Download 1 Chapter 5: Work and Energy (pages 159 182) Dat

Chapter 5: Work and Energy (pages 159 ­ 182) Date ______
Energy and matter make up the universe.
Matter is substance and energy is the mover of the substance.
Matter we can see, feel and smell.
Energy is both a thing and a process.
Persons and things have energy, but we can usually only see the energy when it is being transformed from one form to another
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Work
Last chapter we talked about how an unbalanced force makes an object accelerate.
This chapter we will talk about a force over a given distance, which is work.
Work = Force x Distance or
W = F d
The unit of force is a _____________. The unit of distance is _____________
The unit of work is _______________________ or a ______________.
Work is done on a object when the object is moved with the action of a force.
Work depends on: 1) The exertion of a force
2) The movement of an object by that force.
No work is done on a chair when a person holds a chair at arms length even though the person exerts a force to support the chair. The chair does not move as a result of the force.
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Work is something that one object does to other objects.
A object does not have work.
Compare the work done lifting a box up one flight of stairs to the work done when lifting two boxes up one flight of stairs.
Two boxes is __________ the work since ____________ the force is needed.
Compare the work done lifting a box up one flight
of stairs to the work done when lifting a box up
two flights of stairs.
_________ the work is done since the boxes are
moved _________ the distance.
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1) If a neighbor pushes a lawnmower four times as far as you do but exerts only half the force, which one of you does more work and by how much?
2) Determine whether work is being done in each of the following examples;
a) A train engine pulling a loaded boxcar initially at rest
b) A tug of war that is evenly matched
c) A crane lifting a car
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Calculating Work
1) How many joules of work is done when a force of 3 N moves a book 4 meters?
2) A weight lifter lifts a set of weights a vertical distance of 2.0 meters. If a constant force of 350 N is exerted on the weights, what is the work done on the weights?
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Energy
When work is done by an archer in drawing a bow, the bent bow has the ability to do work on the arrow.
When work is done to wind a spring mechanism on a clock, the spring has the ability to do work on the gears in the clock.
When work is done on a object, "something" has been given to the object which enables the object to do work.
This something could be the compression of the atoms as a ball hits the floor or it could be the separation of objects are you raise your book over your head.
This something that enables the object to do work is energy.
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Kinetic Energy (KE)
Kinetic Energy is the energy associated with an object in _________.
Doing work on an object can change the energy of motion of that object.
That motion of the object can do work.
Kinetic energy depends on the mass (m) and the speed (v) of the object. The unit is joule (J). (The same as for work)
For objects with linear motion,
Kinetic Energy (KE) = 1/2 mv2
The amount of kinetic energy like the speed of an object is relative to something else.
If you are in a speeding car, your KE relative to the car is _____. You have _______ KE compared to the ground.
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Calculating Kinetic Energy (for objects with linear motion)
1) Two bullets have masses of 3.0 kilograms and 6.0 kilograms. Both are fired with a speed of 40 m/s. Which bullet has more kinetic energy? Why?
What is the ratio of their kinetic energies?
2) A car has a kinetic energy of 432,000 J when traveling at a speed of 23 m/s. What is the mass of the car?
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3) Two cars each with a mass of 1500 kg are driving on a road. One car is moving at 40 mph (18 m/s) and the other car is moving at 80 mph (36 m/s).
What are the kinetic energies of each car?
What is ratio of their kinetic energies?
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Potential Energy
Potential energy is associated with an object that has the potential to move because of its position relative to some other location.
Gravitational potential energy (GPE) is the energy associated with an object due to the object's position relative to a gravitational source (like the Earth).
The amount of GPE is the work done against gravity in lifting it.
If W = F d and once you start the upward motion the upward force = weight so F = Fg = mg
So the amount of W = mgh.
GPE = mgh
GPE depends on the mass (m) of the object, the magnitude of the acceleration due to gravity (g), and the height of the object (h) measured from an arbitrary zero level. The unit is joule (J).
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The GPE depends on the height of an object but not on the path to get the object there. GPE of the 2 balls are the same
because they are at the same height.
The GPE of an object is only significant when it changes like when it does work or it transforms to an energy of another form
When an object fall off a table it gains KE from the gravitational potential energy associated with the object's initial position on the table relative to the floor.
Compare the KE of the balls right before they hit the ground. (Ignore friction)
Compare the linear speed of the balls if one falls and the other rolls.
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Calculating Potential Energy
1) A 40 kg child is in a swing that is attached to ropes that are 2.0 meters long. Find the gravitational potential energy associated with the child relative to the child's lowest position when
a) The ropes are horizontal.
b) The bottom of the circular arc.
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