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. <em>If the object is moving then it is a dynamic equilibrium and if the object is not moving, then there is a stationary equilibrium.</em>
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.
<em>So in this case it must be a dynamic equilibrium.</em> 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.
