SIMPLE MACHINES

Chapter Nine: Simple Machines

Ø 9.1 Types of Simple Machines

Ø 9.2 Mechanical Advantage

Ø 9.3 Levers in the Human Body

Chapter 9.1 Learning Goals

Ø Define machine.

Ø Identify examples of simple machines.

Ø Explain the meaning of input and output.

Investigation 9A

Ø Key Question: How does a lever work?

Levers

9.1 Using Machines

Ø A machine is a device with moving parts that work together to accomplish a task.

Ø A bicycle is a good example.

9.1 Using Machines Ø The input includes everything you do to

make the machine accomplish a task, like pushing on the bicycle pedals.

Ø The output is what the machine does for you, like going fast or climbing a steep hill.

9.1 Forces in Machines

Ø A simple machine is an unpowered mechanical device, such as a lever.

9.1 Review of work Ø  Every process that is done by machines can

be simplified in terms of work:

1. work input: the work or energy supplied to the process (or machine).

2. work output: the work or energy that comes out of the process (or machine).

9.1 Review of energy Ø A rope and pulley

machine illustrates a rule that is true for all processes that transform energy.

Ø The total energy or work output can never be greater than the total energy or work input.

Chapter Nine: Simple Machines

Ø 9.1 Types of Simple Machines

Ø 9.2 Mechanical Advantage

Ø 9.3 Levers in the Human Body

Chapter 9.2 Learning Goals

Ø Define mechanical advantage in terms of input and output forces.

Ø Classify levers as first, second, or third class.

Ø Evaluate the mechanical advantage of different simple machines.

9.2 Mechanical advantage Ø Machines multiply forces.

Ø One person could lift an elephant—quite a heavy load—with a properly designed system of ropes and pulleys!

9.2 Mechanical advantage Ø Mechanical advantage is the ratio of output force to input force.

MA = Fo Fi

Output force (N)

Input force (N)

mechanical advantage

9.2 The Lever Ø A lever includes a stiff structure (the lever) that rotates around a fixed point called the fulcrum.

9.2 The Lever Ø Levers are useful because you can arrange the fulcrum and the input arm and output arm to adjust the mechanical advantage of the lever.

9.2 The Lever Ø Each class of levers is defined by the location of the input and output forces relative to the fulcrum.

9.2 Gears Ø Many machines require that rotating motion be transmitted from one place to another.

Ø Gears change force and speed.

9.2 Designing Gear Machines Ø The gear ratio is the ratio of output turns

to input turns.

Ø You can predict how force and speed are affected when gears turn by knowing the number of teeth for each gear.

To = Ni Ti No

Number of teeth on input gear Turns of output gear

Turns of input gear Number of teeth

on input gear

9.2 Tension Ø Ropes and strings carry tension forces

along their length. Ø If the rope is not moving, its tension is

equal tothe force pulling on each end.

9.2 Ramps Ø A ramp is a simple machine that allows

you to raise a heavy object with less force than you would need to lift it straight up.

Ø The mechanical advantage of a ramp is the ramp length divided by the height of the ramp.

9.2 Screws Ø A screw is a rotating ramp.

Ø You find the mechanical advantage of a screw by dividing its circumference by the lead.

9.2 Screws Ø A wedge is like a ramp that

can work while in motion (a ramp is always stationary).

Ø A wedge has a side that slopes down to a thin edge.

Ø The mechanical advantage for a wedge is inversely related to the size of the wedge angle.

9.2 Wheel and axle Ø A wheel rotates around a

rod called an axle.

Ø The mechanical advantage is the ratio of the radius of the wheel to the radius of the axle.

Ø The wheel and axle move together to move or lift loads.

Solving Problems

Ø A crowbar is a type of lever that you use to pull a nail out of a piece of wood.

Ø If the handle of a crowbar is 40 centimeters and the foot is 2 centimeters, what is its mechanical advantage?

1.   Looking for: Ø  …mechanical advantage of lever

2.   Given Ø  …input arm = 40 cm; output arm = 2 cm

3.   Relationships: Ø  M.A. = Length of input arm

Length of output arm

4.   Solution Ø  M.A. = 40 cm ÷ 2 cm = 20

Solving Problems

Chapter Nine: Simple Machines

Ø 9.1 Types of Simple Machines

Ø 9.2 Mechanical Advantage

Ø 9.3 Levers in the Human Body

Chapter 9.3 Learning Goals

Ø Locate human body parts that act as levers.

Ø Classify human body as first, second, or third class levers.

Investigation 9B

Ø Key Question: What types of levers does your body have?

Levers and the Human Body

9.3 Levers in the human body Ø The human body is a complex machine that includes a number of simple machines—levers.

Ø Your arms and legs, for example, work as levers to move and lift objects.

9.3 Levers in the human body Ø A classic example of a

third class lever is a broom.

Ø A broom does not multiply force, but it does multiply speed.

Ø Since your limbs are third class levers, they multiply speed to do tasks quickly.

9.3 Levers in the human body Ø Human arms and legs

are examples of third class levers because the input forces are between a fulcrum and the output force.

Ø The output force is what you accomplish with your hands and feet.

Where is the input force and the fulcrum?

9.3 Levers in the human body Ø In the human body, all bones act as levers and each joint can serve as a fulcrum. Ø  When lifting your head, your neck works as

a first-class lever. Ø  When you stand on your toes, the feet act

as second-class levers. Ø  When biting, your jaw works as a third-

class lever.

Investigation 9C

Ø Key Question: Ø  What is mechanical

advantage, and how do ropes and pulleys give you mechanical advantage?

Mechanical Advantage

Prosthetics in Action Ø Hugh Herr, Ph.D., a

physicist and engineer studies biomechanics and prosthetic technology. In addition to holding several patents in this field, he has developed highly specialized feet for rock climbing that are small and thin.