The Work-Energy Theorem: Net Work Is the Change in Kinetic Energy

A block on a level floor. You push it horizontally with a force F over a distance d, against kinetic friction μ, and it may already be moving at v1. Every force gets its own work term and friction's term is negative; add them and you have the net work. The bars show that the net work and the change in kinetic energy are one bar drawn twice — that is the theorem, and it holds just as well when the net work is negative and the block ends up slower than it started. Because the push is horizontal the normal force is exactly m·g, so friction is μ·m·g; angle the push and none of these numbers survives.

KE at the start0.0 J
KE at the end122.3 J
Change in KE122.3 J
Friction force μmg19.6 N
Net force F − μmg20.4 N

What happensNet work 122.3 J, change in kinetic energy 122.3 J.

The push is horizontal, so the normal force is exactly m·g and friction is μ·m·g. One coefficient of friction is used for the sliding block, so the “nothing moves” case is a statement about this model and not about a real floor, which needs a larger force to break a body loose than to keep it sliding.
Work by your push  Wpush = F · d
240.0 J
Work by friction  Wfric = -μmg · d
-117.7 J
Net work  Wnet = Wpush + Wfric
122.3 J
Final speed  v2 = sqrt(v12 + 2Wnet/m)
5.53 m/s
Mass · m8.0 kg
Your push · F40 N
Friction coefficient · μ0.25
Push distance · d6.0 m
Starting speed · v10.0 m/s
Gravity is fixed at g = 9.81 m/s2, the standard value rounded to three significant figures. It is defined, not measured; real local gravity spans about 9.78 to 9.83 m/s2.