Chapter 15 – Work, Power Chapter 15 – Work, Power & Simple Machines& Simple Machines

Chapter 15 – Work, Power Chapter 15 – Work, Power & Simple Machines& Simple Machines

Essential Questions: I. What is Work? (In Physics Terms!)II. What is Power? (In Physics Terms!)III. How do machines make work easier and how efficient are they?IV. What are the 5 types of simple machines?V. What are compound machines?

15-1 What is Work? Work

Def. – Work is done when a force acts on an object along the parallel direction the object moves

In order for work to be done, a force must be exerted over a distance. Ex – you can push on a wall for

hours, you’ll be real tired, but you haven’t done any work – in the scientific sense, anyway…

15-1 Work Work

The amount of work done in moving an object is equal to the force applied to the object along the direction the object moves times the distance through which the object moves

Distancex Force Work

Units Force is measured in Newtons, Distance

is measured in meters. So, the unit is Newton X meters. A Newton•meter is known as a Joule (J)

15-1 Work A 700 N person climbs a 50 m cliff. How

much work did she perform?

GIVEN:

W = F * d

F = 700 N

d = 50 m

WORK:

W = F * d

W = (700 N) (50 m)

W = 35,000 J

15-1 Work An object weighing 200 N is lifted 0.5 m.

How much work was required?

GIVEN:

W = F * d

F = 200 N

d = 0.5 m

WORK:

W = F * d

W = (200 N) (0.5 m)

W = 100 J

15-1 Work A dog does 50 N-m (Joules) of work

dragging a 0.05 N bone. How far did the bone move?

GIVEN:

W = F * d

W = 50 J

F = 0.05 N

WORK:

W = F * d

d = W

F

d = (50 J)

(0.05 N)

d = 1,000 m

15-1 Work Mrs. O’Gorman’s superhuman strength

allows her to lift a pickup truck 2.0 m above the ground. How much force was required if 25.0 Joules (J) of work was done?

GIVEN:

W = F * d

W = 25.0 J

d = 2.0 m

WORK:

W = F * d

F = W

d

F = 25.0 J

2.0 m

F = 12.5 N

15-2 Power Power

Def: The rate at which work is done, or the amount of work per unit time.

Power tells you how fast work is being done – so it is a rate – similar to the way speed, velocity and acceleration are rates. Power is work per unit time.

Any measurement per unit time is a rate!!

Formula:

Time

Work Power

15-2 PowerPower

rate at which work is donemeasured in watts (W)

t

WP

P: power (W)

W: work (J)

t: time (s)

15-2 PowerFormula:Since work’s formula is force X Distance,

the formula for Power may ALSO be written as:

Time

Distancex Force Power

15-2 PowerUnits

Work is measured in Joules (J), So, the unit for Power is a Joule per second (J/s).

The short way to write a J/s is a Watt (W).

15-2 PowerWhen do we use Watts in our

Daily Lives?They are used to express

electrical power.Electric appliances and

lightbulbs are rated in Watts.Ex: A 100 Watt light bulb does

twice the work in one second as a 50 Watt lightbulb.

15-2 Power A small motor does 4000 J of work in

20 sec. What the power of the motor in Watts?

GIVEN:

W = 4000 J

T = 20 sec

P = ?

WORK:

P = W ÷ t

P = 4000 J ÷ 20 s

P = 200 J ÷ s

So P = 200 W

15-2 Power

GIVEN:

P = 2400 W

W = 120,000 J

T = ?

WORK:

t = W ÷ P

t = 120,000 J ÷ 2400 W

t = 50 sec

An engine moves a remote control car by performing 120,000 J of work. The power rating of the car is 2400 W. How long does it take to move the car?

15-2 Power

GIVEN:

P = ?

F = 450 N

d = 1.5 m

t = 3.0 sec

WORK:

A figure skater lift his partner who weighs 450 N, 1.5 m in 3.0 sec. How much power is required?

P

F x d

tP = F x d tP = 450 N x 1.5 m

3.0 secP = 625 J (N•m)

3.0 sec P = 225 W

15-2 Power A sumo wrestler lifts his competitor, who

weighs 300 N, 2.0 m using 300 Watts of power. How long did it take him to accomplish this show of strength?

GIVEN:

F = 300 N

d = 2.0 m

P = 300 W

t = ?

WORK:

P = W ÷ t

W = F x d

W = (300 N)(2.0 m) = 600 J

t = 600 J ÷ 300 W

t = 2.0 s

P

W

t

15-3 Machines

Machine – def. – Any device that changes the size of a force, or its direction, is called a machine.

Machines can be anything from a pair of tweezers to a bus.

