Unit 6 – Lecture 1

WorkWork = force applied over a certain distance

[force and distance are in the same direction]W = F * dForce (F) is measured in NewtonsDistance (d) is measured in metersWork = N*m – AKA: Joules (J)

NOTE: If the object does not move in the direction of the force, NO WORK IS ACCOMPLISHED.

Practice: W = F * dHow much work is accomplished by moving a

600N crate 7 m?How much work is accomplished by a 4500N

piledriver falling 25 m?How far do you have to push a 300 N crate to

accomplish 6000 J of work?What is the force applied to accomplish 20

Joules of work over a 50 meter distance?How much work is accomplished by an 80 N

person walking 10 m upstairs?

4200 Joules

112,500 Joules

20 meters

0.4 Newtons

800 Joules

PowerPower is a rate of the amount of work per

second.P = W / tWork is measured in Joules [Newton

meters]time is measured in secondsP = Joules/second[power is measured in watts]

PracticeTwo 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?

Neither [same]

Ben [takes less time]

Practice How much power will it take to move a 10 kg

mass at an acceleration of 2 m/s2 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.Just to help you out…

F = m * aW = F * dP = W / t

F = 10 * 2 F = 20 NW = 20 * 10 W = 200

JoulesP = 200/5 P = 40 watts

MachinesMachines are devices that make work easier.

They can do this by:Redirecting a forceMultiplying a forceBoth redirecting & multiplying a force

Work & MachinesMachines have:

a Work Input (WIN) and

a Work Output (WOUT).in an “ideal” machine, these would be

equal.

Work & Machines – cont’dWork Input:

the effort force multiplied by the effort distanceEffort Force (Fe ) [aka Input Force (Fi)]

= the force you put into a machineEffort distance (de)

= the distance the machine moves because of the effort force

WIN = Fe * de

Work & Machines – cont’dWork Output:

the resistance force multiplied by the resistance distanceResistance Force (Fr ) [aka Output Force (Fo)]

the force applied by the machine to another object

Resistance Distance (dr) = the distance the machine moves the objectbecause of the resistance force

WOUT = Fr * dr

Work & Machines – cont’dThere is an inverse [opposite] relationship between

the work which is done by a person and that done by the machine.In normal operations,

the person using the machine moves the machine with little force (Fe) through a large distance (de).

The machine moves an object with a large force (Fr) over a small distance (dr).

In essence, you are moving the machine an extra distance so the machine will apply extra force.

PracticeWhat force do you apply to a machine?How far does the machine move another object?Which components comprise the Work Output?Which of these do YOU increase on a machine?

effort distance, effort force, resistance distance, resistance force

Which of these does the machine increase?effort distance, effort force, resistance distance, resistance force

Mechanical AdvantageMechanical Advantage – the number of times

the machine multiplies the force you have applied to it.[your force * MA = the output force]MA = F0 / Fi

Because we put force above and below, there are no units for MA. It is a number (coefficient) showing how many times the force is multiplied.

EfficiencyEfficiency – work output divided by the work

inputefficiency = (WOUT / WIN) * 100

will be expressed as a percentage

HONORS ONLY

Practice MA = Fo / FiWhat is the mechanical advantage of a

machine that applies 35 N for the 25 N of force put into it?

What is the MA of a machine that applies 100 N for the 15 N of force put into it?

What is the Force Output of a machine with a MA of 12 when you put 9 N of force into it?

1.4

6.7

108 N

MachinesA Simple Machine accomplishes the work in

one motion.

A Compound Machine is made up of two or more simple machines. Most machines are compound machines.

Simple MachinesThere are six types of simple machines:

LeverWheel-and-AxlePulleyInclined PlaneWedgeScrew

HomeworkComplete

Work I and Work II worksheets