Using the free-body diagram, calculate the net force acting on the sled. Is the sled in a state of dynamic equilibrium?

The net force acting on the sled is zero. The sled is in a state of dynamic equilibrium.

Explanation:

The two vertical forces, Fg and Fn, cancel each other out, but I choose to "ignore" them in my explenation, although they remain present at all times.

Since the net force acting on the sled is zero, the sled is in some state of equilibrium...

Whether the sled or a stationary equilibrium or in a dynamic equilibrium, depends on the the motion of the object. If the object is moving then it is a dynamic equilibrium and if the object is not moving, then there is a stationary equilibrium.

Look closely at the picture of the sled and the dogs, and try to find out if the sled is standing still or if the sled is moving with a constant speed.

From the movement of snow around the dogs, you can tell, the sled is moving. It clearly is not stationary or left standing still.

So in this case it must be a dynamic equilibrium. The combined pull of all dogs is shown by Ft = 225 N.

Apparently, the friction is displayed as an opposite force shown as Ff = - 225 N. These two horizontal forces cancel each other out.

Strangely enough, this can be compared with a situation like a "tug of war", in which both sides pull with equal force. Although there can be an enormous tension in the cord, nothing moves The same is true here although the sled is moving with a constant speed...

EXTRA

Given is that the sled is moving with a constant speed.

No imagine two situations and try to predict what will happen Here we go...

Situation 1 The friction force remains Ff = - 225 N but the dogs suddenly start to pull harder, say with a force of 275 N.

Now the net Force will be greater then zero. In that case it will be 275 - 225 = 50 N in favour of the dogs, which would result in an exeleration of the sled, and thus an increase of the speed...

Situation 2 The friction force becomes greater Ff = - 300 N, but the dogs pull with the same force of 225 N.

Now the net Force will be less then zero. In that case it will be 225 - 300 = -75 N in favour of the friction Force, which would result in slowing down the sled. Perhaps the sled operator applied the brake on the sled, which caused a deceleration of the sled, and thus the speed decreases.

electromagnetic shielding is the practice of reducing the electromagnetic field in a space by blocking the field with barriers made of conductive or magnetic materials. shielding is typically applied to enclosures to isolate electrical devices from their surroundings, and to cables to isolate wires from the environment through which the cable runs. electromagnetic shielding that blocks radio frequency electromagnetic radiation is also known as rf shielding.

the shielding can reduce the coupling of radio waves, electromagnetic fields and electrostatic fields. a conductive enclosure used to block electrostatic fields is also known as a faraday cage. the amount of reduction depends very much upon the material used, its thickness, the size of the shielded volume and the frequency of the fields of interest and the size, shape and orientation of apertures in a shield to an incident electromagnetic

from wikipedia