Define the following termsA. WorkB. PowerC. Machine
S-34
SPS8 Students will determine relationships among force, mass, and motion.
e. Calculate amounts of work and mechanical advantage using simple machines
Chapter 14
Work, Power, and
Machines
When does a force do work?How are work and power related?
14.1 Work and Power
Work – force applied over a distanceFor work to be done force must act in
the same direction as the movementAny part of the force that doesn’t
point in the direction of motion doesn’t do any work
12.1 Force
14.1 Work and PowerWhen does a force do work??
Force
displacement
Force
displacement
No Work Done
Force
Work Donedisplacement
The equation for work is
W=work in joulesF = force in newtonsd = distance in meters
The unit is the same as energy because work is the transfer of energy
12.1 Force
14.1 Work and PowerWhen does a force do work??
W=Fd
Practice Work Problem 1
A vertical force of 72 N is used to lift a box to a height of 3.0 m. How much work is done?
Variables?Fill in EquationAnswer
12.1 Force
14.1 Work and PowerWhen does a force do work??
W=Fd
F=72 N d=3 m
W=(72N)(3m)W=216J
Practice Work Problem 2
A force of 25 N does 302 J of work. How far did the object get pushed?
Variables?Fill in EquationAnswer
12.1 Force
14.1 Work and PowerWhen does a force do work??
W=Fd
F=25 N W=302J
302J=(25N)dd=12.08m
Power is the rate of doing work.For something to be more powerful, it
must Do more work in the same period of time Do the same amount of work faster
12.1 Force
14.1 Work and PowerHow are work and power related?
The equation for power is
P = power in wattsW = work in joules t = time in seconds
Power is sometimes measured in horsepower, 1 HP = 746W
12.1 Force
14.1 Work and PowerWhen does a force do work??
P=W/t
Practice Power Problem 1
216 J of work are done to lift a box in 2.0s. How much power is needed to lift the box?
Variables?Fill in EquationAnswer
12.1 Force
14.1 Work and PowerWhen does a force do work??
P=W/t
W=216J t=2.0s
P=(216J)/(2s)P=108W
Practice Power Problem 2
If an engine can generate 79822 W of power, how long would it take to do 1173 jof work?
Variables?Fill in EquationAnswer
12.1 Force
14.1 Work and PowerWhen does a force do work??
P=W/t
P=79822W W=1173J
79822W=(1173)/tt=0.015s
During the worlds strongest man competition, this man moved a 20,000-N truck a distance of 12-m in 60-s.
A. How much work did he do.B. How much power did he
generate?
S-35How are work and power related?
How do machines make work easier?How are work input and work output related for a machine?
14.2 Work and Machines
Machine – a device that changes force Change the size of a force Change the direction of a force
Increase force by increasing the distance
Decrease force by decreasing the distance
12.1 Force
14.2 Work and MachinesHow do machines make work easier?
Definitions Input force – the force you exert on a
machine Output force – the force exerted by the
machine Input distance – how far you move the
machine Output distance – the distance that the
machine move the objectWork input – the input force x input
distanceWork output – the output force x
output distance
12.1 Force
14.2 Work and MachinesHow do machines make work easier?
In a perfect machine – work input would equal work output
But because of friction – the work output is ALWAYS less than the work input
12.1 Force
14.2 Work and MachinesHow are work input and work output related for a machine?
How does the actual mechanical advantage of a machine compare to its ideal mechanical advantage?Why is the efficiency of a machine always less than 100 percent?
14.3 Mechanical Advantage and Efficiency
Mechanical Advantage – the number of times that the machine increases input force
Actual Mechanical Advantage – the ratio of output force to input force
12.1 Force
14.3 MA & EfficiencyHow does the AMA of a machine compare to its IMA?
AMA outputforceinputforcefffffffffffffffffffffffffffffffffffff
Sample Problem
A lever is used to lift a heavy weight. The weight is 200-N, and a force of 25 N is required to lift it. What is the Actual Mechanical Advantage?
Variables?Fill In?Answer?
