Presentation on theme: "Chapter 5 Lesson 2. What is a machine? AA machine is a device that makes doing work easier MMachines can be simple. SSome, like knives, scissors,"— Presentation transcript:
What is a machine? AA machine is a device that makes doing work easier MMachines can be simple. SSome, like knives, scissors, and doorknobs, are used everyday to make doing work easier.
Making Work Easier MMachines can make work easier by increasing the force that can be applied to an object. AA second way that machines can make work easier is by increasing the distance over which a force can be applied. MMachines can also make work easier by changing the direction of an applied force.
Increasing Force AA car jack is an example of a machine that increases an applied force. TThe upward force exerted by the jack is greater than the downward force you exert on the handle.
HHowever, the distance you push the handle downward is greater than the distance the car is pushed upward. TThe jack increases the applied force, but doesn't increase the work done.
Force and Distance TThe work done in lifting an object depends on the change in height of the object. TThe same amount of work is done whether the mover pushed the furniture up the long ramp or lifts it straight up. IIf work stays the same and the distance is increased, then less force will be needed to do the work.
Changing Direction SSome machines change the direction of the force you apply. TThe wedge-shaped blade of an ax is one example.
The Work Done by Machines WWhen you use an ax to split wood, you exert a downward force as you swing the ax toward the wood. TThe blade changes the downward force into a horizontal force that splits the wood apart.
WWhen you use a machine such as a crowbar, you are trying to move something that resists being moved. IIf you use a crowbar to pry the lid off a crate, you are working against the friction between the nails in the lid and the crate.
YYou also could use a crowbar to move a large rock IIn this case, you would be working against gravity—the weight of the rock.
Input and Output Forces TTwo forces are involved when a machine is used to do work. TThe force that is applied to the machine is called the input force. FF in stands for the effort force. TThe force applied by the machine is called the output force, symbolized by F out.
TTwo kinds of work need to be considered when you use a machine—the work done by you on the machine and the work done by the machine. TThe work done by you on a machine is called the input work and is symbolized by W in. TThe work done by the machine is called the output work and is abbreviated W out.
Conserving Energy WWhen you do work on the machine, you transfer energy to the machine. WWhen the machine does work on an object, energy is transferred from the machine to the object. TThe amount of energy the machine transfers to the object cannot be greater than the amount of energy you transfer to the machine.
Work Conservation of Energy can never get more work out than you put in trade-off between force and distance W in = W out F e × d e = F r × d r
Ideal Machines SSuppose a perfect machine could be built in which there was no friction. NNone of the input work or output work would be converted to heat. FFor such an ideal machine, the input work equals the output work.
SSuppose the ideal machine increases the force applied to it. TThis means that the output force, F out, is greater than the input force, F in. RRecall that work is equal to force times distance.
IIf F out is greater than F in, then W in and W out can be equal only if the input force is applied over a greater distance than the output force is exerted over.
Mechanical Advantage TThe ratio of the output force to the input force is the mechanical advantage of a machine. TThe mechanical advantage of a machine can be calculated from the following equation.
Force Effort Force (F e ) force applied to the machine “what you do” Resistance Force (F r ) force applied by the machine “what the machine does”
Mechanical Advantage Mechanical Advantage (MA) number of times a machine increases the effort force MA > 1 : force is increased MA < 1 : distance is increased MA = 1 : only direction is changed
Mechanical Advantage WWindow blinds are a machine that changes the direction of an input force. AA downward pull on the cord is changed to an upward force on the blinds.
TThe input and output forces are equal, so the MA is 1.
Ideal Mechanical Advantage TThe mechanical advantage of a machine without friction is called the ideal mechanical advantage, or IMA. TThe IMA can be calculated by dividing the input distance by the output distance.
Efficiency Efficiency measure of how completely work input is converted to work output always less than 100% due to friction
Calculating Efficiency IIn an ideal machine there is no friction and the output work equals the input work. So the efficiency of an ideal machine is 100 percent. TThe efficiency of a real machine is always less than 100 percent.
Increasing Efficiency MMachines can be made more efficient by reducing friction. This usually is done by adding a lubricant, such as oil or grease, to surfaces that rub together. AA lubricant fills in the gaps between the surfaces, enabling the surfaces to slide past each other more easily.
Mechanical Advantage A worker applies an effort force of 20 N to open a window with a resistance force of 500 N. What is the crowbar’s MA? GIVEN: F e = 20 N F r = 500 N MA = ? WORK : MA = F r ÷ F e MA = (500 N) ÷ (20 N) MA = 25 MA FrFr FeFe
Mechanical Advantage Find the effort force needed to lift a 2000 N rock using a jack with a mechanical advantage of 10. GIVEN: F e = ? F r = 2000 N MA = 10 WORK : F e = F r ÷ MA F e = (2000 N) ÷ (10) F e = 200 N MA FrFr FeFe