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Chapter 15


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? – PowerPoint PPT presentation

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Title: Chapter 15

Chapter 15 Work, Power 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
  • In order for work to be done, a force must be
    exerted over a distance.
  • Ex you can push on a wall for hours, youll be
    real tired, but you havent 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
  • Units
  • Force is measured in Newtons, Distance is
    measured in meters. So, the unit is Newton X
    meters. A Newtonmeter 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. OGormans 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

15-2 Power
  • Power
  • rate at which work is done
  • measured in watts (W)

P power (W) W work (J) t time (s)
15-2 Power
  • Formula
  • Since works formula is force X Distance, the
    formula for Power may ALSO be written as

15-2 Power
  • Units
  • 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 Power
  • When do we use Watts in our Daily Lives?
  • They are used to express electrical power.
  • Electric appliances and lightbulbs are rated in
  • 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
  • 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?

GIVEN P 2400 W W 120,000 J T ?
WORK t W P t 120,000 J 2400 W t 50 sec
15-2 Power
  • A figure skater lift his partner who weighs 450
    N, 1.5 m in 3.0 sec. How much power is required?

GIVEN P ? F 450 N d 1.5 m t 3.0 sec
P F x d t P 450 N x 1.5 m 3.0
sec P 625 J (Nm) 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

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
15-3 Machines
  • Machine def. Any device that changes the size
    of a force, or its direction, is called a
  • 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 machine
  • Work 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 Machines
  • Lets 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 Machines
  • Again, the formula for work is
  • Work force x distance
  • So, 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 Machines
  • Why is it that machines cant 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 Machines
  • If 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 machines
  • Efficiency is usually expressed as a percentage

15-3 Machines
  • Efficiency
  • measure of how completely work input is converted
    to work output
  • 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 ramps efficiency?

GIVEN Fi 500 N di 4.0 m Fo 1500 N do 1.0
WORK Win (500N)(4.0m) 2000 J Wout
(1500N)(1.0m) 1500 J E 1500 J 100
2000 J E 75
15-3 Machines
  • Mechanical Advantage is another way of expressing
    how efficient a machine is.
  • Mechanical advantage is the ratio of resistance
    force to the effort force.

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
15-4 Simple Compound Machines
  • Simple Machines
  • There are six types of simple machines. They are
  • 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 1
  • Def 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 Axle
  • Def - Made of 2 circular objects of different
    sizes attached together to rotate around the same

15-4 Simple Compound Machines
  • Compound Machine
  • Def - A combination of 2 or more simple machines