machines-from complex ones such as a car to relatively simple ones such as a car jack, a hammer, or...
TRANSCRIPT
Work and
Energy
Machines-from complex ones such as a car to relatively simple ones such as a car jack, a hammer, or a ramp-help people get things done everyday.
Work, Power, and Machines
If you try to lift a car without a jack, you might exert a lot of force without moving the car at all.
Exerting the force might seem like hard work.
Work
Work—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.
Work
Work is done only when force causes a change in the position or the motion of an object in the direction of the applied force.
Work
Work is a transfer of energy from one object to another.
Work
Energy—The capacity to do work.
Work
Work is calculated by multiplying the force by the distance over which the force is applied.
W = Fd(Work equals force times distance
Work
The SI unit for work, like energy, is the Joule (J).
1 J = 1 Nm = 1 kg m²/s²
Work
When talking about work it must be clearly stated from where the work is coming from and on what is the work being done.
Work
A crane uses an average force of 5200 N to lift a girder 25 m. How much work does the crane do on the girder?
A: 1.3 x 105 J
Crane
While rowing in a race, John uses his arms to exert a force of 165 N per stroke while pulling the oar 0.800 m. How much work does he do in 30 strokes?
A: 3960 J
Row Boat
A mechanic uses a hydraulic lift to raise a 1200 kg car 0.5 m off the ground. How much work does the lift do on the car?
A: 6000 J
Mechanic Lift
Three vehicles take three different paths up a hill, Which path requires the most energy to get to point D?
Different Paths
As something is going uphill work only depends on the vertical distance and not the slope.
Work
Running up stairs doesn’t require more work than walking up slowly, but it is more exhausting.
The amount of time it takes to do work is an important factor when considering work and machines.
Power
Power—A quantity that measures the rate at which work is done or energy is transformed.
Power
Power is calculated by dividing the total amount of work done, by the amount of time that it takes to do the work.
P = W/t(Power equal work divided by time)
Power
Running takes less time than walking, how does reducing the time in the power equation affect the power, if the same amount of work is done?
Power
Power is measured in the SI unit called watts (W)
A watt is the amount of power to exert 1 Joule of energy in 1 second.
1 W = 1 J/1 s
Power
Every second, a certain coal-fired power plant produces enough electricity to do 9 x 108 J (900 MJ) of work. What is the power output of this plant in watts? Megawatts?
A: 9 x 108 W/900 MW
Power plant
You and two friends apply a net force of 425 N to push a piano up a 2.0 m long ramp. How much work in joules has been done when you reach the top of the ramp?
A: 850 J
Moving
Suppose you are moving a 300 N box of books. Calculate you power output if you exert a force of 60.0 N pushing the box across the floor 12.0 m in 20.0 s.
A: 36 W
Moving books
After you push the box of books (300 N) across the floor you lift it one meter into the back of a truck in 3.00 s. How much power did you exert lifting the box?
A: 100 W
Moving books pt 2
Is it easier to lift a car yourself, or use a jack to lift the car?
Even using a jack to lift a car you still exert the same amount of work to lift the car. The jack just makes the force less to lift the car.
Mechanical Advantage
Machines help us do work by redistributing the work that we put into them.
Machines can change the direction of the force applied.
Mechanical Advantage
Machines can also increase or decrease the force applied by changing the distance which the force is applied.
This process is called multiplying the force.
Mechanical Advantage
Compare the amount of work required to lift a box straight onto the bed of a truck with the amount of work to push the same box up a ramp.
Mechanical Advantage
When the mover lifts the object straight up he must apply a force a little greater than the weight of the box. When he uses a ramp he can apply a smaller force but over a greater distance.
Mechanical Advantage
Whether the mover lifts the box straight up or pushes it up a ramp, he exerts about the same amount of work.
Mechanical Advantage
A machine allows the same amount of work to be done by either decreasing the distance while increasing the force, or increasing the distance while decreasing the force.
Mechanical Advantage
When using a ramp, how long should the ramp be?
A really long ramp, will decrease the force, but the distance is really long, and a steep ramp won’t be very helpful.
