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⚛️ NEET Physics❓ Question

Does friction always do negative work? Also why is work zero when I hold a heavy bag still?

Ananya Reddy11th Sample

29 Sept 2026 · 1 min read

Work, Energy and Power. I made a table of "who does positive / negative / zero work" and several rows don't feel right to me.

W=F⃗⋅d⃗=Fdcos⁡θW = \vec F\cdot\vec d = Fd\cos\theta. I get:

  • Lifting a book at constant speed by hh: work done by me is +mgh+mgh, by gravity is −mgh-mgh, net zero. This fits the work-energy theorem (no change in KE).
  • Holding a heavy bag still for a minute: displacement zero so W=0W=0, even though my arm feels tired. OK. Though it feels wrong.
  • Walking with a bag horizontally: the force from my hand is upward and displacement horizontal, so θ=90∘\theta=90^\circ and W=0W=0.

In my notes I wrote "friction always does negative work, that is why it dissipates energy". But then I saw a question: a box on the floor of a truck that accelerates forward without the box slipping. What does friction do to the box? It's the only horizontal force, it accelerates the box, so it must be doing positive work on it. That contradicts my line.

Also is work done by a centripetal force always zero?

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  • [+]Aman YadavDropperSample· 20d ago

    Friction can do positive, negative or zero work. Your truck example: static friction on the box is forward (the direction of the truck's acceleration) and the box moves forward relative to the ground, so W>0W>0. It speeds the box up. Kinetic friction when a block slides on a floor opposes sliding, usually negative work on the sliding body. Zero when the contact point isn't moving (a rolling wheel).

    • [+]Dev Agarwal12thSample· 20d ago

      Hmm so for the truck box, relative to the truck the box doesn't move. Is the work then zero in the truck's frame?

      • [+]Aman YadavDropperSample· 20d ago

        In the truck's frame the box is at rest, and friction is balanced by the pseudo force, so net work zero. In the ground frame it's positive. Work depends on the frame because displacement does. The work-energy theorem holds in whichever inertial frame you choose and the truck is accelerating, so the ground is the clean frame to use.

    • [+]Ananya Reddy11thSample· 20d agoAuthor

      Updated the table: friction can be positive, negative or zero. Dropping "always negative" from my notes. Thank you!

  • [+]Ishaan Bose11thSample· 20d ago

    Holding a bag: in physics, work needs displacement. Your muscles do burn energy though because muscle fibres keep contracting and relaxing internally, so energy goes out as heat even if the bag doesn't move. Physics work on the bag is zero, but biology of your arm is a different thing. Same reason pushing a wall until you're sweaty gives you zero work on the wall.

  • [+]Meera Patel11thSample· 20d ago

    Centripetal force is always perpendicular to velocity in uniform circular motion, so W=F⃗⋅v⃗ dt=0W=\vec F\cdot\vec v\,dt=0. KE stays constant. Similarly the magnetic force on a charge and the normal force on a block sliding on a fixed surface do zero work. If the circle is non-uniform there is a tangential force too, and that part does work.

  • [+]Kabir Joshi11thSample· 20d ago

    Quick trick for graph questions: work is the area under a force-displacement graph (signed). For a spring, W=∫kx dx=12kx2W=\int kx\,dx=\tfrac12kx^2. For a variable force F(x)F(x) in 1D, W=∫F dxW=\int F\,dx. NEET likes to give a straight-line F vs x and ask for the area of a trapezium.

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