1. Work and Energy
Previously we learned that the change in an object's motion is related to both force and how long the force acts.

2. "How long“ meant time. Recollect, the quantity
Work and Energy
"How long“ meant time. Recollect, the quantity
force x time
is called impulse.

3. Work and Energy
But "how long" can also mean distance

4. Work and Energy
When we consider the quantity force x distance, we are talking about an entirely different concept. This concept is called work.

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The SI unit for work is the joule (J)

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Example 1:
You exert a 50.0Nforce on a wall and the wall does not move. You have not done any work because there was no displacement.
W=Fd=(50.0N)(0m)=0

8. Work and Energy
Calculation example 1
Problem: You pushed a crate along a rough floor for a distance of 3.0m. You have exerted a constant force of 25.0N during the process. How much work have you done on the crate?

9. Work and Energy
Calculation example 1
Solution:
W=Fd=(25.0N)(3.0m)=75J

10. Work and Energy
In order for force to do work on an object it must be acting in the direction of motion.

11. Work and Energy
Example:
You exert a 50.0N downward vertical force on a box, and your friend exerts a 10.0N horizontal force on the same box, and the box moves horizontally in the direction of his force through a distance of 1.0m.

12. Work and Energy
Example:
You have not done any work on the box, but your friend has done 10.0J of work on the box.

13. Work and Energy
Work can be done when a force acts against another force.

14. Work and Energy
Examples
When you lift an object against Earth’s gravitational pull.
When you stretch a spring
When you push an object across a surface.

15. Work and Energy
Work can also be done to change the speed of an object.

16. Work and Energy
For example, if you pull a sled over a frictionless surface you will increase it’s speed.

17. Work and Energy
Power –

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Example:
You pull a box across a counter with a force of 10.0N for a distance of 3.00m. It takes you 2.0s to accomplish this. How much power have you generated?

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20. Work and Energy
When work is done by an archer in drawing a bowstring, the bent bow acquires the ability to do work on the arrow.
When work is done to raise the heavy ram of a pile driver, the ram acquires the ability to do work on the object it hits when it falls.
When work is done to wind a spring mechanism, the spring acquires the ability to do work on various gears to run a clock, ring a bell, or sound an alarm.

21. Work and Energy
In each case, something has been acquired that enables the object to do work.
It may be in the form of a compression of atoms in the
material of an object
a physical separation of attracting bodies;
a rearrangement of electric charges in the molecules of a substance.

22. Work and Energy
In each case, something has been acquired that enables the object to do work.
It may be in the form of a compression of atoms in the material of an object
a physical separation of attracting bodies
a rerrangement of electric charges in the molecules of a substance.

23. Work and Energy
This "something" that enables an object to do work is energy. Like work, energy is measured in joules. It appears in many forms.

24. Work and Energy
For now we will focus on the two most common forms of mechanical energy-the energy due to the position of something, or the movement of something.
Mechanical energy can be in the form of either potential energy or kinetic energy.

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Potential Energy
An object may store energy by virtue of its position. The energy that is stored and held in readiness is called potential energy (PE) because in the stored state it has the potential for doing work.

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Potential Energy
A stretched or compressed spring, for example, has a potential for doing work.
When a bow is drawn, energy is stored in the bow. The bow can do work on the arrow.

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A stretched rubber band has potential energy because of its position. If the rubber band is part of a slingshot, it is also capable of doing work.

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The chemical energy in fuels is also potential energy. It is actually energy of position at the submicroscopic level.

29. Work and Energy
This energy is available when the positions of electric charges within and between molecules are altered, that is, when a chemical change takes place.

30. Work and Energy
Any substance that can do work through chemical action possesses
potential energy. Potential energy is found in fossil fuels, electric batteries,
and the food we eat.

31. Work and Energy Gravitational potential energy Figure 8.3
The potential energy of the 100-N boulder with respect to the ground below is 200 J in
each case because the work done in elevating it 2 m is the same whether the boulder is
(a) lifted with 100 N of force,
(b) pushed up the 4-m incline with 50 N of force, or
(c) lifted with 100 N of force up each 0.5-m stair. No work is done in moving it horizontally,
neglecting friction

32. Work and Energy
gravitational potential energy= weight x height
PE= mgh

33. Work and Energy
Work is required to elevate objects against the earth's gravity. The potential energy due to elevated positions is called gravitational potential energy.

34. Work and Energy
Push on an object and you can set it in motion. If an object is moving,
then it is capable of doing work. It has energy of motion, or kinetic energy (KE). The kinetic energy of an object depends on themass of the object as well as its speed.

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36. Work and Energy Example:
A ball with a mass of 0.50kg is moving with a velocity of 20.0m/s
What is its kinetic energy?

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Solution:

38. Work and Energy Law of Conservation of Energy
More important than knowing what energy is, is understanding how
it behaves-how it transforms. We can better understand nearly
every process or change that occurs in nature if we analyze it in
terms of a transformation of energy from one form to another.

39. Work and Energy Law of Conservation of Energy
As you draw back the stone in a slingshot, you do work stretching
the rubber band . The rubber band then has potential energy.
When released, the stone has kinetic energy equal to this potential
energy.

40. Work and Energy Law of Conservation of Energy
It delivers this energy to its target, perhaps a wooden fence
post. The slight distance the post is moved multiplied by the average
force of impact doesn't quite match the kinetic energy of the stone.
The energy score doesn't balance.

41. Work and Energy Law of Conservation of Energy
But if you investigate further,
you'll find that both the stone and fence post are a bit warmer. By how much? By the energy difference. Energy changes from one form to another. It transforms without net loss or net gain

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45. Work and Energy
Energy cannot be created or destroyed. It can be transformed from one form into another, but the total amount of energy never changes.

46. Work and Energy Example 1
You drop a 20.0 kg rock from a height of 20.0m above ground.
What is its potential energy right before you drop it?
What is its kinetic energy right before you drop it?
What is its potential energy right before it hits the ground?
What is its kinetic energy right before it hits the ground?
What is its speed right before it hits the ground?

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54. Work and Energy Ideal spring
As you stretch an ideal spring (no mass, no heat generated) you are doing work on it. Because work is being done on it, it is gaining potential energy.

55. Work and Energy
An ideal spring has no mass, and does not generate any heat when it moves.
A spring that has no net force acting on it is said to be at its equilibrium position.
To stretch or compress it from its equilibrium position, an unbalanced force must act on it.
As the distance from the equilibrium increases, the magnitude of the force needed to move the spring through a given distance also increases.

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59. Work and Energy Example – A spring has a force constant of 100N/m
What is the force that the spring exerts when it compressed through a distance of 0.05m?

60. Work and Energy Example – A spring has a force constant of 100N/m
What is the work done on the spring when it is compressed through a distance of 0.05m?

61. Work and Energy Example – A spring has a force constant of 500N/m
What is the work done on the spring when it is compressed through a distance of 0.2m?
How much potential energy does the spring have when it is compressed through a distance of 0.2m?
The spring is released and pushes a 0.10kg puck across a horizontal frictionless surface that ends in a ramp. How fast is the puck moving when it leaves the ramp?
The puck slides up the ramp and comes to a stop. What is its height above the horizontal surface when it stops?

62. Work and Energy