A 25 kg box is being pulled at a constant velocity with a tension force of 65 N. what is the coefficient of friction between the box and the ground?

The box is in equilibrium, so Newton's second law says

n + (-w) = 0

65 N + (-f ) = 0

where n denotes the magnitude of the normal force, w denotes the weight of the box, and f denotes the magnitude of the friction force.

The box has a weight of

w = (25 kg) (9.80 m/s²) = 245 N

so n = 245 N, too.

The friction force has magnitude

f = 65 N

and is proportional to the normal force by a factor of µ, the coefficient of kinetic friction. So we have

65 N = µ (245 N)   →  µ ≈ 0.26

most voltage drop calculators want the distance between the source and destination. then internally, they double it so both the source and return wires are calculated. but you need to know which distance the calculator uses so you're not half of what you should be or double what you should be.

most circuit breakers will not take a #6 directly, so the electrician will have to splice on a smaller pigtail. you also have to upsize the grounding wire when you upsized the ungrounded (hot) wire (see note in 250.122). this will preclude using most cable assemblies, as a cable with #6 copper will most likely only have a #10 grounding conductor. you need a #6 grounding conductor if the circuit is 20a. a #8 cable is no better, as it will have a #10 grounding conductor too. but most 20a breakers will take a #8 directly.

in your related post, you didn't say what type of load this is. if it is a light or heater, voltage drop won't be a big issue. if this is a motor, the current needs are huge at startup and the voltage drop can pose a problem. if this is a motor, knowing if the motor is under much load during startup will answer the voltage drop effect (a saw is easier to start than a compressor).

. west