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Thank you,
Vicki Hughes
Labs and Activities for this Presentation:
(none)
Work and Machines
WORK
Work = the use of force to move an object.
Not all force that is used to move an object does work.
For work to be done, the force must be applied in the
same direction that the object moves.
WORK
Force
If a force is applied in a
different direction than
the object moves, no
work is done.
NOT
WORK
direction
Force
Work, Force, and Distance
Work is directly related to both the force applied and the distance
moved.
Work = Force x Distance
Who is doing more work (pretend the weights are the same)?
Arnold
Sylvester
http://www.schooltube.com/video/85de91bb7097c101fbda/Eureka-Episode-8-Work.
Practice Problem #1
Work = Force x Distance
Who is doing more work (weights are different)?
Balloons = 0.5 N
Height = 2.5 meters
Work = 0.5 N x 2.5 m
= 1.25 Nm
Steel balls = 500 N
Height = 2.5 meters
Work = 500 N x 2.5 m
= 1250 Nm
Work, Force, and Distance
Example: Todd pushed a 500 N box 4 meters across the floor. How
much work did he do?
Work = Force x Distance
Work = 500 N x 4 meters = 2000 Nm
Newton meters can also be called Joules (J). So…
Work = 500 N x 4 meters = 2000 J
Machines
A simple machine is any device that makes work easier by changing
the size or direction of a force.
Machines
• Machines that increase force
(Examples: doorknobs, ramps,
nutcrackers)
• Machines that increase the distance over
which a force is applied
(Examples: paddles and hammers)
• Machines that change only the direction of
force
(Examples: Rope systems on flagpoles)
Machines
An inclined plane is a sloped surface and changes the direction of
the force.
Wedge
Screw
Ramp
Machines
A lever is a bar that pivots on a point (fulcrum) changes the
direction of the force.
Bottle Opener
Scissors
Crowbar
Seesaw
Machines
A pulley is grooved wheel that holds a rope or cable and multiplies
force over distance.
Fixed Pulley
Moveable Pulley
Machines
A wheel and axle is a wheel on a fixed point that multiplies force
over distance.
Steering Wheel
Door Knob
Screwdriver
Machines
Input force = force that you apply to a machine.
Output force = force that the machine applies to an object.
Machines
Mechanical Advantage of Machines
Mechanical Advantage = Output Force
Input Force
Ideal Mechanical Advantage (IMA) = multiplication of input force
that would be achieved in the absence of friction.
IMA = Input Distance
Output Distance
Machines
In the illustration below, a man pushes a heavy box up a ramp.
Calculate the IMA.
IMA = Input Distance = 6 m = 3
Output Distance 2 m
This means that the output force is 3 times that of the input
force.
Practice Problem #2
A man uses a force of 30 N to push a 300-N box up a long
ramp. What is the mechanical advantage of the ramp?
MA = Fout = 300 N = 10
Fin
30 N
ACT 23
Practice Problem #3
A woman uses a lever to move a load weighing 100 N
over a distance of 0.1 m from the fulcrum. The woman
applies a force of 20 N. How far from the fulcrum does
she need to apply the input force? (ignore friction)
Force due to load x distance from fulcrum = input force x distance from fulcrum
Load = Output
F1 x d1 = F2 x d2
d2 = F1 x d1
F2
d2 = 100 N x 0.1 m
20 N
d2 = 0.5 m
Air Pressure and Fluids
Pressure is the result of force acting on a given area.
Buoyancy is the force a fluid exerts on an object pushing it up.
Archimedes’ Principle = size of the buoyant force of an object is
equal to the amount of fluid it displaces.
The buoyant force of this rock is 3 ml.
23 ml – 20 ml = 3 ml
Oceanic and Atmospheric Pressure
Fluids exert more pressure at greater depths due to all of the fluid
above pressing down.
Lower pressure
Higher pressure
Oceanic and Atmospheric Pressure
Divers must ascend slowly with frequent pauses to allow their
bodies to adjust to the decrease in pressure.
What would happen to the diver’s body cells if he
ascended to quickly?
http://www.youtube.com/watch?v=LfCOnGHheok
Oceanic and Atmospheric Pressure
Air in the atmosphere acts as a fluid
and exerts pressure that increases
with depth.
Why do people sometimes get headaches when traveling up a
http://www.youtube.com/watch?v=3bNI8TNkltQ
high mountain?
Pascal’s Law
Pascal’s Law states that a change in pressure at any
point in an enclosed fluid is transmitted equally
throughout the fluid.
pressure released
pressure applied
pressure transmitted
Bernoulli’s Principle
Bernoulli’s principle states that whenever the speed
of flow increases, the fluid’s pressure increases.
ACT 24
Buoyancy and the Titanic
http://www.youtube.com/watch?v=dnU3_GKSoxc
Any
Questions?