work, energy, & power (chapter 4). student learning objectives compare and contrast mechanical...
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Work, Energy, & Power
(Chapter 4)
Student Learning Objectives
• Compare and contrast mechanical work, energy, and power
• Name examples of simple machines
• Describe renewable energy sources.
What is mechanical work?
Mechanical work is the use of energy to move an object a distance.
Depends on total displacement, not path
http://www.physicsclassroom.com/mmedia/energy/au.cfm
W = Fd
Practice1) Is mechanical work a scalar or vector quantity? Explain.
2) How much work would be done on a 150 lb (667 N) barbell?
a) When it is lifted 2 ft (0.61 m)?
b) While it is held overhead?
c) When it is lowered 2 ft (0.61 m)?
3) What is the work done to lift a 15 kg child up one meter?
What is mechanical energy?
Mechanical energy is associated with objects that are in motion or may become in motion.
Object has ability to do mechanical work.
Kinetic Energy
Kinetic energy is the energy an object has when it is in motion.
The work done to change the motion of a moving object = DKE
KE = (½)mv2
Practice
Which has the most kinetic energy?
Which has the most momentum?
Which has the greatest inertia?
18-Wheel Truck 10,000 lbs (4535 kg) Parked
Football Player 300 lbs (136 kg) Running 10 mph (4.47 m/s)
Olympic Sprinter 150 lbs (68 kg) Running 22 mph (9.83 m/s)
Small Car 2,640 lbs (1,200 kg) Rolling (1 m/s)
Potential EnergyGravitational potential
energy is the energy an object has that may fall or travel downward from a height.
The work done by or against gravity = DGPE
GPE = mgh
Practice
A 60 kg person stands on top of a 4 m tall ladder.
a) What is the gravitational potential energy of the person?
b) How much work would the person do on a lazy observer lying on the floor if the person falls from the ladder?
An object can have both GPE and KE, at the same time.
Energy may change.
http://www.physicsclassroom.com/mmedia/energy
Image Credit: Physics Classroom
How is conservation of mechanical energy used?
Energy cannot be created or destroyed. The total amount of energy in the universe is conserved.
Mechanical energy is conserved when no energy is lost or gained by the system.
Mechanical energy and work are interchangeable.
Energy Work
For Systems on a HillGPEtop = KEbottom
Elliptical Orbits
Elliptical orbits are maintained by conservation of energy.
Planets move in elliptical orbits with the Sun at one focus of the ellipse.
PerihelionAphelio
n
Equal Areas in Equal Time Intervals
Orbits Practice
1) Where is KE maximum in an elliptical orbit?
2) Where is GPE maximum in an elliptical orbit?
3) Where do you think Earth is when it is the first day of summer in the northern hemisphere, aphelion or perihelion?
Orbits PracticeThe orbital periods of the planets depend on the semi-major axis of the orbit.
P2 = a3
Which planet has the longer orbital period?
Saturn: a = 9.54 AU
Jupiter: a = 5.2 AU
http://www.exploratorium.edu/ronh/age/
a
1) A 10 kg steel ball is dropped 12 meters from the roof of a building onto a concrete sidewalk.
a) Calculate the initial energy.
b) What is the KE the instant before impact?
c) What happens to this KE?
Practice(Always assume mechanical energy is conserved)
More Practice2) A 68 kg skier begins to ski at the top of a hill that is
20 meters high.
a) How much kinetic energy does the skier have 75% of the way down?
b) When are GPE and KE equal?
c) If the skier runs into Teresa at the bottom of the hill, how much mechanical work may be done on Teresa?
d) What is the skier's speed at the bottom of the hill?
What is Mechanical Power?
Power is the rate at which work is done or energy is used.
Car Engines & Light bulbs
P = W t
P = E t
Practice1) How much power is needed to lift a 150 lb (667 N)
barbell 2 ft (0.61 m), in 0.25 seconds? How many horsepower is this? 1 hp = 746 Watts
2) What is the energy used by a 60 Watt light bulb in 2 hours?
3) What exactly is a kWh?
What does a machine do?
Machines transform mechanical energy into mechanical work.
Simple machines can multiply input force.
Lever
Ramp (Inclined Plane)
Pulley
Torque is work that results in rotational motion. (lever)
Force acts through a distance
Torque is equal on each side of the fulcrum (pivot)
t = Fd
Practice1) If a 667 N person sits 3 meters from the pivot, where
would a 533 N person need to sit to balance a seesaw?
More Practice 2) A wrench measures 12 inches on the long side and 2
inches on the short side. If a person applies 300 N to the long side of the wrench, how many Newtons are exerted on the other end of the wrench?
Pivot
12 in
2 in
What is a renewable energy source?
Renewable energy sources can be replenished in a short period of time.
Wind
Water
Solar
Geothermal
Biofuel
Practice
1) Wind transforms _____ energy into _____ energy.
2) Most renewable/clean energies are transformed into _____ energy.