kg is attached to the spring, the new force applied is

And so, the stretch of the spring is

And since the initial lenght of the spring is

The final length will be

The net force is 270 N
Explanation:
We can solve this problem by using Newton's second law, which states that the net force on an object is equal to the product between its mass and its acceleration:

where
F is the force
m is the mass
a is the acceleration
In this problem, we have
m = 90.0 kg

Substituting, we find the net force on the object:

Learn more about Newton's second law:
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Radiation fog is the fog that is formed when the heat absorbed the Earth's surface is released into the atmosphere producing fog. This only occurs when the air is clear and calm. In the center of an anticyclone, the conditions of the air are clear and calm which is favorable for the formation of radiation fog. The center of cyclones, on the other hand, is turbulent and cloudy which prevents the formation of radiation fogs.
That's called "refraction".