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## Chapter 15

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### 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

1
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?

2
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, youll be
real tired, but you havent done any work in
the scientific sense, anyway

3
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)

4
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
5
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
6
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
7
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
8
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

9
15-2 Power
• Power
• rate at which work is done
• measured in watts (W)

P power (W) W work (J) t time (s)
10
15-2 Power
• Formula
• Since works formula is force X Distance, the
formula for Power may ALSO be written as

11
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).

12
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
Watts.
• Ex A 100 Watt light bulb does twice the work in
one second as a 50 Watt lightbulb.

13
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
14
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
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
WORK
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
16
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
17
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.

18
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!!!

19
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.

20
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

21
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.

22
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

23
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.
• Efficiency is usually expressed as a percentage
().

24
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.

25
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
m
WORK Win (500N)(4.0m) 2000 J Wout
(1500N)(1.0m) 1500 J E 1500 J 100
2000 J E 75
26
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.

27
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
28
15-4 Simple Compound Machines
• Simple Machines
• There are six types of simple machines. They are
the
• 1 - Inclined plane
• 2 - Wedge
• 3 - Screw
• 4 - Lever
• 5 - Pulley
• 6 - Wheel and axle

29
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.

30
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.

31
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!!!

32
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.

33
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

34
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

35
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
• Ex - a hammer or tweezer

36
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.

37
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