Chapter 14 work and power power point kremkus

CHAPTER 14
Work, Power and Machines

14.1 Work and Power
• Work requires motion .
• Work is the product of force and distance.
• Figure 1 work is only being done when the weight
lifter is lifting the barbell.
• Therefore work requires motion
• For a force to do work on an object some of the
force must act in the same direction as the object
moves. No movement, No work is done.

14.1 Work and Power

• Work depends on direction.
• Amount of work done on an object
depends on the direction of the force
and the direction of the movement.
• A force does not have to act entirely in
the direction of movement to do work.
•

14.1 Work

• Suitcase ex p. 413
• Pulling a suitcase the force acts upward and to
the right along the handle
• The suitcase moves only to the right along
ground.
• Horizontal portion of the force acting in the
direction of motion
• Only the horizontal part of the applied force the
part in the direction of movement does work
• Any part of a force that does not act in the
direction of motion does no work on an
object.

Calculating Work
• Work = Force x Distance
•W = F × D
• Units= Joule (J) Is the SI Unit of work
• Force =Newtons (N)
• Distance = meters (m)
• Newton – meters = Joules

Calculating work sample problem
A body builder lifts a 1600 N barbell
over his head and is lifted to a height of
2.0 meters. How much work has he
done?
W=F×D
W=1600N × 2m
W= 3200N·m=3200J

14.1 What is Power?

• Power- is the rate of doing work. The
amount of work done in a certain amount of
time.
• Need to use more power to do work at a
faster rate.
• To increase power you can increase the
amount of work done in a given time or
you can do a given amount of work in
less time.

14.1 Power

• Calculate Power
• Formula: Power = Work p= w
• time t
•
• SI Units= Watts (W) = 1 J/S
• Horsepower (hp) another common unit
• 1 hp = 746 watts

• Power = Watts [W]
• Work = Joules [J]
• Time = Seconds [s]
• Distance= Meters [m]

14.1 Power

• Sample Problems:
• 1. How much power does a light bulb contain if it does
600J of work in 5 Seconds?
• P= W Solving for Power. P = 600J = 120 W = 100 W
• t 5s sig. figs

• 2. If a car has 700W of power and does 2000J of work.
How much time was involved?
• Solving for time. t = W Changed the power formula
• P
• t = 2000J = 2. 86 s = 3 s with sig. figs
• 700W

14.2 Work and Machines
• Machine – A device that changes a
force.
• Machines make work easier to do
• -change the size of a force needed
• -the direction of a force
• -or the distance over which a force
acts.


• 1. Increasing Force
• Example: Jack handle
• A small force exerted over a large distance
becomes a large force exerted over a small
distance.
• However, if a machine increases the
distance over which you exert a
force, then it decreases the amount of
force you need to exert.


• 2. Increasing Distance
• Example: oars
• - decreases the applied force, but
increases the distance over which the force
is exerted.
• - However, a machine that decreases the
distance through which you exert the
force increases the amount of force
required.


• 3. Changing Direction
• Some machines change the direction
of the applied force
• Pulling on an oar – other ends moves
opposite


• Work input and work output
• -because of friction, the work done by a
machine is always less than the work done
on the machine.
• Work input of a machine
• Input force – force you exert on a machine.
• Input distance – the distance the input force
acts through.


• Work Input – the work done by the
input force acting through the input
distance
• Work input= input force × input
distance
• Explain perpetual motion page 419 (read
on own )

14.2 Work and Machnes

• Work output of a machine
• Work output force the force exerted by a
machine.
• Output distance – the distance the output
force is exerted through
• Work output- the output force multiplied
by the output distance.
• Remember you cannot get more work
out of a machine than you put in it.


• Explain movement of oar through the
water.
• Newton’s 3rd Law
• -fluid friction slows its motion.
• Output work is always less than
input work due to friction.

14.3 Mechanical Advantage and
Efficiency
• The mechanical advantage of a machine
is the number of times that the machine
increases an input force.
• The actual mechanical advantage (AMA)-
is the mechanical advantage determined by
measuring the actual forces acting on a
machine.
• AMA = the ratio of the output force to the
input force.

14.3 Mechanical Advantage and Efficiency

• AMA= output force
• input force
• Sample Problem:
• If you exert 100N on a jack to lift a
10,000N car, what would be the jack’s
AMA? (do on board)

14.3 Mechanical Advantage and Efficiency

• Ideal Mechanical Advantage – of a machine
is the mechanical advantage in the
absence of friction.
• To increase the mechanical advantage of a
machine you would reduce the friction.
• This would allow the mechanical advantage
of a machine be at its maximum possible
value.
• However, friction is always present and this
is why the AMA of a machine is always
less than the IMA.

