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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

• Explain the meaning of work.

• Explain how work and energy are related.

• Calculate work.

• Calculate power.

• 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

• Can be calculated by the following equation

• Work = force x distance (W= Fd)

• W= work

• F= force

• d= distance

• 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

• 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

• 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

• Units for power include

• J/s

• W

• Stands for watt

• Is the main unit used for power

• 1W= 1J/s

• 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

• Explain how machines make work easier.

• Explain how to calculate mechanical advantage.

• Define efficiency.

• Describe the six types of simple 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

• 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

• 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%

• 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

• 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

• 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

• 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

• 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

• 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

• 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

• 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

• 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

• Is an inclined plane wrapped around a cylinder

• Example- spiral staircases, jar lids

• 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

• 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 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

• 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

• 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

• 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

• 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

• 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

• 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

• 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

• 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 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

• 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

• Describe how energy is conserved when changing from one form to another.

• Apply the law of conservation of energy to familiar situations.

• 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

• 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

• 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