Download 10.3 Kinetic Energy

Work and Energy
 10.1
Machines and Mechanical Advantage
 10.2 Work
 10.3 Energy and Conservation of Energy
Chapter 10 Objectives
1.
2.
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5.
Calculate the mechanical advantage for a lever or rope
and pulleys.
Calculate the work done in joules for situations
involving force and distance.
Give examples of energy and transformation of energy
from one form to another.
Calculate potential and kinetic energy.
Apply the law of energy conservation to systems
involving potential and kinetic energy.
Chapter 10 Vocabulary Terms
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machine
energy
input force
output force
thermal energy
ramp
gear
screw
rope and pulleys
closed system
work
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lever
friction
mechanical system
simple machine
potential energy
kinetic energy
radiant energy
nuclear energy
chemical energy
mechanical energy
 mechanical
advantage
 joule
 pressure
 energy
 conservation of
energy
 electrical energy
 input output
 input arm output
 arm
 fulcrum
10.1 Machines and Mechanical Advantage
Key Question:
How do simple
machines work?
10.1 Machines
The ability of humans to build
buildings and move mountains
began with our invention of
machines.
 In physics the term “simple
machine” means a machine
that uses only the forces
directly applied and
accomplishes its task with a
single motion.
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The best way to analyze what a machine does is
to think about the machine in terms of input and
output.
10.1 Machines
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Mechanical advantage is the ratio
of output force to input force.
For a typical automotive jack the
mechanical advantage is 30 or
more.
A force of 100 newtons (22.5
pounds) applied to the input arm of
the jack produces an output force
of 3,000 newtons (675 pounds)—
enough to lift one corner of an
automobile.
10.1 Mechanical Advantage
10.1 Mechanical Advantage
Mechanical
advantage
Input force (N)
MA = Fo
Fi
Output force (N)
Mechanical
advantage
MAlever = Li
Lo
Length of input arm
(m)
Length of output arm
(m)
10.1 Mechanical Advantage of a Lever
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Where should the fulcrum of a lever be
placed so one person weighing 700 N
can lift the edge of a stone block with a
mass of 500 kg?
 The lever is a steel bar three meters long.
 Assume a person can produce an input force equal to
their own weight.
 Assume that the output force of the lever must equal
half the weight of the block to lift one edge.
10.1 Calculate position
10.1 Wheels, gears, and rotating machines
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Axles and wheels provide advantages.
Friction occurs where the wheel and axle touch or where the wheel
touches a surface.
Rolling motion creates less wearing away of material compared with
two surfaces sliding over each other.
 With gears the trade-off is
made between torque and
rotation speed.
 An output gear will turn with
more torque when it rotates
slower than the input gear.
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Ramps reduce input force by
increasing the distance over
which the input force needs to
act.
A screw is a simple machine that
turns rotating motion into linear
motion.
A thread wraps around a screw
at an angle, like the angle of a
ramp.
10.1 Ramps and Screws
10.2 Work
Key Question:
What are the
consequences of
multiplying forces in
machines?
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In physics, work has
a very specific
meaning.
In physics, work
represents a
measurable change
in a system, caused
by a force.
10.2 Work
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If you push a box with a force of one newton
for a distance of one meter, you have done
exactly one joule of work.
10.2 Work
10.2 Work (force is parallel to distance)
Force (N)
Work (joules)
W=Fxd
Distance (m)
10.2 Work (force at angle to distance)
Force (N)
Work (joules)
W = Fd cos (q)
Angle
Distance (m)
10.2 Work done against gravity
Mass (g)
Work (joules)
W=
mgh
Height object raised (m)
Gravity (m/sec2)
10.3 Why the path doesn't matter
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10.3 Calculate work
A crane lifts a steel beam
with a mass of 1,500 kg.
Calculate how much work is
done against gravity if the
beam is lifted 50 meters in
the air.
How much time does it take
to lift the beam if the motor of
the crane can do 10,000
joules of work per second?
10.3 Energy and Conservation of Energy
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Energy is the ability to make things change.
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A system that has energy has the ability to do
work.
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Energy is measured in the same units as work
because energy is transferred during the action
of work.
Mechanical energy is the energy possessed by
an object due to its motion or its position.
 Radiant energy includes light, microwaves, radio
waves, x-rays, and other forms of
electromagnetic waves.
 Nuclear energy is released when heavy atoms in
matter are split up or light atoms are put
together.
 The electrical energy we use is derived from
other sources of energy.
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10.3 Forms of Energy
10.3 Potential Energy
Mass (kg)
Potential Energy
(joules)
Ep = mgh
Height (m)
Acceleration
of gravity (m/sec2)
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A cart with a mass of 102 kg is
pushed up a ramp.
The top of the ramp is 4 meters
higher than the bottom.
How much potential energy is
gained by the cart?
If an average student can do 50
joules of work each second,
how much time does it take to
get up the ramp?
10.3 Potential Energy
10.3 Kinetic Energy
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Energy of motion is called kinetic energy.
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The kinetic energy of a moving object depends on
two things: mass and speed.
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Kinetic energy is proportional to mass.
10.3 Kinetic Energy
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Mathematically, kinetic energy increases as the
square of speed.
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If the speed of an object doubles, its kinetic energy
increases four times. (mass is constant)
10.3 Kinetic Energy
Mass (kg)
Kinetic Energy
(joules)
Ek = 1
2
mv2
Speed (m/sec)
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Kinetic energy becomes important in calculating
braking distance.
10.3 Kinetic Energy
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A car with a mass of 1,300
kg is going straight ahead
at a speed of 30 m/sec (67
mph).
The brakes can supply a
force of 9,500 N.
Calculate:
a) The kinetic energy of the
car.
b) The distance it takes to
stop.
10.3 Calculate Kinetic Energy
As energy takes different forms and changes
things by doing work, nature keeps perfect track
of the total.
 No new energy is created and no existing
energy is destroyed.
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10.3 Law of Conservation of Energy
10.3 Energy and Conservation of Energy
Key Question:
How is motion on a track
related to energy?
Application: Hydroelectric Power