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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