mfe chapter 4 notes

Work, Power, and Simple Work, Power, and Simple MachinesMachines

WorkWork PowerPower MachinesMachines Simple and Compound MachinesSimple and Compound Machines

WorkWork

Work is defined as a force acting Work is defined as a force acting through a distance.through a distance.

Work=Force x DistanceWork=Force x Distance W=Fd=Newtons x meters=N-m=JouleW=Fd=Newtons x meters=N-m=Joule A Joule (J) is an amount of work or A Joule (J) is an amount of work or

energy that is equivalent to applying a 1 energy that is equivalent to applying a 1 Newton force to an object for a distance Newton force to an object for a distance of 1 meter. of 1 meter.

Energy is the ability to do work.Energy is the ability to do work.

Work (continued)Work (continued)

Work= Force (F) x Distance (x)

PowerPower

Power is the rate at which work is Power is the rate at which work is done.done.

Power=Work/TimePower=Work/Time P=W/t=Joule/s=WattP=W/t=Joule/s=Watt 1 Kilowatt (kW)= 1000 Watts1 Kilowatt (kW)= 1000 Watts

MachinesMachines

A machine is a device that makes A machine is a device that makes work easier.work easier.

Work input is the work that is put Work input is the work that is put into the machine (the input distance into the machine (the input distance times the input or effort force).times the input or effort force).

Work output is the work that a Work output is the work that a machine does (the output distance machine does (the output distance times the output or resistant force).times the output or resistant force).

Machines (continued)Machines (continued)

Machines make work easier by Machines make work easier by changing the input force or distance or changing the input force or distance or by changing the direction of input force.by changing the direction of input force.

Work is conserved.Work is conserved. Theoretically, the work put into a Theoretically, the work put into a

machine must equal the work that machine must equal the work that comes out of a machine. comes out of a machine.

In practice this never happens due to In practice this never happens due to energy being lost to friction.energy being lost to friction.

Machines (continued)Machines (continued)

A comparison of input work to output A comparison of input work to output work is called efficiency.work is called efficiency.

The efficiency of a machine is equal The efficiency of a machine is equal to work output/work input.to work output/work input.

Due to friction, no machine is 100% Due to friction, no machine is 100% efficient.efficient.

Machines with the smallest amount Machines with the smallest amount of friction are the most efficient.of friction are the most efficient.

Machines (continued)Machines (continued)

The mechanical advantage of a The mechanical advantage of a machine is also important to know.machine is also important to know.

Mechanical advantage tells how many Mechanical advantage tells how many times the input force is multiplied.times the input force is multiplied.

The greater the mechanical The greater the mechanical advantage, the less force is required advantage, the less force is required to do the work.to do the work.

This comes at the cost of distance, This comes at the cost of distance, however.however.

Simple MachinesSimple Machines

All machines are either one or more All machines are either one or more simple machine.simple machine.

There are 6 simple machines:There are 6 simple machines: Inclined plane, wedge, screw, lever, Inclined plane, wedge, screw, lever,

pulley, and wheel and axle.pulley, and wheel and axle.

Simple Machines (continued)Simple Machines (continued)

An inclined plane is a flat, sloped An inclined plane is a flat, sloped surface like a ramp.surface like a ramp.

The longer the inclined plane is, the The longer the inclined plane is, the less force is needed to raise the less force is needed to raise the object because of the decreased object because of the decreased slope.slope.

Therefore, longer inclined planes Therefore, longer inclined planes with less slope yield greater with less slope yield greater mechanical advantages.mechanical advantages.

Simple Machines (continued)Simple Machines (continued)

Simple Machines (continued)Simple Machines (continued)

A wedge is a moving inclined plane.A wedge is a moving inclined plane. Many wedges are made from two Many wedges are made from two

inclined planes stuck together like an inclined planes stuck together like an ax.ax.

The longer and thinner the wedge, The longer and thinner the wedge, the greater its mechanical the greater its mechanical advantage.advantage.

Simple Machines (continued)Simple Machines (continued)

Simple Machines (continued)Simple Machines (continued)

A screw is an inclined plane wrapped A screw is an inclined plane wrapped around a central bar to form a spiral.around a central bar to form a spiral.

A screw gets its mechanical A screw gets its mechanical advantage from the length of the advantage from the length of the inclined plane wrapped around it.inclined plane wrapped around it.

The closer together the threads on a The closer together the threads on a screw, the greater its mechanical screw, the greater its mechanical advantage. advantage.

Simple Machines (continued)Simple Machines (continued)

Simple Machines (continued)Simple Machines (continued)

A lever is a rigid bar that is free to A lever is a rigid bar that is free to pivot about a fixed point called a pivot about a fixed point called a fulcrum.fulcrum.

A good example is the see-saw.A good example is the see-saw.

Simple Machines (continued)Simple Machines (continued)

Simple Machines (continued)Simple Machines (continued)

There are three classes of levers.There are three classes of levers. A first class lever has a fulcrum in A first class lever has a fulcrum in

the middle.the middle. A second class lever has the A second class lever has the

resistance in the middle.resistance in the middle. A third class lever has the effort in A third class lever has the effort in

the middle.the middle. Just remember FRE.Just remember FRE.

Simple Machines (continued)Simple Machines (continued)

Simple Machines (continued)Simple Machines (continued)

Notice that different classes of levers Notice that different classes of levers change the direction of the input change the direction of the input force.force.

This can sometimes make work This can sometimes make work easier.easier.

The mechanical advantage of a lever The mechanical advantage of a lever is generated by changing the length is generated by changing the length of the force or effort arm relative to of the force or effort arm relative to the resistant arm of the lever.the resistant arm of the lever.

Simple Machines (continued)Simple Machines (continued)

A pulley is a rope, belt, or chain A pulley is a rope, belt, or chain wrapped around a wheel.wrapped around a wheel.

A fixed pulley can change the A fixed pulley can change the direction of input force.direction of input force.

Pulleys can be combined in systems.Pulleys can be combined in systems. The mechanical advantage of a The mechanical advantage of a

moveable pulley system is equal to moveable pulley system is equal to the number of lifting lines.the number of lifting lines.

Simple Machines (continued)Simple Machines (continued)

A wheel and axle is a simple machine A wheel and axle is a simple machine made from two circular objects of made from two circular objects of different sizes.different sizes.

The wheel is the larger of the two.The wheel is the larger of the two. The mechanical advantage of a The mechanical advantage of a

wheel is generated by the difference wheel is generated by the difference in the radii.in the radii.

Compound MachinesCompound Machines

A compound machine is a A compound machine is a combination of two or more simple combination of two or more simple machines.machines.