PY 131 Chapter 7: Energy

What is the precise definition of work in physics?

The work done by a force on an object is the product of the distance moved by the object and the component of the force in the direction of the object's displacement.

What is the SI unit of work?

The SI unit of work is the joule – symbol J.

Can a force do negative work?

Yes, a force can do negative work if it is directed opposite the displacement.

What is the work-energy principle?

The net work done on an object is equal to the change in the object's kinetic energy.

What are the SI units of kinetic energy?

The SI units of kinetic energy are joules.

What is the equation for power?

Power = work done / time interval.

What is the SI unit of power?

The SI unit of power is the watt, W, or J/s.

What is the work-energy principle for an object that only changes direction?

Objects whose velocity only changes direction do not change their kinetic energy.

What is the distinction between conservative and nonconservative forces?

Conservative forces are forces whose work is independent of the path taken between two points, while nonconservative forces are forces whose work is dependent upon the path taken.

Name three examples of conservative forces.

Gravity, electric (Coulomb) force, and spring (elastic) force.

Name three examples of nonconservative forces.

Tension, friction, and air resistance.

What is potential energy?

Potential energy is the energy possessed by an object due to its position in a force field.

What is the associated potential energy for every conservative force?

Every conservative force has an associated potential energy U.

What is the change in potential energy when an object is moved from an initial location to a new location?

The change in potential energy is the negative of the work done by the force moving between those positions.

What is the work done by gravity when an object with mass m is raised to a height h above the floor?

The work done by gravity is W_G = -mgh.

What is the gravitational potential energy of an object with mass m raised to a height h above the floor?

The gravitational potential energy of the object is U_G = mgh.

What is the conservation of mechanical energy?

The total mechanical energy, E = K + U, of an object is a constant when only conservative forces do work upon it.

What is the equation for the net work done by all conservative forces acting upon an object?

W_net = -ΔU = -U_final + U_initial.

What is the equation for the conservation of mechanical energy when only conservative forces do work upon an object?

K_final + U_final = K_initial + U_initial.

What is the total mechanical energy of an object?

The total mechanical energy of an object is the sum of its kinetic energy and potential energy.