Unit 12 - Mechanical Advantage (day 1)

Unit 12 - Mechanical Advantage (day 1)

By S Harris

You need to consider the work done by you and the work done by the machine.

• The work done by you on the machine is called INPUT WORK, W_in.

• The work done by the machine is called the OUTPUT WORK, W_out.

Types of work​

​

The number of times a machine multiplies the effort force is the mechanical advantage (MA) of the machine.

• you divide the load (F_o) by the effort (F_i)​

What is mechanical advantage?​

​Load

​MA

​Effort

weight is actually a force that acts on a mass and is due to the force of gravity.

• Weight is measured in newtons.

Weight

Step 1: determine the input (effort) and output (load).

Step 2: solve for MA​

Example:

A piano weighs 500 N. If you were to use a pulley to lift the piano with a force of about 150 N, then what is the mechanical advantage? ​

​MA =

load

---------- =

effort

500 N

---------- =

150 N

​3.3

Multiple Choice

A crate weighs 950 N. If you can use a pulley system to lift that crate with a force of only 250 N, then what is the mechanical advantage?

• 3.8

• 26

• 700

• 1200

Machines can increase force or speed and change the direction, but it will not/cannot change the amount of work done on the object.

RECALL

Types of Simple Machines​

• A straight, slanted or sloping surface,

• Easier to lift something by increasing the distance over which the force is applied.

• In other words, it reduces the amount of force needed by Increasing the distance over which the force is applied..

Inclined Plane

• An inclined plane wrapped in a spiral around a cylinder with a wedge at the tip.

• As you turn the screw, the threads pull the screw into the wood.

• The inclined plane slides through the wood.

• Makes work easier because...

  • It applies much force with little effort

  • It holds things together​

Screws

• An inclined plane with one or two sloping sides that moves.

• Wider or thicker at one end than at the other.

Wedge

Makes work easier by cutting, splitting or prying apart objects as it moves (including air and water).

• Wedges change the direction of the force.

How does it make work easier?

• A bar that is free to pivot or turn around a fixed point called a fulcrum.

• Force applied to a lever is called the effort force.

• Object moved is the load.

Lever

TYPES OF LEVERS

​

The fulcrum is between the effort force (input) and the load/resistance force (output).

• in the middle = fulcrum​

1st Class Levers

​

1st Class Examples​

​Seesaw

Scissors

​Shovel

Crowbar

​

the output force (load) is in the middle of the lever with the input force (effort) and fulcrum at each end.

2nd Class​

2nd Class Examples​

Wheelbarrow

​Stapler

Nail clipper

​

the effort (input force) is in between the fulcrum and the load (output force).​

3rd Class​

3rd Class Examples​

fishing pole

tongs

Your arm

broom

• Makes work easier by:

  • Multiplying your effort force and changing the direction of your force

How do these help?

• A grooved wheel with a rope, chain, or cable running along the groove.

• The two sides of the pulley are the effort arm and the resistance arm.

Pulley

• Fixed pulley:

  • ​Cannot multiply the effort force, only changes the direction of the effort force.

• Movable Pulley

  • Can multiply the effort force and change the direction of the effort force.

  • Because effort force increases, the distance must increase to conserve energy. You pull more.​

How do pulleys work?

​

​A machine consisting of two wheels of different sizes that rotate together.

• A wheel is connected to a post (axle)​

Wheel and Axle

Makes work easier because it applies more force or lifts a heavy load with less effort.

• Usually the effort is exerted on the larger wheel. Then the smaller wheel (axle) exerts the resistance force.​

How does it make work easier?