BellringerBellringer

Compare and explain in complete

sentences what is work

HomeworkHomework

CALCULATE KINETIC ENERGY OF AN OBJECT

DROPPED FROM A BUILDING 10 M HIGH, MASS 50 kg

JUST BEFORE IT HITS THE GROUND

ObjectivesObjectives

Define work and identify the units Describe the conditions that must exist for a force to

do work on an object Calculate the work done on an object Describe and calculate power Compare the units of watts and horsepower as they

relate to power

WorkWork Definition

- quantity of energy transferred by a force when it is applied to a body and causes that body to move in the direction of force

Formula- W = F x D

Units- Newton meter (N/m) or a joule (J)

Work ContinuedWork Continued Two factors

1. size of force, and direction it is applied

ex: pulling a suitcase

* any part of a force that does not act in the direction of motion does no work on an

object

2. movement of something by that force

Work Cont.Work Cont.

- A weight lifter who holds a barbell weighing 1000 N does NO work

- Why?

- he may get very tired, but if the barbell is not moved by the force he exerts, he does no work on the barbell

- work is done on his muscles when he raises the barbell

Work ProblemsWork ProblemsQ: A crane uses an average force of 5200 N to lift a

girder 25 m. How much work does the crane do on the girder?

A: W = (5200N)(25m) = 1.3 x105 J

Q: While rowing in a race, John uses his arms to exert a force of 165 N per stroke while pulling the oar 0.800 m. How much work does he do in 30 strokes?

A: W = (30)(165N)(0.800m) = 3960 J 4.0 x 103

Work Problems Cont.Work Problems Cont.

Q: Jake, a 235 N track athlete completes his race, which totals 1575 J, what is the total distance Jake ran?

A: 1575 J = 6.70 m

235 N

Q: Joey, performed 900 J of work, while lifting a box 12 meters. What force did Joey exert on the box?

A:75 N

PowerPower Definition

- a quantity that measures the rate at which work is done

Formula- P = W/t

Unit- Joule per second (J/s) or Watts (W) - Horsepower (Hp) = 746 W

Power ProblemsPower Problems

Q: Using a jack, a mechanic does 5350 J of work to lift a car 0.500 m in 50.0 s. What is the mechanic’s power output?

A: P = 5350J/ 50.0s = 107 W

Q: Anna walks up the stairs on her way to class, She weighs 565 N and the stairs go up 3.25 m vertically. Calculate the power output if she climbs the stairs in 12.6 s. What is her power output if she climbs the stairs in 10.5 s?

A: P = (565N)(3.25m)/12.6s =146 W P = (565N)(3.25m)/10.5s = 175 W

ObjectivesObjectives Describe what a machine is and how it makes work

easier Relate the work input to a machine to the work

output of the machine Compare a machines actual mechanical advantage

to its ideal mechanical advantage Calculate the ideal and actual mechanical and actual

mechanical advantages of various machines Explain why efficiency of a machine is always less

than 100% Calculate the machines efficiency

MachinesMachines

Definition- a device that changes force

ex. jack, nutcracker, an oar

How does a machine change force?- 3 ways a machine makes work easier to perform

- change the size of force needed- the direction of a force- the distance over which a force acts

Changing the ForceChanging the Force Increasing the force you applied

ex. jack applied to a car

- small force exerted over a large distance becomes a large force exerted over a short distance

Changing the DistanceChanging the Distance Increasing distance

ex. rowing a boat with oars

- small movement of oar at the hands makes a large distance the oar in the water will move.

*remember the trade off: small distance large force*

Changing the DirectionChanging the Direction Changing the direction of the applied force

ex. rowing a boat

- pulling back on the handle of the oars causes its other end to move in the opposite direction.

Work InputWork Input Work Input

- the work done on a machine as the input force acts through the input distance

- input force: force exerted on a machine- input distance: distance the input force acts through

- equals the input force multiplied by the input distance

ex. rowing a boat- input distance < output distance- input force > output force

Work OutputWork Output Work Output

- the work done by a machine as the output force acts through the output distance

- output force: force exerted by a machine

- output distance: distance the output force is exerted through

*due to friction the work done by a machine is always less than the work done on the machine

Mechanical AdvantageMechanical Advantage Definition

- a quantity that measures how much a machine multiplies force or distance

Two types Actual: measures the actual forces action on a

machine

- AMA = output force

input force Ideal: measures the mechanical advantage in the absence

of friction

- IMA = input distance

output distance

Mechanical Advantage ProblemsMechanical Advantage ProblemsQ: Alex pulls on the handle of a claw hammer with a

force of 15 N. If the hammer has a actual mechanical advantage of 5.2, how much force is exerted on a nail in the claw?

