CHAPTER 5: WORK AND MACHINES

WORK WORK IN THE SENSE OF SCIENCE IS DIFFERENT THAN WHAT MOST PEOPLE CONSIDER WORK AS BEING. WORK IN THE SENSE OF SCIENCE IS DIFFERENT THAN WHAT MOST PEOPLE CONSIDER WORK AS BEING. Work: the transfer of energy when a force makes an object move. Work: the transfer of energy when a force makes an object move.

TWO CONDITIONS FOR WORK 1) The applied force must make the object move. 2) The movement must be in the same direction as the applied force.

For Example When lifting books off the ground, you do work on the books because books move upward and the force is also upward. When lifting books off the ground, you do work on the books because books move upward and the force is also upward. If you hold the books in your arms and do not move, then no work is being done. If you hold the books in your arms and do not move, then no work is being done. If you start to move, you will not be doing any work on the books because you are moving horizontally, but the force of your arms is still upward. If you start to move, you will not be doing any work on the books because you are moving horizontally, but the force of your arms is still upward.

ENERGY and WORK When work is done, a transfer of energy always occurs. When work is done, a transfer of energy always occurs. In other words, ENERGY IS THE ABILITY TO DO WORK. In other words, ENERGY IS THE ABILITY TO DO WORK. An object can transfer energy to another object by doing work on that object. An object can transfer energy to another object by doing work on that object. Energy is always transferred from the object that is doing the work to the object on which the work is done. Energy is always transferred from the object that is doing the work to the object on which the work is done.

WORK and POWER Work is calculated as: Work is calculated as: Work (J) = Force X distance or Work (J) = Force X distance or W = Fd W = Fd Power is the rate at which energy is transferred or the amount of work done per second and is measured in W or watts. Power is the rate at which energy is transferred or the amount of work done per second and is measured in W or watts. P=Work / time or P =E/t P=Work / time or P =E/t

TWO SYSTEMS OF WORK The English System The English System Force F= pounds Force F= pounds Distance d= feet Distance d= feet Time t = seconds Time t = seconds Work W = Fd = footpounds = ftlb Work W = Fd = footpounds = ftlb Power P = W/t = ftlb/s Power P = W/t = ftlb/s Horsepower 1HP = 550 ftlb/s Horsepower 1HP = 550 ftlb/s

METRIC SYSTEM Force F = Newton = N = 1kgm/s² Force F = Newton = N = 1kgm/s² Distance d = meter = m Distance d = meter = m Time t = second Time t = second WORK W = Fd = Newtonmeter = Nm = J WORK W = Fd = Newtonmeter = Nm = J POWER P= W/s = NM/s = J/s = Watt =W POWER P= W/s = NM/s = J/s = Watt =W Horsepower 1 HP = 746W Horsepower 1 HP = 746W

WORK – POWER – HORSEPOWER DIAGRAM

WORK W = F d

WORK – POWER – HORSEPOWER DIAGRAM WORK W = F d POWER P = W / t

WORK – POWER – HORSEPOWER DIAGRAM WORK W = F d POWER P = W / t METRIC SYSTEMMETRIC SYSTEM

WORK – POWER – HORSEPOWER DIAGRAM WORK W = F d POWER P = W / t METRIC SYSTEMMETRIC SYSTEM ENGLISH SYSTEMENGLISH SYSTEM

WORK – POWER – HORSEPOWER DIAGRAM WORK W = F d POWER P = W / t METRIC SYSTEMMETRIC SYSTEM ENGLISH SYSTEMENGLISH SYSTEM F=Newton =N d=meter F=pound d=foot

WORK – POWER – HORSEPOWER DIAGRAM WORK W = F d POWER P = W / t TIME METRIC SYSTEMMETRIC SYSTEM ENGLISH SYSTEMENGLISH SYSTEM F=Newton =N d=meter F=pound d=foot

WORK – POWER – HORSEPOWER DIAGRAM WORK W = F d Work = Nm = J Newtonmeter = Joule POWER P = W / t TIME METRIC SYSTEMMETRIC SYSTEM ENGLISH SYSTEMENGLISH SYSTEM F=Newton =N d=meter F=pound d=foot

WORK – POWER – HORSEPOWER DIAGRAM WORK W = F d Work = footpound ftlb Work = Nm = J Newtonmeter = Joule POWER P = W / t TIME METRIC SYSTEMMETRIC SYSTEM ENGLISH SYSTEMENGLISH SYSTEM F=Newton =N d=meter F=pound d=foot

WORK – POWER – HORSEPOWER DIAGRAM WORK W = F d Work = footpound ftlb Work = Nm = J Newtonmeter = Joule P = Nm/s = J/s = Watt POWER P = W / t TIME METRIC SYSTEMMETRIC SYSTEM ENGLISH SYSTEMENGLISH SYSTEM F=Newton =N d=meter F=pound d=foot