Calculating the ideal mechanical advantage
• It is easier to calculate the actual mechanical
advantage because it depends only on the
locations of the forces and the distances they act
on.
• IMA = Input distance
• Output distance
• Sample Problem:
• What is the IMA of a 5m long ramp that rises 1m
off the ground at its end? (answer on board)

• If the input distance is greater than the output distance
the IMA has to be greater than 1.

14.3 Efficiency
• Efficiency of a machine is the percentage of the work
input that becomes work output.
• Will always be less than 100% because of friction.
• Formula:
• Efficiency = Work output ×100
• Work input

• Sample Problem: What is the efficiency of a machine that
has a work input of 40J and a work output of 35J?
(answer on board)

14.4 Simple Machines
• Machines (mechanical devices) are
combinations of 2 or more of the 6
different simple machines.
• The 6 types of simple machines are:
• 1. the lever 2. the wheel and axle
• 3. the inclined plane
• 4. the wedge 5. the screw
• 6. the pulley


• 1. The Lever
• A rigid bar that is free to move around a
fixed point.
• The fulcrum- is the fixed point the bar
rotates around.
• There are 3 classes of levers based on the
locations of the input force, the output
force, and the fulcrum.

14.4 Simple Machines- Levers

• Input arm of a lever is the distance
between the input force and the
fulcrum.
• Output arm of a lever is the distance
between the output force and the
fulcrum.
• You can calculate the IMA for a lever
by dividing the input arm by the output
arm.

14.3 Simple machines - Levers

• 3 Types of Levers
• 1. 1st Class Lever- identified by the position
of the fulcrum.
• The fulcrum is always located between the
input force and the output force.
• The mechanical advantage can be greater
than 1, equal to 1, or less than 1 depending
on the location of the fulcrum.
• Ex: Screwdriver, seesaw, scissors

14.4 Simple Machines-Levers

• 2nd Class Levers-
• Output force is located between the input
force and the fulcrum
• The mechanical advantage is always
greater than 1.
• Ex: Wheel barrow
• Handle is the input force, wheel is the
fulcrum, and the load lifted is the output
force.

14.3 Simple Machines- Levers

• 3rd Class Levers-
• The input force is located between the
fulcrum and the output force.
• The output distance over which it exerts its
force is always longer than the input
distance you move the lever through.
• The mech. Advantage is less than 1.
• Ex: baseball bat, hockey stick, golf
club, broom, rake


• Wheel and Axle
• Consists of two discs or cylinders, each one
with a different radius.
• Input force can be exerted on the wheel or
axle depending on the machine.
• Calculate IMA- divide the radius where the
input force is exerted by the radius where
the output force is exerted.
• Mech. Adv. can be greater or less than 1.


• Inclined Planes
• A slanted surface along which a force
moves an object to a different elevation.
• Distance along ramp is the input distance.
• The height of the ramp is the output
distance.
• IMA is the distance along the inclined plane
divided by its change in height.


• Wedges and Screws
• Like inclined planes because they have a
sloping surface but wedges and screws
move.
• Wedges- used to raise an object or split an
object apart.
• A v-shaped object whose sides are two
inclined planes sloped toward each other.
• Mech. Adv. is greater than 1.

14.3 Simple Machine

• Thinner wedges have a greater IMA than a
thick wedge as long as their widths are the
same.
• Screws
• An inclined plane wrapped around a
cylinder.
• Screws with threads that are closer
together have a greater IMA.


• Pulleys- pull with less force than is needed
to lift the load upward.
• A simple machine that consists of a rope
that fits into a groove in a wheel.
• They produce an output force that is
different in size, direction, or both, from that
of the input force.
• The IMA of a pulley or pulley system is
equal to the number of rope sections
supporting the load being lifted.


• Fixed Pulleys- is a wheel attached in a
fixed location.
• It is only able to rotate in place.
• Direction of the force is changed, but the
size of the force isn’t.
• IMA is 1. The input and output force are
about the same.
• Examples: Flagpole, blinds


• Moveable Pulley- A pulley is attached to
the object being moved not to a fixed
location.
• It reduces the input force needed to lift a
heavy object.
• Examples: sails, platforms (window
washing).
• Pulley system- combining fixed and
moveable pulleys.
• Large mech. Adv., depends on how the
pulleys are arranged.


• Compound Machine- A combination
of 2 or more simple machines that
operate together.

Chapter 14 work and power power point kremkus

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