Chapter 5

Work and Machines

Work

Transfer of energy that occurs when a force makes an object move.

Work – Two Conditions

1. The object must move.2. The motion must be in the

same direction as the applied force.

Is Work Done?

Picking up a stack of books.Holding a stack of books.Walking while holding a stack

of books.

Work and Energy

When work is done, a transfer of energy occurs (energy = ability to do work)

Ex. When you pick up a box, you transfer energy from your muscles to the box, increasing its PE

Calculating Work

Work = Force X distanceW=FdF = Nd = mW = N-m = J

Power

Is the rate at which work is done.

Amount of work done in a certain amount of time.

Calculating Power

Power = Work/timeP = W/tW = Jt = secP = J/sec = Watts (W)

Power and Energy

Energy can be transferred without involving work.

Ex. Light bulb transfers energy into heat and light.

P = E/t

Machines

A device that makes doing work easier.

Work by:

–Increasing F (car jack)

–Increasing d (ramp)

–Changing direction (ax)

Work done by Machines

Two forces involved

–Effort Force (Fe) – the force applied to a machine.

–Resistance Force (Fr) – the force applied by the machine to overcome resistance.

Work done by Machines

Two types of work involved:

–Input work (Win) – work done by you on a machine.

–Output work (Wout) – work done by the machine.

Conserving Energy

What does the Law of Conservation of Energy state?

You transfer energy to the machine, and the machine transfers energy to an object.

Conserving Energy

A machine cannot create energy, so Wout is never greater that Win.

Can Wout = Win ? Why?

Ideal Machines

What is an ideal Machine?If a machine allows you to

apply less force, then how do you get the same amount of work out of the machine?

Mechanical Advantage (MA)

It is the number of times a machine multiplies the effort force.

IMA vs AMA

Calculating MA

MA = resistance force/effort force

MA = Fr/Fe

MA has no units. Why?

Efficiency

A measure of how much of the work put into a machine is converted into useful output work.

Why is work output always less than work input?

How can you efficiency?

Calculating Efficiency

Efficiency = (Wout/Win) X 100%

Simple Machines

A machine that does work with only one movement.

Six Types: 1)lever, 2)pulley, 3)wheel and axle, 4) inclined plane, 5) screw, and 6) wedge.

Lever

A bar that is free to pivot about a fixed point or fulcrum.

Effort arm = distance from fulcrum where effort force is applied.

Resistance arm = distance from fulcrum where resistance force is applied

Types of Levers

Based on positions of effort force, resistance force, and fulcrum.

Types of Levers

1st class – fulcrum is in the middle (crowbar)

2nd class – resistance is in the middle (wheelbarrow)

3rd class – effort is in the middle (baseball bat)

IMA of Levers

IMA = length of effort arm/length of resistance arm

IMA = Le/Lr

Pulleys

A grooved wheel with a rope, chain or cable running along the groove.

How is a pulley a modified 1st class lever?

Fixed Pulleys

Attached to something that does not move

Only change the direction of the force

IMA = 1

Moveable Pulleys

One end of the rope is fixed and the wheel is free to move.

Multiplies effort forceIMA = 2

Block and Tackle

A system of pulleys made of fixed and moveable pulleys

IMA = # of supporting rope segments. Only count effort segment if effort force is in the direction of the object’s movement.

Wheel and Axle

Machine consisting of two wheels of different sizes that rotate together.

Modified form of a lever.IMA = rw/ra

Gears

Modified Wheel and Axle with two wheels of different sizes with interlocking teeth.

Large wheel = effort gear, small wheel = resistance gear.

Larger effort gear = more turns of resistance gear = effort force

Inclined plane

Sloping surface that reduces the amount of force required to do work.

Increases distanceIMA = length/height

Screw

Inclined plane wrapped, in a spiral, around a cylinder.

Wedge

Moving inclined plane with one or two sloping sides.

Changes the direction of effort force

Compound Machines

When two or more simple machines are used together.

Examples?