Work & Machines

Topics

• Work and Power– Definition, Calculation, and Measurement

• Using Machines– Nature of Machines– Mechanical Advantage– Efficiency of Machines

• Simple Machines

Work

• Definition: Work is the application of a force to an object that results in the movement of the object over a certain distance.– For work to occur, object must move– The motion of the object must be in the same

direction as the applied force on the object

• Work and energy are related. Energy is the ability to do work. Motion results when work is being done.

Example Is Work Being Done?

The teacher pushes on the wall until she is tired.

No. The wall did not move.

A book falls off the table and hits the floor.

Yes. Gravity applied a force and moved the book in the direction of the floor.

The waiter carries a tray of food

No. The force to hold the tray is not applied in the direction of the motion.

A rocket acclerates through space

Yes. The force of the rocket thrust is causing the rocket to move.

Calculating Work

• Work equals the product of a force (F) and the distance (d) over which the force is applied.

• Work (W) = Force (F) x Distance (d)

• SI Unit: Joules = kg x m2/s2

Example

• A mover pushes a box for 5 meters across the floor. He applies a force of 100N directly to the floor. How much work does he do?

Power

• Definition: Amount of work done in a certain amount of time; or Rate at which work is done.

• Calculating Power:

• Measured in watts (w)

Power = Work (Energy)

time

Using Machines

• What is Machine

• Mechanical Advantage

• Efficiency of Machine

Using Machines

• Definition of Machine – Device that makes work easier to do.

• Machines increase applied force (input force) and/or change direction of applied force to make work easier.

• Because W = Fd, therefore, increasing distance reduces the amount of force needed to do the work.

Mechanical Advantage• Mechanical Advantage (MA) is the number

of times a machine multiplies the input FORCE (NOT WORK).

• Input Force (FI) – force applied to machine, aka, Effort Force

• Output Force (FO) – force applied by machine to do the task or overcome resistance, aka, Resistance Force

• When a machine takes a small input force and increases the magnitude (strength) of output force, a mechanical advantage is produced.

Calculating MA

• Example, if a machine increases an input force of 10N to an output force of 100N, the machine has a mechanical advantage of ( )??

Efficiency in Machines

• In machines, when talking about efficiency, it is WORK (NOT FORCE)

• There are two types of WORK

• Work Input (Win) = Work done by you on a machine.

• Work Output (Wout) = Work done by the machine

Ideal Machine

• Win = Wout

• But according to Conservation of Energy, no energy is created or lost. Therefore machines cannot create energy.

• Wout is never greater than Win.

• Some energy is transferred in the form of heat due to friction.

• Lubricant can make machines more efficient by reducing friction

Efficiency

• Because the amount of work put into a machine is always greater than the amount of work done by the machine, machines are never 100 percent efficient.

• Definition: Measure of how much of the work put into (Win) a machine is changed into useful output (Wout) by the machines

Calculating Efficiency

• Efficiency is a comparison between the work output and the work input of a machine.

EfficiencyWork = X 100%Work Output

Work Input

Simple Machines

• Definition: A machine that does work with only one movement.

• To make work easier, simple machines change forces by:– Change the direction– Change the strength (magnitude)– Change both

Types of Simple Machines

• Lever

• Inclined Plane

– Wedge

– Screw

• Wheel and Axle

– Gear

– Pulley

Lever

• Definition: A bar that is free to pivot about a fixed point called fulcrum.

• The bar may be straight or curved.• Three Parts:

– Effort arm (Force) – part of lever where effort force is applied

– Resistance arm (Load) – part of lever where resistance force is applied

– Fulcrum – Support or balance

Three Types of Lever

• Depending on the location of the resistance arm (resistance force) and the effort arm (effort force) relative to the fulcrum, there are three types of lever:

First-Class Lever• Definition: Fulcrum is located between the

effort and resistance forces; effort being further than resistance; multiples and changes direction of force.

Examples

• Crowbars, scissors, pliers, seesaws.

Second-class Lever• Resistance force is located between the

effort force and fulcrum; always multiples force.

• No change in direction of force. Greater MA is resulted when fulcrum is closer to the load

Examples

• Nutcrackers, wheel barrows, doors, and bottle openers

Third-class Lever• Effort force is between resistance force

and fulcrum; does NOT multiply force, but does increase distance over which force is applied.

• Always produce a gain in speed and distance and a corresponding decrease in force.

Examples

• Tweezers, hammers, brooms, shovel, your arm

Inclined Plane

• Definition: A sloping surface that reduces the amount of force required to do work.

• It is easier to move a weight from a lower to higher elevation

• MA of inclined plane is equal to the length of the slope divided by the height of the slope.

• Less force is required if a ramp is longer and less steep

Although it takes less force for car A to get to the top of the ramp, all the cars do the same amount of work.

A B C

Wedge

• Definition: Inclined plane with one or two sloping sides. If two sides, the planes are joined back to back.

• Wedges are used to split things.

• Examples: Axe, knife, your teeth, saw, doorstop

Screw

• Definitions: inclined plane wrapped in a spiral around a cylindrical post

MA of an screw can be calculated by dividing the number of turns per inch.

Screw

• Three parts: head, shaft, and tip.

• Head: part where you exert force

• Shaft: has ridges called threads that wind around the screw.

• Tip is usually sharp.

• Uses: fasten things together.

• Examples: jar lid, screws, drill bids

Wheels and Axle

• Definition: machine with two wheels of different size rotating together.

• Consists of a large wheel rigidly secured to a smaller wheel or shaft called axle.

• Example: doorknob,

MA of Wheel and Axle

• The mechanical advantage of a wheel and axle is the ratio of the radius of the wheel to the radius of the axle

51

Gear

• Modified form of wheel and axle.

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

• Example: Can opener, bicycle, egg beater

Pulley

• Definition: Grooved wheel with a rope, simple chain, belt or cable running along the groove is a pulley.

Types of Pulley

• Fixed pulley is attached to something that doesn’t move. Force is not multiplied, but direction is changed.

• Movable pulley has one end of the rope fixed and the wheel free to move; multiplies force.

• Block and tackle (compound) system of pulleys of fixed and movable pulleys.

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