12.1 Force
14.3 MA & EfficiencyHow does the AMA of a machine compare to its IMA?
AMA outputforceinputforcefffffffffffffffffffffffffffffffffffff
Output Force=200 N Input Force=25 N
AMA 200N25Nffffffffffffffffff
AMA 8
Idea Mechanical Advantage – the mechanical advantage in the absence of friction
Because of friction, the IMA is always greater than the AMA
A ratio of the input distance to the output distance
12.1 Force
14.3 MA & EfficiencyHow does the AMA of a machine compare to its IMA?
IMA input distanceoutput distancefffffffffffffffffffffffffffffffffffffffffffffffff
Sample Problem
A woman drives her car up onto wheel ramps to perform some repairs. If she drives a distance of 1.8 m to raise the car 0.3 m, what is the IMA of the ramp?
Variables?Fill In?Answer?
12.1 Force
14.3 MA & EfficiencyHow does the AMA of a machine compare to its IMA?
IMA input distanceoutput distancefffffffffffffffffffffffffffffffffffffffffffffffff
Input Dis.=1.8m Output Dis.=0.3m
IMA 1.8m0.3mfffffffffffffff
IMA 6
A man uses a crowbar to break off a 40,000 N piece of rock. The rock moves a total distance of 0.08 m. The man pushes on the crowbar with a force of 4050 N while pushing the crowbar 0.80 m.
A. What is the AMA of the crowbar?
B. What is the IMA of the crowbar?
S-36How does the actual mechanical advantage of a machine compare to its ideal mechanical advantage?
A girl cranks an ice cream churn by pushing on the handle with 25 N of force. The churn applies a force of 95 N. When the crank turns 1 complete circle, the ice cream spins 0.25 turns
A. What is the AMA of the churn?
B. What is the IMA of the churn?
S-37How does the actual mechanical advantage of a machine compare to its ideal mechanical advantage?
Efficiency – the percentage of work input that becomes work output
12.1 Force
14.3 MA & EfficiencyWhy is the efficiency of a machine always less than 100 percent?
Efficiency Work OutputWork Inputffffffffffffffffffffffffffffffffffffffffx 100%Efficiency
Force out x Distance outForce in x Distance in
ffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffx 100%
A pulley system is set up to lift a 200-N box. The box is lifted 5 m when a force of 25 N is applied over a distance of 45 m.
1. What is the work input?2. What is the work output?3. What is the efficiency of the
machine?
12.1 Force
14.3 MA & EfficiencyWhy is the efficiency of a machine always less than 100 percent?
W=FdWin=(25N)(45)Win=1125JW=FdWout=(200N)(5m)Wout=1000J
Efficiency 1000J1125Jfffffffffffffffffffx 100%Efficiency
Work OutputWork Inputffffffffffffffffffffffffffffffffffffffffx 100%Efficiency 88.89%
What are the six types of simple machines?
14.4 Simple Machines
12.1 Force
14.3 MA & EfficiencyWhat are the six types of simple machines?
Lever – a rigid bar free to move around a fixed point
12.1 Force
14.3 MA & EfficiencyWhat are the six types of simple machines?
Wheel and Axle – two disks or cylinders, with different radii
12.1 Force
14.3 MA & EfficiencyWhat are the six types of simple machines?
Inclined Planes – a slanted surface
12.1 Force
14.3 MA & EfficiencyWhat are the six types of simple machines?
Wedges and Screws are moving inclined planes
Wedge – V shaped object
12.1 Force
14.3 MA & EfficiencyWhat are the six types of simple machines?
Screw – inclined plane wrapped around a cylinder
12.1 Force
14.3 MA & EfficiencyWhat are the six types of simple machines?
Pulley – a rope that fits into a groove in a wheel
12.1 Force
14.3 MA & EfficiencyWhat are the six types of simple machines?
Compound Machine – a combination of two or more simple machines
S-38Why is the efficiency of a machine always less than 100 percent?
A man uses a crowbar to break off a 40,000 N piece of rock. The rock moves a total distance of 0.08 m. The man pushes on the crowbar with a force of 4050 N while pushing the crowbar 0.80 m.
What is the efficiency of the crowbar?
S-39Why is the efficiency of a machine always less than 100 percent?
Bob has a job pulling out nails. It is not a very exciting job. When Bob applies a force of 5.5-N to his hammer, the nail move 0.03 m with a force of 25 N. Bob has to move the hammer 0.15 m.
A. What is the AMA of the hammer?
B. What is the IMA of the hammer?
C. What is the efficiency of the hammer?
S-40Test Day!