Mechanical Advantage
Mechanical Advantage—A quantity that measures how much a machine multiplies force or distance.
Mechanical Advantage
Mechanical advantage is defined as the ratio of the output force to the input force.
Mechanical Advantage
Machines with mechanical advantage greater than “1” help you move heavier objects with a smaller force, but increases distance.
Mechanical Advantage
Determine the mechanical advantage of an automobile jack that lifts a 9900 N car with an input force of 150 N.
A: 66
Automobile Jack
A sailor uses a rope and pulley to raise a sail weighing 140 N. The sailor pulls down with a force of 140 N on the rope. What is the mechanical advantage?
A: 1
Sailing
Alex pulls on the handle of a claw hammer with a force of 15 N. If the hammer has a mechanical advantage of 5.2, how much force is exerted on the nail?
A: 78 N
Hammer Claw
The most basic of all machines of all are called simple machines.
Other machines are either modifications of simple machines or combinations of several simple machines.
Simple Machines
Simple Machine—One of the six basic types of machines, which are the basis for all other forms of machines.
Simple Machines
Simple machines are divided into two families, the lever family and the inclined plane family.
Simple Machines
Levers are divided into three classes:First-class levers
Second-class levers
Third-class levers
The Lever Family
All levers have a rigid arm that turns around a point called the fulcrum.
Force is transferred from one part of the arm to another; in that way, the input force can be multiplied or redirected.
The Lever Family
1st Class Lever:Most common type of leverFulcrum located between the point of input force and output force.
The Lever Family
2nd Class Lever:Fulcrum at one end.The input force at other end and load in between.
The Lever Family
3rd Class Lever:Fulcrum at one end.Load at other end and the input force in between.
The Lever Family
Pulleys are modified levers.The point in the middle is like the fulcrum, and the rest of the wheel is the rigid body, that is rotating about the fulcrum.
The Lever Family
If you only use one pulley, the mechanical advantage is “1”.
Using multiple pulleys can increase the mechanical advantage.
The Lever Family
What is the Mechanical Advantage for the pulley system shown in the image? If the crate weighs 250 N how much force must be applied to the rope to lift it?
A: 5A: 50 N
Multiple Pulley System
A wheel and axle is a lever or pulley connected to a shaft.
Made of a lever or pulley (wheel) connected to a shaft (axle), when the wheel turns the axle turns.
The Lever Family
A loading ramp is an inclined plane, simple machine.
When you push an object parallel to the ramp the ramp redirects the force to lift the object up.
The Inclined Plane Family
An inclined plane turns a small input force into a large output force by spreading it over a large distance.
The Inclined Plane Family
A wedge is a modified inclined plane.
One example of a wedge is an axe blade.
The Inclined Plane Family
Another example of a wedge is the doorstop.
As the door closer tries to close the door it pushes the doorstop into the ground increasing friction.
The Inclined Plane Family
A screw is an inclined plane wrapped around a cylinder.
The Inclined Plane Family
Many devices people use every day are made of more than one simple machine.
Compound Machines
Compound Machine—A machine made of more than one simple machine.
Compound Machines
The world around you is full of energy.
When you see a flash of lightning or hear a clap of thunder, you are observing light and sound energy.
What is Energy?
You have energy just because you are moving.
Even things sitting still have energy waiting to be released.
Without energy, living organisms could not survive.
What is Energy?
When you stretch a slingshot you are doing work on the band, transferring energy to it.
When the elastic snaps back, it may in turn transfer energy to a stone, doing work on the stone.
Energy and Work
Whenever work is done, energy is transformed or transferred to another system.
One way to define energy is as the ability to do work.
Energy and Work
While work is done only when an object experiences a change in position or motion, energy can be present in an object or a system when nothing is happening at all.
Energy and Work
Energy can be observed only when it is transferred from one object or system to another.
Energy and Work
The amount of energy transferred can be measured by how much work is done.
Energy is a measure of the ability to do work, and both are measured in the SI unit of the Joule.
Energy and Work
Stretching a rubber band requires work.
