because of friction, a constant force of 20 N is needed to slide a box across a room to the right at a constant speed, and box is moved 3 m.

What is the friction force and how much work is done by friction force??

How much work is done by gravity?
How much work is done by normal force(upward)?

So, if v(speed) is constant, then acceleration=0 and net force=0????

Yes to your last question. Since it does not accelerate, the friction force equals the applied force, 20 N.

Since one can assume the floor of the room is level, gravity does no work. It acts perpendicular to the direction of motion. The same applies to the upward force.

The friction force does negative work (absorbs energy) equal to work done by the applied force, 20 x 3 = 60 J

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