How Machines Work

Silent Seven

•What is friction?

•Give an example and state what type of friction it is.

•When potential energy is converted to kinetic energy, what is always left?

Work and Power

• Work is the amount of force applied over a given distance

• Power is how quickly work is done

• Simple machines affect the amount of force we apply an give us a mechanical advantage for the amount of work we have to complete

Machine

• A machine is a device that allows you to do work that is easier by changing at least one of three factors:• The amount of force you exert

• The distance the forces is exerted

• The direction in which you exert the force

Input and Output Work

• Input work = input force * input distance

• Input force: the force you exert on a machine

• Input distance: the distance the input force moves the object

• Output work= output force * output distance

• Output force: the force the machine exerts on object

• Output distance: the distance the output force moves the object

Input and Output Work

• Output WORK of the machine CANNOT be greater than the input work of the person doing the work

• If your were digging the hole, which is doing more work, your hands or the shovel?

• However… The output FORCE can be greater, less than or equal to the input FORCE

Input and Output Work

Force Distance Direction Example How?

Output Force is greater than input force

Input distance is greater than output distance

Direction of the force remains the same

Pushing a box up a ramp

You push the box with less force up the ramp than if you were to lift the ramp

Input force is greater than output force

Output distance is greater than input distance

Direction of the force remains the same

Hitting a baseball with a bat

You move your arms a short distance and the bat moves the ball a much larger distance

Input force equals output force

Input distance equals output distance

Direction of force changes

Lifting weights on a machine

You change the direction of your force but the distance traveled or the weight does not change

Mechanical Advantage

• The number of times a machines increases a force exerted on it

• Mechanical advantage =

• Mechanical Advantage tells us how machines make things easier

Mechanical Advantage• Mechanical advantage =

• If the output force is greater than the input force, the M.A. is greater than 1• Ex. = = 3 N

• Force has increased

• If the input force is greater than the output force, the M.A. is less than 1• Ex. = = 0.5 N

• Distance has increased

What happed to the forces?

How was work made easier?

Mechanical Advantage

Example

the output force is greater than the input force

The force increased

greater than 1 N

= = 3 N

input force is greater than the output force

The distance increased

Less than 1 N

Ex. = = 0.5 N

input force equals the output force

The direction changed

1 N Ex. = = 1 N

Efficiency

• Every machine wastes some work while it is trying to overcome friction

• Compares the output and the input work of a machine

• Is expressed as a percent

• The higher the percent the more efficient the machine is

• * 100 % = % efficiency

Calculating Efficiency

• You do 200 Joules of work pushing a shopping cart. The wheels and the axels on the cart do 150 Joules of work to move the cart. What is the efficiency of the shopping cart?

To solve… We know:

• W input = 200 J

• W output = 150 J

• Our formula is * 100 % = % efficiency

• % efficiency * 100 %

=

75% efficiency The rest of the work goes to friction

Independent Practice

• In your seats complete the following sheet.

• For homework, complete the following textbook problems:• Pg. 419 lab, 2abc, 3ab

• 433 lab, 3a

• 437 1-10

• Whatever you do not finish is homework

Mechanical Advantage

Practice

Buzzer Beater 1. What is mechanical advantage?

2. What is efficiency?

3. To lift a television, you can apply a force of 50 N to a wedge. The wedge applies a force of 480 N to the weed. What is the mechanical advantage of the wedge?A. 10 N

B. 12 N

C. 15 N

D. 22 N

4. If the input distance was 0.5 meters and the output distance was 1 meter to move the same TV what is the input work and the output work?

5. Using the input and output work for the same TV, what was the efficiency of the wedge?