**Work**

A transfer of energy that occurs when a force makes an object move; measured in joules.

Definition

formula

Work = force x distance

W = F x d

Requirement of Work

the object must move

Arnold lifts a 1250 N barbell 2 m. How much work did he do?

Work = Force x Distance

W = 1250 N x 2m

W = 2500 J

If there is no movement, there is no work!

ex. pushing against a wall

For work to be done, the direction of the applied force must be in the SAME direction of motion!!!

ex. pushing against the front of a moving car and the car keeps moving opposite the direction you are pushing = NO work!

Joule = N x m

If Jane weighs 700 N and does 1400 J of work climbing a ladder. How tall is the ladder?

D = W/F

D = 1400 J / 700 N

D = 2 m

**Power**

**Definition**

The amount of work done in a certain amount of time.

**formula**

Power = work / time

P = W/t

Unit of Power = Watt

1 Watt = 1 J/s

People performing the same task do the same amount of WORK, but the person who does it fastest uses the most POWER.

A RATE is anything divided by time.

Arnold lifts a 1250 N barbell 2 m in 3 s. How much power was used?

Power = Work / Time

Work = Force x Distance

W = 1250 N x 2 m

W = 2500 J

P = 2500 J / 3 s

P = 833 Watts

Jane weighs 700 N and generates 800 watts of power climbing a 6 m ladder, how long would it take her to reach the top?

W = F x d

P = W/t

W = 700 N x 6 m

W = 4200 J

t = W/P

t = 4200/800

t = 5.25 s

**Energy, Work, and Simple Machines**

Chapter 10

Chapter 10

**Machines**

**Levers**

Mechanical Advantage

**Pulleys**

**Wheel & Axle**

**Screw**

**Inclined Plane**

Definition

A device that makes doing work easier.

3 Ways

increasing the force applied to an object

changing the direction of the applied force

increasing the distance over which a force can be applied

Compound Machine

a combination of 2 or more simple machines

**Wedge**

number of times a machine multiplies the effort force applied to it

Efficiency

measure of how much work put into a machine is changed into work done by a machine

Friction

makes machines less efficient

energy is transferred from mechanical energy into thermal energy

no machine is 100% efficient... why???

Oil and other lubricants are used to reduce friction.

but where does the energy go?? is it lost?

If a machine has a mechanical advantage of 1 does it help you do any work?

No. If the effort you put in equals the resistance you feel, then it is not really a machine because it isn't really helping you do work.

Air Resistance

friction due to air

As a pendulum swings, it gradualy comes to a stop because of air resistance.

ex. Newton's Cradle stops swinging.

Formula for Mechanical Advantage

Mechanical Advantage = resistance force / effort force

MA = Fr / Fe

There is no unit for MA, it is just a number.

decreases the amount of effort force needed because it increases the distance over which the effort force acts

the surface of the inclined plane makes a difference in efficiency

ex. sandpaper vs. oiled wood

to increase the mechanical advantage of an inclined plane, increase the length of the plane

As the length of the inclined plane increases, the effort force required to go up the ramp decreases.

Ramp

wheelchair ramp

winding mountian road

screw

MA = Length of ramp / Height of ramp

examples:

loading ramp

wheelchair ramp

winding mountain road

screw

definition

a rigid bar that pivots about a fixed point

3 parts of every lever

Effort Force - where you push or pull on the lever

Resistance Force - what you are trying to move or what is pushing back on you. (also called the load)

Fulcrum - the pivot point (not always in the middle)

3 classes of levers

each class of lever has one of these parts in the middle

1st Class

F - Fulcrum is in the middle

examples:

screwdriver opening a paint can

sea-saw (teeter totter)

2nd Class

Resistance

R - Resistance is in the middle

examples:

wheelbarrow

rolling a trash can to the curb

dolly

3rd Class

E - Effort is in the middle

f

R

e

e

"that spells free, Credit report.com bay bee!"

