Work and Energy

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# Work and Energy - PowerPoint PPT Presentation

Work and Energy. Chapter 12. Students Will Be Able To:. Explain the meaning of work. Explain how work and energy are related. Calculate work. Calculate power. Work.

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### Work and Energy

Chapter 12

Students Will Be Able To:
• Explain the meaning of work.
• Explain how work and energy are related.
• Calculate work.
• Calculate power.
Work
• It is the transfer of energy to a body by the application of a force that causes the body to move in the direction of the force
• It is only done when a force causes a change in the position or the motion of an object in the same direction of the applied force
• Therefore, for work to occur, an object must move
Work
• Can be calculated by the following equation
• Work = force x distance (W= Fd)
• W= work
• F= force
• d= distance
Work
• Units for work include
• N • m
• J
• Stands for joule
• Is the main unit used for work
• kg • m2/s2
• 1J = 1 N • m= 1kg • m2/s2
Example- Work
• Imagine a father playing with his daughter by lifting her repeatedly in the air. How much work does he do with each lift, assuming he lifts her 2.0 m and exerts an average force of 190 N?

W = Fd

= 190N x 2.0m

= 380 N • m

= 380 J

Power
• It is a quantity that measures the rate at which work is done or energy is transformed
• Can be calculated by the following equation
• Power = work/time (P= W/t)
• P= power
• W= work
• t= time
Power
• Units for power include
• J/s
• W
• Stands for watt
• Is the main unit used for power
• 1W= 1J/s
Example- Power
• It takes 100 kJ of work to lift an elevator 18 m. If this is done in 20 s, what is the average power of the elevator during the process?
• 100kJ x 1000J/1kJ = 100000 J
• P = W/t

= 100000 J / 20 s

= 50000 J/s

= 50000 W

Students Will Be Able To:
• Explain how machines make work easier.
• Explain how to calculate mechanical advantage.
• Define efficiency.
• Describe the six types of simple machines.
Machines
• Machines multiply and redirect forces
• Help do work by redistributing the work that is put into them
• Can also change the direction of the input force
• Able to increase or decrease force by changing the distance over which the force is applied
• Different forces can do the same amount of work
• Due to decreasing the distance while increasing the force OR decreasing the force while increasing the distance
• Mechanical advantage tells how much a machine multiplies force or increases distance
• Calculated by the following equation
• Input force is the force applied to the machine
• Output force is the force applied by the machine to overcome resistance
• Amount of energy the machine transfers to the object cannot be greater than the amount of energy transferred to the machine
• Some energy transferred is changed to heat due to friction
• An ideal machine with no friction would have the same input work and output work
• Calculate the mechanical advantage of a ramp that is 5.0 m long and 1.5 m high.
• Input distance= 5.0 m and output distance= 1.5 m
• Mechanical advantage = 5.0m/ 1.5m

= 3.33

Efficiency
• Is a measure of how much of the work put into a machine is changed into useful output work by the machine
• Calculating efficiency
• Efficiency = output work/input work X100%
• Efficiency of a machine is always less than 100%
• Lubricants can make machines more efficient by reducing friction
Example- Efficiency
• A sailor uses a rope and an old, squeaky pulley to raise a sail that weighs 140 N. He finds that he must do 180 J of work on the rope in order to raise the sail by 1 m (doing 140 J of work on the sail). What is the efficiency of the pulley? Express your answer as a percentage.
• Efficiency = output work/input work X100%

