Work

Kinetic Energy

Potential Energy

Energy

Work is done whenThere is an application of a forceThere is movement of something by that force

Work = force x distance (W=Fd)SI Unit = Joule (J) = Newton meter (Nm)

Work

Work-Energy relationshipWhenever work is done on an object, its energy

is changedWhenever energy is changed from one form to

another work is doneWork-Energy Theorem

Change in energy = work done

Work and Energy

Gravitational Potential Energydue to the height of an object with mass

Elastic Potential Energy The energy in springy things

Kinetic Energydue to motion

Energy is measured in Joules (J)

Forms of Mechanical Energy

Kinetic EnergyThe energy of an object that is due to the object’s motion is called kinetic energy.

Kinetic energy depends on speed and mass.

2

2

1

21

kinetic energy = mass speed2

KE mv

Kinetic Energy – It’s moving

A dog can run at a speed of 16.0 m/s. What is the KE of the dog if its mass is 20.0-kg?

Kinetic Energy problem

Potential Energy

Potential Energy is the energy associated with an object because of the position, shape, or condition of the object.Gravitational potential energy is the potential energy stored in the gravitational fields of interacting bodies.Gravitational potential energy depends on height from a zero level.

PEg = mghgravitational PE = mass free-fall acceleration

height

Legend has it that Isaac Newton “discovered" gravity when an apple fell from a tree and hit him on the head. If a 0.20-kg apple fell 7.0 m before hitting Newton, what was its change in PE during the fall?

GPE Problem

The total amount of energy in a system is constant in a closed, isolated system.

The energy can change form.Energy is neither created nor destroyed.Kbefore + Ug before = Kafter + Ug after

Law of Conservation of Energy

Identify the relevant points in the problemAt one point is you know enough to calculate the

energy, at the other point you want to know something.

Write down the conservation of energy formula

Write down the forms of energy at each point.

Substitute in the formulas for each type of energy.

Substitute numbers with units.Solve for the answer – check units.

Solving conservation of energy problems

Maya is changing the tire of her car on a steep hill 20.0 m high. She trips and drops the 10.0-kg spare tire, which rolls down the hill with an initial speed of 2.00 m/s. What is the speed of the tire at the top of the next hill, which is 5.00 m high? (Ignore the effects of rotational KE and friction.)

Conservation of Energy Problem

Momentum is always conserved in a collision if there is no outside force.

Mechanical Energy is also conserved in an elastic collision.pinitial = pfinal ; KEinitial = KEfinal

Mechanical Energy is not conserved in an inelastic collision.pinitial = pfinal ; KEinitial > KEfinal

Energy in a Collision