When you release the rubber band, the energy used to stretch the band is then released to move it.
Potential Energy
Where is the energy between the time you do work on the rubber band and the time you release it?
The rubber band stores energy in a form called potential energy.
Potential Energy
Potential Energy—The energy that an object has because of the position, shape, or condition of the object.
Potential Energy
Potential energy is sometimes called energy of position because it results from the relative positions of objects in a system.
Potential Energy
The energy stored in any type of stretched or compressed elastic material is called elastic potential energy.
Potential Energy
When a stem breaks an apple falls.
The energy that could potentially do work on the apple results from its position above the ground.
Potential Energy
Any system of two or more objects separated by a distance contains gravitational potential energy resulting from the gravitational attraction between the objects.
Potential Energy
Gravitational potential energy depends on both height and mass.
Potential Energy
Gravitational potential energy is the work that the gravitational force from Earth could do on an object.
PE = mgh(Potential energy equals mass
times gravity times height)
Potential Energy
Height can be relative.Height is usually measured from the ground, but sometimes it is better to measure it from the lowest position.
Potential Energy
Calculate the gravitational potential energy of a car with a mass of 1200 kg at the top of a 42 m high hill.
A: 4.9 x 105 J
Car on a Hill
A science student holds a 55 g egg out a window. Just before the student releases the egg, it has a potential energy of 8.0 J. How far is the student’s egg above the ground?
A: 15 m
Egg drop
A diver has 3400 J of potential energy after stepping onto a diving platform 6.0 m above the water. What is the diver’s mass?
A: 58 kg
Diver
As an apple is falling it has energy due to its motion, it can do work when it lands.
This energy is calledkinetic energy
Kinetic Energy
Kinetic Energy—The energy of a moving object due to the object’s motion.
Kinetic Energy
Kinetic energy depends on the object’s mass and speed.
KE = ½ mV²(Kinetic Energy equals ½ mass
times velocity squared)
Kinetic Energy
Car crashes are more dangerous at higher speeds because of the kinetic energy involved.
Kinetic Energy
Calculate the kinetic energy of a 1500 kg car moving at (a) 29 m/s, and at (b) 18 m/s.
A: (a) 6.3 x 105 JA: (b) 2.4 x 105 J
Car Kinetic Energy
A 35 kg child has 190 J of kinetic energy after sledding down a hill. What is the child’s speed at the bottom of the hill?
A: 3.3 m/s
Sledding
Law of Conservation of Energy—The total energy of an isolated system remains constant.
Law of Conservation of Energy
Same child (35 kg), same hill, same run (190 J) how tall is the hill?
A: 0.55 m
Sledding Pt.2
As objects are falling they have both potential and kinetic energy.
The sum of the potential energy and kinetic energy is called mechanical energy.
Other Forms of Energy
Mechanical Energy—The amount of work an object can do because of the object’s kinetic and potential energy.
Other Forms of Energy
Some types of nonmechanical energy include:Chemical EnergySolar EnergyThermal EnergyElectrical Energy
Other Forms of Energy
We learned from last semester that molecules are constantly moving, they have molecular kinetic energy.
This energy can be detected with the temperature of objects, also called thermal energy.
Other Forms of Energy
Chemical reactions involve potential energy.
This energy is called chemical energy, and is usually from electrons going from orbital to orbital or breaking bonds.
Other Forms of Energy
Plants use Photosynthesis, which is a process of converting solar energy, from the sun, into chemical energy, making sugars.
Other Forms of Energy
The sun releases solar energy, by means of chemical reactions, nuclear fusion and nuclear fission.
These nuclear reactions convert chemical energy into thermal and light energy, warming the Earth and lighting our days.
Other Forms of Energy
Electricity is a form of energy, electrical energy.
Electricity is brought about by moving electrons through different mediums.
Other Forms of Energy
Light carries energy across empty space, through electromagnetic waves.
The different colors of light is due to the amount of energy being carried.
Other Forms of Energy
Red light is oscillating slower, because it has less energy than blue light.
Other Forms of Energy