= 140 J/ 180 J X 100%

= 77.8%

Simple Machines
• One of the six basic types of machines, which are the basis for all other forms of machines
• Divided into two families
• Lever family
• Simple lever
• Pulley
• Wheel and axle
• Inclined plane family
• Simple inclined place
• Wedge
• Screw
Levers
• Have a rigid arm that turns around a point called the fulcrum
• Input arm is the part of the lever on which effort force is applied
• Output arm is the part of the lever that exerts the resistance force
• Divided into three classes based on the location of the fulcrum and of the input and output forces
First Class Levers
• Most common type
• Fulcrum is located between the effort and resistance forces (input and output forces)
• Multiplies and changes the direction of the force
• Example- claw hammer, pliers
Second Class Levers
• Fulcrum is located at one end of the arm and the input force (effort force) is applied to the other end
• Always multiplies force
• Example- wheelbarrow, nutcrackers, hinged doors
Third Class Levers
• Effort force is between the output force (resistance force) and fulcrum
• Responsible for multiplying distance rather than force
• They have a mechanical advantage of less than 1
• Example- human forearm
Pulleys
• Are modified levers
• The point in the middle of a pulley is like the fulcrum of a lever
• A single, fixed pulley has a mechanical advantage of 1
• It is attached to something that doesn’t move
• Force is not multiplied but direction is changed
• A moveable pulley has one end of the rope fixed and the wheel free to move
• It multiplies force
• Multiple pulleys are sometimes put together in a single unit called a block and tackle
Wheel and Axles
• Is a machine with two wheels of different sizes rotating together
• Is a lever or pulley connected to a shaft
• The steering wheel of a car, screwdrivers, and cranks are common wheel-and-axel machines
Inclined Planes
• Is a sloping surface that reduces the amount of force required to do work
• Multiplies and redirects force
• Turns a small input force into a large output force by spreading the work out over a large distance
• Less force is required if a ramp is longer and less steep
Wedge
• Is a modified inclined plane
• Functions like two inclined planes back to back
• Turns a single downward force into two forces directed out to the sides
• Example- nails
Screw
• Is an inclined plane wrapped around a cylinder
• Example- spiral staircases, jar lids
Compound Machines
• Is a machine made of more than one simple machine
• Examples of compound machines
• scissors, which use two first class levers joined at a common fulcrum
• a car jack, which uses a lever in combination with a large screw
Students Will Be Able To:
• Explain the meaning of energy.
• Distinguish between kinetic and potential energy.
• Recognize the different ways that energy can be stored.
• Calculate kinetic and potential energy.
Energy
• Energy is the ability to do work or to cause change
• When you do work on an object, you transfer energy to that particular object
• Whenever work is done, energy is transformed or transferred to another system
• Energy is measured in joules
• Work is also measured in joules
Potential Energy
• Is the energy that an object has because of the position, shape, or condition of the object
• Is stored energy
• 3 types exist
• Chemical
• Energy stored in chemical bonds between atoms
• Elastic
• Energy stored in stretched or compressed elastic materials
• Gravitational
• Energy stored in the gravitational field which exists between two or more objects
Gravitational Potential Energy
• Depends on mass, height, and acceleration due to gravity
• Can be calculated by the following equation
• PE = mass x free fall acceleration x height (PE = mgh)
• PE= gravitational potential energy
• m= mass
• g= free fall acceleration (9.8 m/s2)
• h= height
• Usually measured from the ground but can also be measured from two objects
Example- Gravitational PE
• A 65 kg rock climber ascends a cliff. What is the climber’s gravitational potential energy at a point 35 m above the base of the cliff?
• PE = mgh

= 65 kg x 9.8 m/s2 x 35 m

= 22295 kg•m2/s2

= 22295 J

Kinetic Energy
• Is the energy of a moving object due to the object’s motion
• Depends on mass and speed
• Depends on speed more than mass
• Can be calculated by the following equation
• Kinetic Energy = ½ mass x speed squared

KE = ½ mv2

Example- KE
• What is the kinetic energy of a 44 kg cheetah running at 31 m/s?
• KE = ½ mv2

= ½ (44 kg)(31m/s)2

= 21142 kg•m/s2

= 21142 J

Other Forms of Energy
• The amount of work an object can do because of the object’s kinetic and potential energies is called mechanical energy
• Mechanical energy is the sum of the potential energy and the kinetic energy in a system
• In addition to mechanical energy, most systems contain non-mechanical energy
• Non-mechanical energy does not usually affect systems on a large scale
Other Forms of Energy
• Atoms and molecules have kinetic energy
• The kinetic energy of particles is related to heat and temperature
• Chemical reactions involve potential energy
• The amount of chemical energy associated with a substance depends in part on the relative positions of the atoms it contains
• Living things get energy from the sun
• Plants use photosynthesis to turn the energy in sunlight into chemical energy
Other Forms of Energy
• The sun gets energy from nuclear reactions
• The sun is fueled by nuclear fusion reactions in its core
• Electricity is a form of energy
• Electrical energy is derived from the flow of charged particles, as in a bolt of lightning or in a wire
• Light can carry energy across empty space
• Light energy travels from the sun to Earth across empty space in the form of electromagnetic waves
Energy Transformations
• Energy readily changes from one form to another
• Potential energy can become kinetic energy
• As a car goes down a hill on a roller coaster, potential energy changes to kinetic energy
• Kinetic energy can become potential energy
• The kinetic energy a car has at the bottom of a hill can do work to carry the car up another hill
Energy Transformations
• Mechanical energy can change to other forms of energy
• Mechanical energy can change to non-mechanical energy as a result of friction, air resistance, or other means
Students Will Be Able To:
• Describe how energy is conserved when changing from one form to another.
• Apply the law of conservation of energy to familiar situations.
The Law of Conservation of Energy
• Energy may change from one form to another, but the total amount of energy never changes
• Energy doesn’t appear out of nowhere
• Whenever the total energy in a system increases, it must be due to energy that enters the system from an external source
• Energy doesn’t disappear,but it can be changed to another form
Energy Systems
• Scientist study energy systems
• Boundaries define a system
• Systems may be open or closed
• When the flow of energy into and out of a system is small enough that it can be ignored, the system is called a closed system
• Most systems are open systems, which exchange energy with the space that surrounds them
Mass  Energy
• Mass as thought of as energy when discussing nuclear reactions
• The total amount of mass and energy is conserved
• 2 processes involved
• Nuclear fusion
• Two nuclei are fused together
• Nuclear fission
• Two nuclei are broken apart
Food Energy
• Chemical potential energy from food that is stored in your body is used to fuel the processes that keep you alive
• The food calorie is used to measure how much energy you get from various foods
• One calorie is equivalent to 4180 J