AP Physics - Work and Energy

Work

  • Work

    • The work done by a constant force acting on an object is equal to the product of the magnitudes of the displacement and the component of the force parallel to that displacement

    • Units

      • Force

        • N / lb. / dyne

          • Nm = Joule

          • Lb. ft = ft lb.

          • Dyne cm = erg

      • Displacement

        • m / ft / cm

    • Is a scalar

      • Can be positive or negative

        • Work ON system = positive

        • Work BY system = negative

    • NOT a vector

  • If the force is at an angle to the displacement, a more general form for the work must be used

Power

  • Power

    • The rate at which work is done

    • P = W / t

The Work-Energy Theorem: Kinetic Energy

  • Kinetic Energy

    • Energy in motion

Potential Energy

  • Potential Energy

    • Energy of position

    • Gravitational

      • PEg, Ug

      • The energy an object has based on its position within a gravitational field

    • Elastic

      • PEe, Ue, Us

      • Ue ∝ X²

      • Ue ∝ K (spring constant)

        • Elasticity: stiffness

      • Ue = ½ K X²

Conservation of Energy

  • Law of Conservation of Energy

    • The total energy of an isolated system is always conserved

  • Conservative Force

    • A force is said to be conservative if the work done by it in moving an object is independent of the object’s path

  • Non-Conservative Force

    • A force is said to be nonconservative if the work done by it in moving an object does depend on the object’s path

      • Friction