Unit 6: Energy - Ch. 9

Energy

• the ability of an object to cause change in its surroundings

• it makes things happen in the universe

When can we observe the effect of energy

When energy is:

1. Transported

2. Transferred

3. Transformed

Work

• The product of a force acting on an object and the distance through which the object is moved parallel to the force:

  • work = force × distance (𝑊 = 𝐹×𝑑)

• When work is done on an object, its velocity changes.

• The unit of work is the newton-meter (𝑁𝑚), also

  called the joule (𝑱).

  • [𝑊] = [𝐹] × [𝑑] = 𝑁 ⋅ 𝑚 = 𝑱

Positive work is done

The force is parallel to the motion (angle between force and motion = 0°)

Negative work is done

The force is opposite to the motion (angle between force and motion = 180°)

Power

• The rate at which work is done.

• Equals the amount of work done divided by the time interval during which the work is done.

  • Power = (work done) ÷ (time interval) => P = W ÷ t

• The unit of power is the joule per second, also known as the watt (𝑾) where 1 𝑊 = 1 𝐽/𝑠

Mechanical Energy

Mechanical energy is energy an object has due to:

• Its motion (kinetic energy)

• Its position (gravitational potential energy)

• Its shape (elastic potential energy)

Potential Energy

• Energy that is stored and held in readiness is called potential energy (PE) because it has the “potential” for doing work.

• There are two kinds of mechanical potential energy: elastic potential energy and gravitational potential energy.

elastic potential energy

Energy an object has because it is stretched or squeezed out of its usual shape

Gravitational Potential Energy

The energy an object has because it can fall

PE_g = mgh

kinetic energy

A moving object can do work because it has energy of motion

depends both on the mass of the object and its speed.

KE = ½mv²

Work-Energy Theorem

states that it takes work to change the kinetic energy of an object.

𝑊 = Δ𝐾𝐸

Work causes a change in kinetic energy, just as impulse causes a change in momentum.

work done by friction

𝑊 = −𝑓𝑑 = − ½ 𝑚𝑣_i²

Law of Conservation of Energy

• Energy within the system may change form, but the total energy stays the same

• energy can never be created from nothing, nor destroyed into nothing.

𝐸_i = 𝐸_f

𝑃𝐸_i + 𝐾𝐸_i = 𝑃𝐸_f + 𝐾𝐸_f

Conservative forces

Can convert between kinetic and gravitational potential energy with no loss, 100% efficiency, so mechanical energy is conserved when conservative forces act.

Ex: Gravity

nonconservative forces

Some of the mechanical energy is changed into types that are not mechanical energy, such as thermal energy (heat)

Ex: friction