AP®PhysicsCollege BoardPhysics 1: Algebra-BasedFlashcardsUnit 3: Work, Energy & PowerTranslational Kinetic Energy & Potential Energy

Translational Kinetic Energy & Potential Energy (College Board AP® Physics 1: Algebra-Based): Flashcards

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  • Define translational motion.

    Translational motion is motion without rotation, in which every point on the object moves with the same velocity.

  • Define translational kinetic energy.

    Translational kinetic energy is the energy that an object has due to its translational motion.

  • State the equation for translational kinetic energy.

    K = \frac{1}{2} m v^{2}

    • K = translational kinetic energy (J)

    • m = mass (kg)

    • v = velocity (m/s)

  • How does kinetic energy change if the velocity doubles?

    It quadruples, because translational kinetic energy is directly proportional to the square of the velocity.

  • Why does the direction of an object's velocity not affect its translational kinetic energy?

    Translational kinetic energy is a scalar quantity with magnitude only, so only the magnitude of the velocity is needed.

  • For a fixed velocity, translational kinetic energy is directly proportional to the .......... of the object.

    For a fixed velocity, translational kinetic energy is directly proportional to the mass of the object.

  • True or False?

    The translational kinetic energy of an object is the same in every frame of reference.

    False.

    Velocity is relative, so the translational kinetic energy of an object is also relative. Observers in different frames measure different values.

  • When does a system have potential energy?

    When both of these are true:

    • the system is composed of two or more objects

    • the objects interact with each other only through conservative forces

  • What is potential energy associated with?

    The positions of objects within a system. It is independent of motion.

  • What does a change in potential energy measure?

    The work done on the system or by the system.

  • True or False?

    The change in potential energy depends on the path taken between two positions.

    False.

    The work done by a conservative force is path-independent, so the change in potential energy is also path-independent.

  • How is the point of zero potential energy chosen?

    It is not absolute. The observer defines it to simplify the analysis, generally at the starting position of the object.

  • Potential energy is a .......... quantity with magnitude only.

    Potential energy is a scalar quantity with magnitude only.

  • What happens to the potential energy of the object–Earth system if a shelf holding the object is removed?

    The gravitational force makes the object fall, and the system's potential energy is transformed into the object's translational kinetic energy.

  • Define elastic potential energy.

    Elastic potential energy is the energy stored within a material (e.g. a spring) when it is stretched or compressed.

  • State the equation for the elastic potential energy of an ideal spring.

    U_{s} = \frac{1}{2} k (∆x)^{2}

    • U_{s} = elastic potential energy (J)

    • k = spring constant (N/m)

    • ∆x = distance stretched or compressed from the equilibrium length (m)

  • How does elastic potential energy change if the extension doubles?

    It quadruples, because U_{s} is proportional to (∆x)^{2}.

  • True or False?

    A compressed spring has negative elastic potential energy.

    False.

    The extension or compression is squared, so elastic potential energy can never be negative.

  • Why is a suddenly breaking wire under large stress dangerous?

    Its stored elastic potential energy is converted into kinetic energy. The greater the extension, the greater the speed when it breaks (v \propto ∆x).

  • Elastic potential energy is a .......... quantity with magnitude only.

    Elastic potential energy is a scalar quantity with magnitude only.

  • State the equation for gravitational potential energy change near a surface.

    ∆U_{g} = m g ∆y

    • ∆U_{g} = change in gravitational potential energy (J)

    • m = mass of the object (kg)

    • g = gravitational field strength (N/kg)

    • ∆y = vertical distance moved (m)

  • Why is gravitational potential energy near a surface calculated as a change?

    Gravitational potential energy and height are relative. The zero point is chosen to suit the situation, so only the change in gravitational potential energy is calculated.

  • What happens to the gravitational potential energy of a system when an object is lifted and when it is lowered?

    • Lifted: the system gains gravitational potential energy

    • Lowered: the system loses gravitational potential energy

  • True or False?

    The equation ∆U_{g} = m g ∆y is valid at any distance from a planet.

    False.

    The equation assumes the gravitational field is constant, which holds only near the planet's surface.

  • The gravitational force is a .......... force, so gravitational potential energy is path-independent.

    The gravitational force is a conservative force, so gravitational potential energy is path-independent.

  • How can the speed of a falling object be found from its change in gravitational potential energy?

    Equate the loss in gravitational potential energy to the gain in kinetic energy (negligible air resistance):

    m g ∆y = \frac{1}{2} m v^{2}

    v = \sqrt{2 g ∆y}

  • State the equation for the gravitational potential energy of two spherical masses.

    U_{g} = - G \frac{m_{1} m_{2}}{r}

    • U_{g} = absolute gravitational potential energy (J)

    • G = universal gravitational constant (6.67 × 10-11 N·m2/kg2)

    • m_{1}, m_{2} = masses of the two objects (kg)

    • r = separation between the centers of mass (m)

  • Why is the absolute gravitational potential energy of two masses always negative?

    The zero point is defined at infinite separation, where the gravitational potential energy is at its maximum. At any finite separation the value is lower, so it is negative.

  • What happens to the gravitational potential energy of two masses as their separation increases?

    It increases, becoming less negative, toward its maximum value of zero at infinite separation.

  • As two masses are moved farther apart, work is done .......... the gravitational field.

    As two masses are moved farther apart, work is done against the gravitational field.

  • True or False?

    Doubling the separation of two masses doubles their gravitational potential energy.

    False.

    U_{g} is inversely proportional to r, so doubling the separation halves the magnitude of U_{g}. The value becomes less negative.

  • How is the total gravitational potential energy of a system with more than two objects found?

    It is a scalar, so the total is the sum of the gravitational potential energies of each pair of objects.

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