1st

2nd

3rd

My trick to remember the classes of levers...

MA of Levers

MA = Effort arm length

Resistance arm length

the LONGER the EFFORT arm,the easier it is to do work because it INCREASES the MA

for example, opening a paint can is way easier with a longer screwdriver than with a shorter one.

You always want to be pushing or pulling

on the side with the longest arm!

A lever is used to lift your mom's car to change a flat tire. The length of the effort arm is 4 feet. The length of the resistance arm is 2 feet. What is the MA of the lever?

MA = Effort arm / Resistance arm

MA = 4 feet / 2 feet

MA = 2

Spongebob needs a lever to raise a giant 10,000 N crabby patty off the ground and he only has 200 N of force. What is the mechanical advantage or ratio of the resistance force to the effort force?

MA = resistance / effort

MA = 10,000N / 200 N

MA = 50

The ratio is 50 to 1

examples:

swinging a baseball bat, hockey stick, broom, golf club

lifting weights

an inclined plane that wraps around a metal rod

Definition

**2 Purposes**

holds things together

raise heavy objects

MA is just the same as for an inclined plane

MA = Length of Incline

Height

The longer the incline, the greater the MA

**Definition**

an inclined plane with one or two sloping sides

examples

knife

door stop

If you sharpen a knife what happens to the MA and the effort required to cut something?

MA (or how helpful the machine is) increases.

The wedge becomes more useful.

The effort required to cut through something decreases because the machine is more useful.

**definition**

a simple machine that consists of a grooved wheel with a rope, chain, or cable that runs along a groove, changes the direction of the effort force, and can be fixed or movable.

2 Types

fixed

movable

Fixed Pulleys

are attached to something that doesn't move, such as a ceiling or wall

only change the direction of the force

effort is not multiplied

MA is only 1

Movable Pulleys

one end of the rope is fixed

the wheel is free to move

does multiply force

MA = number of supporting ropes

do not count the rope if you pull down on it, only the ones holding up the weight or that you are pulling up on

Block & Tackle Pulley

is a system of pulleys consisting of fixed and movable pulleys

200 N

How much force does the man have to pull with to raise the 200N block?

60 N

How much force does the hand need to pull down with?

MA = count the rope segments,

doesn't count if the person is pulling down on it

**definition**

simple machine that consists of two different-sized wheels that rotate together

Formula

MA = radius of wheel

radius of axle

What is the MA of this car wheel and axle?

Radius = 5 in

Radius = 30 in

MA = Radius of wheel / radius of axle

MA = 30 in / 5 in

MA = 6

Radius of Wheel = 50 cm

Radius of Axle = 10 cm

Force of Bucket = 400 N

MA = radius of wheel / radius of axle

MA = 50 cm / 10 cm

MA = 5

Resistance / MA = Effort

400 N / 5 = 80 N

Vocabulary:

Work-

Energy-

Kinetic Energy-

Work-Energy Theorem-

Joule-

Power-

Watt

Machine-

Effort Force-

Resistance Force-

Mechanical Advantage-

Ideal Mechanical Advantage--

Efficiency-

Compound Machine-

product of force and displacement in the direction of the force.

non-material property capable of causing changes in matter.

energy of object due to its motion

.

work done on object is equal to the change in its kinetic energy

SI unit of energy equal to one Newton-meter

rate of doing work

unit of power, one joule per second

device that changes force needed to do work

force extended on a machine

force exerted by a machine

ratio of resistance force to effort force in a machine

the ratio of effort distance to resistance distance

ratio of output work to input work

machine consisting of two or more simple machines

Kinetic Energy

energy of object due to its motion

Formula

Kinetic Energy = one half the mass x velocity squared

KE = 1/2*m*v^2

measured in Joules

INTERESTING?!

Work-Energy Theorem

Work is equal to the change in KE!

W = KE

W = KE final - KE initial

How can we calculate Work if we apply a force at an angle?

Work at an Angle