A: output force = (5.2)(15N) = 78 N

Q: If you exert 100 N on a jack to lift a 10,000 N car, what would be the jack’s actual mechanical advantage (AMA)

A: AMA= 10,000 N / 100 N = 100

Mechanical Advantage ProblemsMechanical Advantage Problems

Q: Calculate the ideal mechanical advantage (IMA) of a ramp that is 6.0 m long and 1.5 m high?

A: IMA = 6.0m / 1.5m = 4.0

Q: The IMA of a simple machine is 2.5. If the output distance of the machine is 1.0 m, what is the input distance?

A: Input distance = (2.5)(1.0m) = 2.5 m

Efficiency of MachinesEfficiency of Machines Definition

- a quantity, usually expressed as a percentage, that measures the ratio of useful work input

Formula

- Efficiency = useful work output

work input

- % of work input that becomes work output

- due to friction, efficiency of any machine is always less than 100%

Efficiency ProblemsEfficiency ProblemsQ: Alice and Jim calculate that they must do 1800 J of

work to push a piano up a ramp. However, because they must also overcome friction, they must actually do 2400 J of work. What is the efficiency of the ramp?

A: 1800 J/ 2400 J x 100 = 75%

Q: If the machine has an efficiency of 40%, and you do 1000 J of work on the machine, what will be the work output of the machine?

A: Work Output = (Efficiency x work input) / 100% Work Output = (40% x 1000 J) / 100% = 4.0 x 102 J

ObjectivesObjectives Name, describe and give an example of each of the

six types of simple machines Describe how to determine the ideal mechanical

advantage of different types of simple machines Define and identify compound machines Recognize simple machines within compound

machines

Simple MachinesSimple Machines Definition

- one of the six basic types of machines 2 types or families

1. lever 2. inclined planes

LeversLevers Definition

- a rigid bar that is free to move around a fixed point

ex. screwdriver- all levers have a rigid arm that turns around a

point called the fulcrum- force is transferred from one part of the arm to

another- original input force can be multiplied or

redirected into output force- levers are divided into 3 classes, based on the

locations of the input force, output force, and the fulcrum

Lever Family Cont.Lever Family Cont. 3 classes

1. First class- fulcrum is in the middle of an arm - the input force acts on one end - the other end applies an output force- MA can be: <1, >1, =1

ex. teeter totter, scissors, tongs2. Second class

- fulcrum is at one end of the arm - input force is applied to the other end - output force is in the middle- MA will always > 1

ex. wheel barrow

Lever Family Cont.Lever Family Cont.

3. Third class

- input force is in the middle

- output force is one one end

- fulcrum is on the other end

- multiplies distance rather than force

MA always < 1

ex. baseball bats, hockey sticks, golf clubs, human body

Wheel and AxisWheel and Axis Definition

- simple machine that consists of two disks or cylinders, each one with a different radius

ex. steering wheel, screwdriver

- made of a level or a pulley (wheel) connected to a shaft (axle)

- small input force, multiplied to become a large output force

Inclined PlanesInclined Planes Definition

- slanted surface along which a force moves an object to a different elevation

ex. knife, ax, zipper, wedge, screw- ramp redirects the force applied to lift object upward- turns a small input force into a large output force by

spreading the work out over a large distance- wedge: functions as two inclined planes back to back, turning a downward force into two forces directed out to the sides-screw: an inclined plane wrapped around a cylinder

Pulleys Pulleys Definition

- a simple machine that consists of a rope that fits into a groove in a wheel.

- very similar to a lever- point in the middle of the pulley is like a fulcrum- rest of the pulley acts like the rigid arm

3 Types of Pulleys- fixed pulleys- moveable pulleys- pulley system

Types of PulleysTypes of Pulleys

- fixed pulleys: changes the direction of the input force

- moveable pulleys: changes both the direction and the size of the input force

- pulley system: made of both fixed and moveable pulleys

Compound MachinesCompound Machines Definition

- a machine that is made of more than one simple machine

ex. scissors, jacks, bicycle, washing machine, car, clock