EnergyEnergy

EnergyEnergy Universe is made up of matter

and energy. Energy is the mover of matter. Energy has several forms: – Kinetic– Potential– Electrical– Heat– etc.

WorkNow instead of a force for how long

in time we consider a force for how long in distance.

Work = Force Distance W = F d

The unit for work is the Newton-meter which is also called a Joule.

Work

An applied force acting through a distance parallel to the force

Requires movement in same direction as force– Zero distance, no work– Displacement

perpendicular to applied force, no work

How much work?

A weight lifter lifts a barbell weighing 1000 Newtons 1.5 meters

A weight lifter pushes on a wall with a force of 1000 Newtons for 15 sec?

Power

Power is equal to the amount of work done per unit time.

interval time

donework Power

The unit for power is the Joule/second which is also called a Watt.

Power

The rate at which work is done

Units: watts (W), horsepower (hp)

Example: Walking versus running upstairs

The “power bill” - you pay for energy

Light Bulbs and Appliances

How much energy does a 100 Watt light bulb use in one hour?

How about a 40 Watt light bulb?

interval time

usedenergy electricalRatingPower

Mechanical EnergyWhen work is done on an object, the

object generally has acquired the ability to do work.

This "ability to do work" is called energy and it has the same units as work….Joules.

Two Types of Mechanical Energy– Potential Energy and Kinetic Energy

Potential Energy

The energy that is stored is called potential energy.

Examples: – Rubber bands– Springs– Bows– Batteries– Gravity

Gravitational Potential Energy

PE = Weight height

PE = m g h

Question:– How much potential energy does a

10kg mass have relative to the ground if it is 5 meter above the ground?

Kinetic EnergyKinetic Energy is the energy of

motion.

Kinetic Energy = ½ mass speed2

2mv2

1KE

Question: How much kinetic energy does a 1kg mass have if it is moving at 10 meters/second?

Work/Energy RelationshipIf you want to move something, you

have to do work.

The work done is equal to the change in kinetic energy.

Work = KE

Example Question

When the brakes of a car going 90 km/h are locked, how much farther will it skid than if the brakes lock at 30 km/h?

• (a) 2 times• (b) 3 times• (c) 9 times• (d) 27 times• (e) half as far

Conservation of EnergyEnergy cannot be created or destroyed......it may be transformed from one form

into another......but the total amount of energy never

changes.

Demos– Galileo's incline– Bowling ball pendulum

Pendulum Example Problem

At what point is the kinetic energy the highest?– (A) At the bottom of its path– (B) At the ends of its path– (C) Somewhere in between

Pendulum Example Problem

At what point is the potential energy the highest?– (A) At the bottom of its path– (B) At the ends of its path– (C) Somewhere in between

Example Problem

A 100 kg mass is dropped from rest from a height of 1 meter.

How much potential energy does it have when it is released?

How much kinetic energy does it have just before it hits the ground?

What is its speed just before impact?How much work could it do if it were to

strike a nail before hitting the ground?

100 kg

100 kg

100 kg

1 meter

0mvKE 221

J980)m1)(s/m8.9)(kg100(mghPE 2

Joules 980mvKE 221

Joules 0mghPE

Joules 980DistaceForceDone Work nail

Machines - An Application of Energy Conservation

If there is no mechanical energy losses then for a simple machine...

work input = work output

(F d)input = (F d)output

Examples - levers and tire jacks

Efficiency

Useful energy becomes wasted energy with inefficiency.

Heat is the graveyard of useful energy.

usedenergy

donework fficiencyE

Comparison of Kinetic Energy and Momentum

Kinetic energy is a scalar quantity.

Momentum is a vector quantity.

Example Questions

A 10 lb weight is lifted 5 ft. A 20 lb weight is lifted 2.5 ft. Which lifting required the most work?

(a) 10 lb weight(b) 20 lb weight(c) same work for each lifting(d) not enough information is given to work

the problem

An object of mass 6 kg is traveling at a velocity of 30 m/s. How much total work was required to obtain this velocity starting from a position of rest?

(a) 180 Joules

(b) 2700 Joules

(c) 36 Joules

(d) 5 Joules

(e) 180 N

A 20 Newton weight is lifted 4 meters. The change in potential energy of the weight in Newton.meters is

(a) 20

(b) 24

(c) 16

(d) 80

(e) 5

An object of mass 2 kg is traveling at a velocity of 30 m/s. How much total work was required to obtain this velocity starting from a position of rest?

(a) 180 Joules

(b) 2700 Joules

(c) 36 Joules

(d) 5 Joules

(e) 900 Joules

Example QuestionsA 10 lb weight is lifted 5 ft.

A 20 lb weight is lifted 5 ft.

Which lifting required the most work?

(a) 10 lb weight(b) 20 lb weight(c) same work for each lifting(d) not enough information is given to work

the problem

A 1 kg mass is lifted 1 meter. The change in potential energy of the weight is

(a) 1 N

(b) 9.8 N

(c) 1 J

(d) 9.8 J

(e) 9.8 m/s2