Transcript Energy, Work and Simple Machines

Energy, Work and Simple
Machines
Unit 8
Vocabulary
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Work
Energy
Kinetic energy
Work-energy theorem
Joule
Power
Watt
Machine
Effort force
• Resistance force
• Mechanical advantage
• Ideal mechanical
advantage
• Efficiency
• Compound machine
Objective: Describe the relationship
between work and energy.
• Newton’s Laws: used to analyze motion
– Force and mass were used to determine
acceleration
– Acceleration information was used to determine
velocity or displacement for a time interval
New Approach to Analyzing Motion
• The work-energy approach
• Analyze the affect that work has upon the
energy of a system (or system of objects)
• Determine the resulting velocity or height of
the object from the energy information
• Work-energy theorem: when work is done on
an object, the result is a change in kinetic
energy.
W = ∆KE
What is work?
• when a force acts upon and object and causes
its displacement
– 3 ingredients to work:
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Force
Displacement
Cause
For a force to qualify as having done work, there must
be a displacement and the force must be the cause of
the displacement.
Examples of work….
Work or Not!
• Consider the following examples. Decide if
they represent work or if work is being done.
1. A teacher pushes on a wall and becomes
tired.
2. A book falls off a table and free falls to the
floor.
3. A rocket accelerates through space.
Energy
A massive, fast-moving vehicle can do damage to
the objects around it.
A baseball hit at a high speed can rise high into the
air.
What property of an object can produce a change in
the object itself or the world around it?
Answer: Energy. Both the vehicle and the baseball
possess energy that is associated with their
motion. This is known as kinetic energy and
represented by the symbol KE.
Mathematically speaking….
• W = Fd (work is equal to the product of
Force and displacement)
• KE = ½ mv2
• W = ∆KE
• The relationship between work done and the
change in energy that results was established
by 19th century physicist James Prescott Joule.
• To honor him, the unit of energy is called a
joule (J)
• Example: if a 2kg object moves at 1m/s, it has
a kinetic energy of 1kg·m2/s2 or 1 Joule.
Review: What is a system?
• A system is the object of interest
• The external world is everything else
• Example: one system might be a box in a
warehouse and the external world might
consist of you, Earth’s mass and anything else
external to the box.
• Through the process of doing work, energy
can move between the external world and the
system.
• Direction of energy transfer can go both ways.
– If external world does work on the system, then W
is positive and the energy of the system increases
– If the system does work on the external world,
then W is negative and the energy of the system
decreases.
Calculating Work…
• W=Fd (work is measured in joules too. One joule of work
is done when a force of 1N acts on an object over a
displacement of 1m. )
– Holds only for constant forces exerted in the direction of motion
– What happens if the force exerted is perpendicular to the
direction of the object?
– Consider a planet in circular orbit. The force is perpendicular to
the direction of motion. A perpendicular force does not change
the speed of the planet only its direction. Consequently the
gravitational force does not work on the planet. The kinetic
energy remains constant as well.
– If KE = 0 (constant) then W=0. F and d are at right angles.
Force exerted at an angle
• A force exerted in the direction of motion
does an amount of work
• A force exerted perpendicular to the motion
does NO work.
• What work does a force exerted at an angle
do?
Consider a force applied to a car at an angle.
The net force is doing the work is the component
That acts in the direction of the displacement.
Fx
What work is done by the person pushing the car?
Fy
Any force can be replaced by its components. If we consider a 125 N force exerted
In the direction of the person’s arm, it has 2 components. The magnitude of the
Horizontal component is Fx and the vertical component is Fy . Calculate both components.
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Y:
The negative sign on the Y component indicates that it is downward. The horizontal force
In the x direction is responsible for the displacement of the car and therefore is
Responsible for the work done.
• Work (angle between force and displacement)
W = Fdcosθ
Work is equal to the product of force and
displacement, times the cosine of the angle
between the force and the direction of the
displacement.
• Consider the car in the previous example.
Other agents exert forces on the pushed car.
– Earth’s gravity (downward)
– The ground (upward)
– Friction (horizontal, opposite the direction of
motion)
Which forces produce work?