Work and Simple Machines

What is work? In science, the word work has a

different meaning than you may be familiar with.

The scientific definition of work is: using a force to move an object a distance (when both the force and the motion of the object are in the same direction.)

Work or Not? According to the

scientific definition, what is work and what is not? a teacher lecturing

to her class a mouse pushing a

piece of cheese with its nose across the floor

What’s work?A scientist delivers a speech to

an audience of his peers. A body builder lifts 350 pounds

above his head. A mother carries her baby from

room to room. A father pushes a baby in a carriage.A woman carries a 20 kg grocery

bag to her car?

What’s work?A scientist delivers a speech to

an audience of his peers. NoA body builder lifts 350 pounds

above his head. YesA mother carries her baby from

room to room. No A father pushes a baby in a carriage.

YesA woman carries a 20 km grocery

bag to her car? No

Formula for work

Work = Force x Distance

The unit of force is newtonsThe unit of distance is metersThe unit of work is newton-metersOne newton-meter is equal to one jouleSo, the unit of work is a joule

W=FD

Work = Force x Distance

Calculate: If a man pushes a concrete block 10 meters with a force of 20 N, how much work has he done?

W=FDWork = Force x

Distance

Calculate: If a man pushes a concrete block 10 meters with a force of 20 N, how much work has he done? 200 joules(W = 20N x 10m)

PowerPower is the rate at which work

is done.

Power = Work*/Time *(force x

distance)

The unit of power is the watt.

Check for Understanding

1.Two physics students, Ben and Bonnie, are in the weightlifting room. Bonnie lifts the 50 kg barbell over her head (approximately .60 m) 10 times in one minute; Ben lifts the 50 kg barbell the same distance over his head 10 times in 10 seconds.

Which student does the most work? Which student delivers the most

power?Explain your answers.

Ben and Bonnie do the same amount of work; they apply the same force to lift the same barbell the same distance above their heads.

Yet, Ben is the most powerful since he does the same work in less time.

Power and time are inversely proportional.

 

2. How much power will it take to move a 10 kg mass at an acceleration of 2 m/s/s a distance of 10 meters in 5 seconds? This problem requires you to use the formulas for force, work, and power all in the correct order.

Force=Mass x Acceleration

Work=Force x Distance

Power = Work/Time

2. How much power will it take to move a 10 kg mass at an acceleration of 2 m/s/s a distance of 10 meters in 5 seconds? This problem requires you to use the formulas for force, work, and power all in the correct order.

Force=Mass x Acceleration Force=10 x 2Force=20 N

Work=Force x DistanceWork = 20 x 10

Work = 200 Joules

Power = Work/TimePower = 200/5

Power = 40 watts

History of Work

Before engines and motors were invented, people had to do things like lifting or pushing heavy loads by hand. Using an animal could help, but what they really needed were some clever ways to either make work easier or faster.

The 6 Simple Machines

Lever

Pulley Wheel and Axle

WedgeScrewInclined Plane

Simple MachinesAncient people invented simple

machines that would help them overcome resistive forces and allow them to do the desired work against those forces.

Energy: Ability to do work

Work= Force x Distance

Force: A Push or a Pull

Definitions:

Inclined Plane

Inclined Plane The Egyptians used simple machines to build the

pyramids. One method was to build a very long incline out of dirt that rose upward to the top of the pyramid very gently. The blocks of stone were placed on large logs (another type of simple machine - the wheel and axle) and pushed slowly up the long, gentle inclined plane to the top of the pyramid.

Inclined Planes An inclined plane is

a flat surface that is higher on one end

Inclined planes make the work of moving things easier

Work input and outputWork input is the amount of work

done on a machine. Input force x input distance

Work output is the amount of work done by a machine.Output force x output distance

15 m

3 m

Wout = Win

Fout x Dout = Fin x Din

10N x 3m = 2N x 15m 10 N Fin

DinDout

Inclined Plane -Mechanical Advantage

The mechanical advantage of an inclined plane is equal to the length of the slope divided by the height of the inclined plane.

While the inclined plane produces a mechanical advantage, it does so by increasing the distance through which the force must move.

Screw

The mechanical advantage of a screw can be calculated by dividing the circumference by the pitch of the screw.

Pitch equals 1/ number of turns per inch.

Wedges Two inclined

planes joined back to back.

Wedges are used to split things.

Wedge – Mechanical Advantage The mechanical advantage of a wedge can be found

by dividing the length of either slope (S) by the thickness (T) of the big end.

S

As an example, assume that the length of the slope is 10 inches and the thickness is 4 inches. The mechanical advantage is equal to 10/4 or 2 1/2. As with the inclined plane, the mechanical advantage gained by using a wedge requires a corresponding increase in distance.

T

Fulcrum is between EF (effort) and RF (load)Effort moves farther than Resistance. Multiplies EF and changes its direction

The mechanical advantage of a lever is the ratio of the length of the lever on the applied force side of the fulcrum to the length of the lever on the resistance force side of the fulcrum.

First Class Lever

First Class Lever.

Common examples of first-class levers include crowbars, scissors, pliers, tin snips and seesaws.

RF (load) is between fulcrum and EF Effort moves farther than Resistance.

Multiplies EF, but does not change its direction The mechanical advantage of a lever is the ratio of the distance from the applied force to the fulcrum to the distance from the resistance force to the fulcrum.

Second Class Lever

Second Class LeverExamples of

second-class levers include nut crackers, wheel barrows, doors, and bottle openers.

EF is between fulcrum and RF (load) Does not multiply force Resistance moves farther than Effort. Multiplies the distance the effort force travels

The mechanical advantage of a lever is the ratio of the distance from the applied force to the fulcrum to the distance of the resistance force to the fulcrum

Third Class Lever

Third Class LeverExamples of

third-class levers include tweezers, arm hammers, and shovels.

Pulleys Pulley are wheels

and axles with a groove around the outside

A pulley needs a rope, chain or belt around the groove to make it do work

Diagrams of Pulleys

Fixed pulley: A fixed pulley changes the direction of a force; however, it does not create a mechanical advantage.

Movable Pulley: The mechanical advantage of a moveable pulley is equal to the number of ropes that support the moveable pulley.

COMBINED PULLEY The effort needed to

lift the load is less than half the weight of the load.

The main disadvantage is it travels a very long distance. 

WHEEL AND AXEL The axle is stuck

rigidly to a large wheel. Fan blades are attached to the wheel. When the axel turns, the fan blades spin.

Wheel and Axel The mechanical advantage of a wheel and axle is the

ratio of the radius of the wheel to the radius of the axle.

In the wheel and axle illustrated above, the radius of the

wheel is five times larger than the radius of the axle. Therefore, the mechanical advantage is 5:1 or 5.

The wheel and axle can also increase speed by applying the input force to the axle rather than a wheel. This increase is computed like mechanical advantage. This combination would increase the speed 5 times.

51

GEARS-Wheel and Axel Each gear in a

series reverses the direction of rotation of the previous gear. The smaller gear will always turn faster than the larger gear.

Rube Goldberg Machines Rube Goldberg machines are

examples of complex machines. All complex machines are made

up of combinations of simple machines.

Rube Goldberg machines are usually a complicated combination of simple machines.

By studying the components of Rube Goldberg machines, we learn more about simple machines

When you slip on ice, your foot kicks paddle (A), lowering finger (B), snapping turtle (C) extends neck to bite finger, opening ice tongs (D) and dropping pillow (E), thus allowing you to fall on something soft.

Safety Device for Walking on Icy Pavements