UNIT 5

WORK AND ENERGY

FÍSICA 1 BATXILLERAT

Work (I)

The work (W) done by a force that acts on a body in motion is the product of the force in

the direction of the displacement and the displacement.

The SI unit of work is the joule (J):

1 J = 1 N × 1 m

Work (II)

Motor or drive work:

0 ≤ α ≤ 90°

Positive work: cos α > 0

Resistance or resistive

work:

90°≤ α ≤ 180°

Negative work: cos α < 0

Null work:

α = 90°; cos 90°= 0

Work (III)

Necessary conditions for performing work

That a force acts on a body. That the body moves. That the direction of motion is not perpendicular to the force.

If more than one force acts on a body:

Graphical interpretation of work

The force is constant: The force varies linearly with the distance:

The force is variable, not linear:

By calculating the shaded

area, we obtain the work.

Power

Power (P) is the work performed per unit time.

The SI unit of power is the watt (W):

1 W = 1 J / 1 s

Efficiency

Instantaneous powerMean power

Pu: useful power.

Pc: power consumed.

Energy

Energy (P) is the capacity a body has to carry out work.

The SI unit of energy is the joule (J).

Kinetic energy

Kinetic energy (Ec) is the energy

that a body possesses from the fact

of being in motion.

Theory of work and kinetic energy: the work done by

a resultant force acting on a body is converted into the

kinetic energy of the body.

Conservative and non-conservative forces

Conservative forces are forces for which

the work done depends only on the initial

and final points, independently of the path

followed.

Examples:

Conservative forces: weight, electrical forces, elastic forces.

Non-conservative forces: muscular force, friction, etc.

For a conservative force, W = 0 if

the initial and final points are the

same.

Potential energy (I)

The change in potential energy of a particle is the work, with the opposite sign, done

by a conservative force on the particle:

W = –ΔEp

Potential energy is the energy that a particle has as a result of its position in a zone of

space in which conservative forces apply.

Example:

Potential energy (II)

Gravitational potential energy is the

energy that a particle has as a result of its

position in a zone of space in which

gravitational forces apply:

Ep = mgh

Elastic potential energy is the energy that

an elastic body has as a result of its

state of deformation.

The SI unit for gravitational and elastic potential energy is the joule (J).

Mechanical energy

Mechanical energy is the sum of all the energies that a body can have: kinetic,

gravitational potential, elastic potential, etc.

E = Ec + Ep

The SI unit for mechanical energy is the joule (J).