Machines - Ch. 7

1
Machines - Ch. 7

• II. The Simple Machines
• (Chapter 8)

• Lever
• Pulley
• Wheel Axle

• Introduction to Machines
• Mechanical
• Advantage

2
Machines

• Machine
• device that makes work easier
• changes the size and/or direction of the force

3
Machines

• 2 kinds of work involved when a machine is used
• force applied to the machine
• what you do
• force applied by the machine
• what the machine does

4
Machines

• Work Input (Win)
• Work done on a machine
• You apply a force, called the input force, to the
  machine and move it through a distance

Win F d
5
Machines

• Work Output (Wout)
• work done by a machine
• The machine applies a force, called the output
  force, through a distance

Wout F d
6
Machines

• can never get more work out than you put in
• Work output can never be greater than work input

Win Wout F d F d
7
Machines

• Machines make work easier because they change the
  size or direction of the input force.
• Using a machine does not mean that you do less
  work.a machine allows a smaller force to be
  applied over a longer distance

8
Machines

• Force Distance Trade Off
• Force or distance can increase, but not
  together. When one increases, the other must
  decrease.
• ie. When force increases distance decreases

9
Mechanical Advantage

• Mechanical Advantage (MA)
• number of times a machine increases the force

10
Mechanical Advantage

• A worker use a crowbar to apply an input force of
  20 N to open a window with a output force of 500
  N. What is the crowbars MA?

WORK MA Foutput Finput MA (500 N) (20
N) MA 25
GIVEN Finput 20 N Foutput 500 N MA ?
11
Lever

• Lever
• a bar that pivots at a fixed point, called a
  fulcrum

12
Lever

• Levers are used to apply a force to a load
• There are three classes of levers, based on
  locations of the fulcrum, the load and the input
  force

13
Pulley

• Pulley
• grooved wheel that holds a rope or a cable
• A load is attached to one end of the rope and an
  input force is applied to the other end

F
Le
Lr
14
Wheel and Axle

• Wheel and Axle
• two wheels of different sizes that rotate
  together
• a pair of rotating levers

Wheel
Axle
15
Problems

• You use a 140 cm plank to lift a large rock. If
  the rock is 20 cm from the fulcrum, what is the
  planks MA?

GIVEN Loutput 20 cm Linput 140 cm MA ?
WORK MA Linput Loutput MA (140 cm) (20
cm) MA 7
16
Problems

• A crank on a pasta maker has a radius of 20 cm.
  The turning shaft has a radius of 5 cm. What is
  the MA of this wheel and axle?

GIVEN rinput 20 cm routput 5 cm MA ?
WORK MA rinput routput MA (20 cm) (5
cm) MA 4