WORK – POWER – HORSEPOWER DIAGRAM WORK W = F d Work = footpound ftlb P = ftlb/s Work = Nm = J Newtonmeter = Joule P = Nm/s = J/s = Watt POWER P = W / t TIME METRIC SYSTEMMETRIC SYSTEM ENGLISH SYSTEMENGLISH SYSTEM F=Newton =N d=meter F=pound d=foot

WORK – POWER – HORSEPOWER DIAGRAM WORK W = F d Work = footpound ftlb P = ftlb/s Work = Nm = J Newtonmeter = Joule P = Nm/s = J/s = Watt POWER P = W / t TIME METRIC SYSTEMMETRIC SYSTEM ENGLISH SYSTEMENGLISH SYSTEM F=Newton =N d=meter F=pound d=foot 746 watts = 1 Horse Power = 550 ftlb/s

Example from page 128 You push a refrigerator with a force of 100N. If you move the refrigerator a distance of 5 m, how much work did you do? You push a refrigerator with a force of 100N. If you move the refrigerator a distance of 5 m, how much work did you do?

Section 2: Using Machines Machine: a device that makes doing work easier.

Work is Made Easier... by increasing the force applied. by increasing the force applied. A hammer used as a lever increases force. A hammer used as a lever increases force. by changing the distance over which the force is applied. by changing the distance over which the force is applied. An inclined plane increases distance but decreases force applied to do the same work. An inclined plane increases distance but decreases force applied to do the same work. by changing the direction of an applied force. by changing the direction of an applied force. A pulley changes the direction of the pull on the rope. A pulley changes the direction of the pull on the rope. Applied Force = is the force put into the machine Applied Force = is the force put into the machine

Work Input = Effort Force A force is any push or pull. A force is any push or pull. You are doing work when you use a force to cause motion. You are doing work when you use a force to cause motion. Machines make work easier, but they need energy to do work. Machines make work easier, but they need energy to do work. A person is usually the source of energy for a simple machine. A person is usually the source of energy for a simple machine. The force applied to the machine is called work input or effort force = W in The force applied to the machine is called work input or effort force = W in

Work Output = Resistance Force Machines do work, too. Machines do work, too. The machine exerts a force over a distance. The machine exerts a force over a distance. This is called output force. This is called output force. The work a machine does is called work output = W out The work a machine does is called work output = W out The work output is used to move a “force” you and the machine wish to move or overcome. The work output is used to move a “force” you and the machine wish to move or overcome. This is the resistance force or the load. This is the resistance force or the load.

Machines Work through a Distance The distance that is applied to a machine such as a pulley rope or an inclined plane’s length is called the effort distance. The distance that is applied to a machine such as a pulley rope or an inclined plane’s length is called the effort distance. The distance that the machine moves the object is called the resistance distance. The distance that the machine moves the object is called the resistance distance. Both of these are important when calculating such things as work, mechanical advantage, and efficiency. Both of these are important when calculating such things as work, mechanical advantage, and efficiency.

Recap Work input = work done by the user of the machine Work input = work done by the user of the machine Effort Force = the force done by the user of the machine Effort Force = the force done by the user of the machine Effort Distance = distance put into machine Effort Distance = distance put into machine Work output = work done by machine Work output = work done by machine Resistance Force = he weight of the object to be moved by the simple machine Resistance Force = he weight of the object to be moved by the simple machine Resistance Distance = distance the object is moved Resistance Distance = distance the object is moved

Conserving Energy Energy is always conserved. Energy is always conserved. W out is never greater than W in. W out is never greater than W in. A machine does not transfer all of the energy it receives to the object. A machine does not transfer all of the energy it receives to the object. Some of the energy is transferred to heat through friction. Some of the energy is transferred to heat through friction. So, W in is always greater than W out. So, W in is always greater than W out.

Ideal Machines W in = F in x d in W in = F in x d in W out = F out x d out W out = F out x d out For an ideal machine, meaning no loss to friction, W in = W out For an ideal machine, meaning no loss to friction, W in = W outOr F in x d in = F out x d out F in x d in = F out x d out See page 135 See page 135

Page 135 A hammer claw moves a distance of 1cm to remove a nail. If the output force of 1,500 N is exerted by the claw, and you move the handle of the hammer 5 cm, find the input force. A hammer claw moves a distance of 1cm to remove a nail. If the output force of 1,500 N is exerted by the claw, and you move the handle of the hammer 5 cm, find the input force.

Mechanical Advantage The ratio of the output force to the input force The ratio of the output force to the input force MA = F out /F in MA = F out /F in Ideal mechanical advantage is MA without friction Ideal mechanical advantage is MA without friction IMA = d in /d out IMA = d in /d out

Efficiency Efficiency = the measure of how much of the work put into a machine is changed into useful output work Efficiency = the measure of how much of the work put into a machine is changed into useful output work Efficiency (%) = output work / input work X 100% Efficiency (%) = output work / input work X 100% Ideal machine = 100% Ideal machine = 100% Real machine = less than 100% Real machine = less than 100% Lubricant reduced friction and increases efficiency. Lubricant reduced friction and increases efficiency.