 # The Meaning of Work - What Is Work?

## Presentation on theme: "The Meaning of Work - What Is Work?"— Presentation transcript:

The Meaning of Work - What Is Work?
Work is done on an object when the object moves in the same direction in which the force is exerted.

Work Work = Force x Distance A student drags a backpack 10 meters by applying a force of 50 Newtons. Calculate the work done by the student.

Power Power = Work Time If the student in the previous problem drags the backpack over the period of 20 seconds, calculate the power required If the backpack is dragged over the period of 10 seconds, calculate the power reqiuired.

How Machines Do Work Machine
A machine is a device that allows you to do work in an easier or more effective way A machine makes work easier by changing one or more of three things The force you exert The distance over which you exert the force The direction over which you exert the force Input Force: The force you exert Output force: The force the machine exerts

Calculating Power - What Is Work?
A tow truck exerts a force of 11,000 N to pull a car out of a ditch. It moves the car a distance of 5 m in 25 seconds. What is the power of the tow truck? Plan and Solve What quantity are you trying to calculate? The Power (P) the tow truck uses to pull the car = __ What formula contains the given quantities and the unknown quantity? Power = (Force X Distance)/Time Perform the calculation. Power = (11,000 N X 5.0 m)/25 s Power = (55,000 N•m)/25 s or 55,000 J/25 s Power = 2,200 J/s = 2,200 W

Calculating Power Practice Problem - What Is Work?
A crane lifts an 8,000-N beam 75 m to the top of a building in 30 seconds. What is the crane’s power? 20,000 W or 20 kW

Input and Output Work - How Machines Do Work
The amount of input work done by the gardener equals the amount of output work done by the shovel.

- How Machines Do Work Screwdriver or Faucet

Mechanical Advantage Mechanical Advantage = Output Force Input Force See example pg. 416

Mechanical Advantage - How Machines Do Work
The input force and output force for three different ramps are shown in the graph.

What variable is plotted on the horizontal axis? Input force

Mechanical Advantage - How Machines Do Work Interpreting Data:
If an 80-N input force is exerted on Ramp 2, what is the output force? 400 N

Mechanical Advantage - How Machines Do Work Drawing Conclusions:
Why does the slope represent each ramp’s mechanical advantage? Which ramp has the greatest mechanical advantage? The slope of each ramp’s graph equals the change in output force divided by the change in input force. This is the formula for mechanical advantage. Ramp 1 has the greatest mechanical advantage.

Calculating Efficiency
- How Machines Do Work Calculating Efficiency You do 250,000 J of work to cut a lawn with a hand mower. If the work done by the mower is 200,000 J, what is the efficiency of the lawn mower? Read and Understand What information have you been given? Input Work (Winput) = 250,000 J Output Work (Woutput) = 200,000 J

Calculating Efficiency
- How Machines Do Work Calculating Efficiency You do 250,000 J of work to cut a lawn with a hand mower. If the work done by the mower is 200,000 J, what is the efficiency of the lawn mower? Plan and Solve What quantity are you trying to calculate? The efficiency of the lawn mower = __ What formula contains the given quantities and the unknown quantity? Efficiency = Output work/Input work X 100% Perform the calculation. Efficiency = 200,000 J/250,000 J X 100% Efficiency = 0.8 X 100% = 80% The efficiency of the lawn mower is 80 percent.

Calculating Efficiency
- How Machines Do Work Calculating Efficiency You do 250,000 J of work to cut a lawn with a hand mower. If the work done by the mower is 200,000 J, what is the efficiency of the lawn mower? Look Back and Check Does your answer make sense? An efficiency of 80 percent means that 80 out of every 100 J of work went into cutting the lawn. This answer makes sense because most of the input work is converted to output work.

Calculating Efficiency
- How Machines Do Work Calculating Efficiency Practice Problem You do 20 J of work while using a hammer. The hammer does 18 J of work on a nail. What is the efficiency of the hammer? 90%

Calculating Efficiency
- How Machines Do Work Calculating Efficiency Practice Problem Suppose you left your lawn mower outdoors all winter. Now it’s rusty. Of your 250,000 J of work, only 100,000 J go to cutting the lawn. What is the efficiency of the lawn mower now? 40%

Inclined Plane - Simple Machines
An inclined plane is a flat, sloped surface.

Wedge - Simple Machines
A wedge is a device that is thick at one end and tapers to a thin edge at the other end.

Screws - Simple Machines
A screw can be thought of as an inclined plane wrapped around a cylinder.

Levers - Simple Machines
A lever is a ridged bar that is free to pivot, or rotate, on a fixed point.

Levers - Simple Machines
Levers are classified according to the location of the fulcrum relative to the input and output forces.

Wheel and Axle - Simple Machines
A wheel and axle is a simple machine made of two circular or cylindrical objects fastened together that rotate about a common axis.

Wheel and Axle - Simple Machines
You can find the ideal mechanical advantage of a wheel and axle by dividing the radius of the wheel by the radius of the axle.

Pulley - Simple Machines
A pulley is a simple machine made of a grooved wheel with a rope or cable wrapped around it.

Types of Pulleys Activity
- Simple Machines Types of Pulleys Activity Click the Active Art button to open a browser window and access Active Art about types of pulleys.

Simple Machines in the Body
Most of the machines in your body are levers that consist of bones and muscles.

Compound Machines - Simple Machines
A compound machine is a machine that utilizes two or more simple machines.

Identifying Main Ideas
- How Machines Do Work Identifying Main Ideas As you read the section “What Is a Machine?” write the main idea in a graphic organizer like the one below. Then write three supporting details that further explain the main idea. Main Idea The mechanical advantage of a machine helps by… Detail Detail Detail changing the amount of force you exert changing the distance over which you exert your force changing the direction of the force

- How Machines Do Work Links on Mechanical Efficiency Click the SciLinks button for links on mechanical efficiency.

End of Section: How Machines Do Work

Three Classes of Levers
- Simple Machines Previewing Visuals Before you read, preview Figure 17. Then write two questions that you have about the diagram in a graphic organizer like the one below. As you read, answer your questions. Three Classes of Levers Q. What are the three classes of levers? A. The three classes of levers are first-class levers, second-class levers, and third-class levers. Q. How do the three classes of levers differ? A. They differ in the position of the fulcrum, input force, and output force.

Click the Video button to watch a movie about levers.
- Simple Machines Levers Click the Video button to watch a movie about levers.

Click the Video button to watch a movie about pulleys.
- Simple Machines Pulleys Click the Video button to watch a movie about pulleys.

End of Section: Simple Machines

Graphic Organizer Simple Machine Mechanical Advantage Example
Inclined plane Length of incline ÷ Height of incline Ramp Wedge Length of wedge ÷ Width of wedge Ax Length around threads ÷ Length of screw Screw Screw Distance from fulcrum to input force ÷ Distance from fulcrum to output force Lever Seesaw Wheel and axle Radius of wheel ÷ Radius of axle Screwdriver Pulley Number of sections of supporting rope Flagpole

End of Section: Graphic Organizer