15-3 Machines

There are always 2 types of work involved when using a machineWork Input - The work that

goes into it.Work Output - The work that

comes out of it.The work output can NEVER be

greater than the work input!!!

15-3 Machines

So, if machines do not increase the work we put into them, how do they help us?

Machines make work easier because they change either the size or the direction of the force put into the machine.

15-3 MachinesLet’s analyze this…Machines can not increase the

amount of work, so work either stays the same or decreases.

The formula for work is:Work = force x distance

15-3 MachinesAgain, the formula for work is:

Work = force x distanceSo, mathematically speaking, to

end up with the same or less work:If the machine increases the

force then the distance must decrease.

If the machine increases the distance, then the force must decrease.

15-3 MachinesWhy is it that machines can’t have

more work output than input? Where does all the work disappear to?

A machine loses some of the input work to the force of friction that is created when the machine is used.

Part of the input work is used to overcome the force of friction.

There is no machine that people have made that is 100% efficient

15-3 MachinesIf machines make our work

easier, how much easier do they make it?

The ratio of how much work output there is to the amount of work input is called a machine’s efficiency.

Efficiency is usually expressed as a percentage (%).

15-3 MachinesEfficiency

measure of how completely work input is converted to work output

100%W

WEfficiency

in

out

It is always less than 100% due to the opposing force of friction.

15-3 Machines A worker exerts a force of 500 N to push a

1500 N sofa 4.0 m along a ramp that is 1.0 m high. What is the ramp’s efficiency?

GIVEN:

Fi = 500 N

di = 4.0 m

Fo = 1500 N

do = 1.0 m

WORK:

Win = (500N)(4.0m) = 2000 J

Wout = (1500N)(1.0m) = 1500 J

E = 1500 J × 100 2000 J

E = 75%

1.0m

1500N

4.0m

500N

100%in

out

W

WE

15-3 MachinesMechanical Advantage is

another way of expressing how efficient a machine is.

Mechanical advantage is the ratio of resistance force to the effort force.

forceeffort

force resistance AdvantageMechanical

15-3 Machines A worker exerts a force of 500 N to push a

1500 N sofa 4.0 m along a ramp that is 1.0 m high. What is the mechanical advantage of the ramp?

GIVEN:

Fe = 500 N

Fr = 1500 N

WORK:

MA = F resistance

F effort

MA = 1500N 500 N

MA = 3

1.0m

1500N

4.0m

500N

effort

res

F

FMA

15-4 Simple & Compound Machines

Simple MachinesThere are six types

of simple machines. They are the: 1 - Inclined plane 2 - Wedge 3 - Screw 4 - Lever 5 - Pulley 6 - Wheel and axle

15-4 Simple & Compound Machines 1 - Inclined Plane

Def - A slanted surface used to raise an object.

The force needed to lift the object decreases because the distance through which the object moves increases.

15-4 Simple & Compound Machines

2 - Wedge - Inclined Plane Type #1Def – an

inclined plane that moves in order to push things apart. Tines of a

fork, axe, knife.

15-4 Simple & Compound Machines 3 - Screw - Inclined Plane Type #2 -

Def - An inclined plane wrapped around a central bar or cylinder, to form a spiral. Ex – screw –duh!!!

15-4 Simple & Compound Machines 4 - Lever

Def - A rigid bar that is free to pivot, or move around a fixed point called a fulcrum. Ex – see saw

There are three main types (classes) of levers.

15-4 Simple & Compound Machines

3 classes of levers:First-class levers have

the fulcrum placed between the load and the effort, as in the seesaw, crowbar, and balance scale. Ex - a see-saw or

scissors

15-4 Simple & Compound Machines 3 classes of

levers:Second-class

levers have the load between the effort and the fulcrum. Ex - a wheel

barrow

15-4 Simple & Compound Machines 3 classes of levers:

Third-class levers have the effort placed between the load and the fulcrum. The effort always travels a shorter distance and must be greater than the load. Ex - a hammer or tweezer

15-4 Simple & Compound Machines 5 - Pulley

Def - A rope, chain or belt wrapped around a grooved wheel.

It can change the direction of force or the amount of force needed to move an object.

15-4 Simple & Compound Machines To calculate how

much mechanical advantage a pulley system creates… Count the number of ropes that are attached to the MOVEABLE pulley – that # is your mechanical advantage!!!

15-4 Simple & Compound Machines 6 - Wheel &

AxleDef - Made of

2 circular objects of different sizes attached together to rotate around the same axis.

15-4 Simple & Compound Machines

Compound MachineDef - A combination of 2 or

more simple machines