define the following terms a. work b. power c. machine

• Define the following terms

• A. Work

• B. Power

• C. 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 distance

• For work to be done force must act in the same direction as the movement

• Any 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 joules• F = force in newtons• d = distance in meters

• The unit is the same as energy because work is the transfer of energy

12.1 Force


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 Equation• Answer

12.1 Force


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?


12.1 Force


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 watts• W = work in joules• t = time in seconds

• Power is sometimes measured in horsepower, 1 HP = 746W

12.1 Force


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?


12.1 Force


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?


12.1 Force


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 object

• Work input – the input force x input distance• Work 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

http://en.wikipedia.org/wiki/File:Four_pulleys.svg

• 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


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


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


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


Wheel and Axle – two disks or cylinders, with different radii

12.1 Force


Inclined Planes – a slanted surface

12.1 Force


Wedges and Screws are moving inclined planes

Wedge – V shaped object

12.1 Force


Screw – inclined plane wrapped around a cylinder

12.1 Force


Pulley – a rope that fits into a groove in a wheel

12.1 Force


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?

define the following terms a. work b. power c. machine

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