Notes

Energy (E), Measurement of the ability to do work. SI unit is the joule (J).

Work (W), Change in energy by the transfer of energy from one system to another. Scalar quantity with units of joules (J).

Joules (J), SI unit for energy. Applying a net force of 1N to an object over a displacement of 1m requires 1J of energy.

People forget that forces acting perpendicular to displacement do zero work. Since cos 90 =0 no work occurs when force F and displacement d are perpendiculars even though a perpendicular force can change the object’s direction of motion. For example, if a box is pulled across the level floor, the floor does zero work on the box even though the box is moving. This is because the normal force is perpendicular to the horizontal displacement of the box.

People forget what the sign of work means. Positive work on a system means it receives energy from its surroundings. Negative work on a system means it gave energy to its surroundings. Negative work occurs when the force has a component in the direction opposite the displacement.

Kinetic energy Energy from translational or rotational motion. SI units of joules (J)

Translational motion Linear motion of a rigid body. SI units of m/s

Translational kinetic energy (K), Energy due to linear motion of a rigid body. SI units of joules (J).

Kinetic energy can’t be negative, although the change in kinetic energy can

Some people think doubling speed means doubling the energy

Work-energy theorem Network done on an object equals the object’s change in kinetic energy. Also called the work-energy principle.