Buy that special someone an AP Physics prep book: 5 Steps to a 5: AP Physics 1

Visit Burrito Girl's handmade ceramics shop, The Muddy Rabbit: Yarn bowls, tea sets, dinner ware...

30 October 2013

Can a normal force do work?

At the first-year physics level -- absolutely a normal force can do work.  If an object moves parallel (or antiparallel) to a normal force, then the normal force has done work.  

But how can that be?  A block slides to the left on a flat surface, say.  The normal force is upward, movement is left, so the motion is not parallel to the normal force.  No work is done by Fn.

That's correct.

Even for a block sliding up or down an incline, the normal force does no work.  The normal force by definition is perpendicular to the surface, and the block slides along the surface; no component of the normal force is parallel to the motion.

That's correct, too.

So how in the sam hill can a normal force possibly do any work?

What if the surface itself is moving?

Consider a person standing in an elevator.  The normal force is the force of the elevator floor on the person.  As the elevator moves upward, so does the person... so the normal force is parallel to the person's motion, doing work.  If the elevator moves downward, the normal force is antiparallel to the motion, and so does negative work.

Now I leave you with this... what if the elevator is slowing down as it moves upward, such that the normal force is less than the man's weight.  Does the normal force do positive work, negative work, or zero work?  I'll answer in a few days in the comment section.

5 comments:

  1. You tried to trick us with that last question. Whether an individual force does positive or negative work only depends on direction of motion. Since the normal force and the direction of motion both point up, we get a positive work done by the normal force. However, since the force of gravity is doing a larger negative work, the net work is negative and net work is what equals a change in kinetic energy.

    ReplyDelete
  2. Ah, well... There are no "trick" questions in physics, just *tricky* questions. Michael, you nailed it. It's a common student misconception to conflate motion with acceleration when calculating the sign of work, or to conflate net work with work done by a specific force.

    GCJ

    ReplyDelete
  3. Also, doesn't the normal force do work on an object that bounces into and then rebounds off a surface. Initially negative work to bring it to rest then positive work during rebound to give it kinetic energy again?

    ReplyDelete
  4. Yes to that, too... It's difficult to describe clearly the distance through which the force acts. I've always considered the displacement of the ball's center of mass during the collision, when the ball compresses; this model is consistent with your observation, James, and is sufficient for introductory physics. This is a very confusing question when investigated at a deeper level, because the point of contact doesn't move relative to the surface... or does it?

    ReplyDelete
  5. I've been curious about a situation involving normal force doing work for a while, maybe you can help me straighten it out:

    we know if a block slides down a frictionless ramp, the normal force does not do any work since it is perpendicular to the motion of the block.

    And, as you pointed out, if the surface moves, then normal force might do work. For example: if a block slides down a frictionless ramp and the ramp itself is on a frictionless horizontal surface, then the normal force from the block on the ramp would do some work. (a component of that force must be parallel to the motion of the ramp, causing it to slide backwards as the block falls).

    The issue I'm having is in understanding whether normal force would be conservative or non-conservative.

    I think it must be nonconservative since the path the block takes determines how much work is done: if the block slides down, then some work is done. If the block slides down, then back up, then back down again, more work would be done. Plus, we know the mantra "the only conservative forces in AP Physics 1 are gravity and spring force"

    Now, if it's a nonconservative force, then I don't think energy would be conserved in this case which sounds odd. Here's my reasoning:
    If we take the system to include the block, the ramp, and the earth, the forces acting in and on the system would be as follows:
    - gravity on the block (does work but is conservative)
    - gravity on the earth (doesn't really do any work since the earth doesn't really move at all, it's conservative anyway)
    - gravity on the ramp (does no work since the ramp does not move vertically)
    - normal force from the ramp on the block (no work since it's perpendicular to the block's motion)
    - normal force from the block on the ramp (yes work is done and I think it's nonconservative)
    - normal force from the horizontal surface on the ramp (does no work since the ramp does not move vertically)

    This gives us a single, nonconservative force doing work in the system. We know that work done by nonconservative forces changes the mechanical energy of the system so energy can't be conserved.

    I've seen numerous examples that claim energy WOULD be conserved in cases like these and that makes sense from a logical standpoint. But, based on our definitions for work and energy, I don't see how that can be true. Am I missing another nonconservative force? Would the normal force be considered conservative here? It seems more like an elastic collision than anything else.

    I know we can use conservation of momentum to analyze situations like this but the normal force has me thrown for a loop. Any ideas?

    ReplyDelete