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Transcript 
October 14, 2016
4.1 Work and Machines
work - force applied through a distance
• if you push against an object and it doesn't move, you
have done NO work
Glenco Physical Science 2012
W=Fd
– W = work ....unit J (joules)
– F = force...unit N (newton)
– d = distance ...unit m
• You push a refrigerator with a horizontal force of 100N.
If you move the refrigerator a distance of 5 m while
you are pushing, how much work do you do?
• If you carry books while walking at a constant velocity,
you do no work with your arms because that force is
perpendicular to the direction of motion.
• If the force on an object and that object's motion are
parallel, then the work from the force equals that
force's magnitude multiplied by distance.
• You can only do work while you are applying a force to
the object
October 14, 2016
Machine - is a device that changes the force or increases
the motion from work.
Simple machine - a machine that does work with only
one movement of the machine
compound machine - a combination of two or more
simple machines.
Glenco Physical Science 2012
Efficiency - the ratio of output work to input work
• Often measured in percent
e=
Wout
x 100
Win
e = efficiency...unit %
Wout = output work...unit joules (J)
Win = input work ... unit joules (J)
• Machines can be made more efficient by reducing
friction (add a lubricant)
• All machines are less than 100% efficient
You do 20J of work pushing a crate up a ramp. If the
output work from the inclined plane is 11J, then what is
the efficiency of the inclined plane?
October 14, 2016
Machines are useful because:
>
increase speed
>
change the direction of a force
>
increase force
Glenco Physical Science 2012
Mechanical advantage - the ratio of the output force to
the input force
F
MA = out
Fin
• MA = Mechanical advantage... no unit because its a
ratio
• Fout = Output force...unit Newtons (N)
• Fin = Input force...unit Newtons (N)
A crate weighs 950 N. If you can use a pulley system to
lift that crate with a force of only 250 N, then what is the
mechanical advantage of the pulley system?
October 14, 2016
Homework:
P 107 # 1-3
P110 # 5,6
P112 # 8,14,15
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