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Work, Power & Simple Machines

Science, Grade 6

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Table Of Contents: Work, Power and Simple Machines

1. What Is Work?

2.1. Definition of Work
Work occurs when a force is exerted on an object and causes the object to move. The force exerted and the movement of the object must be in the same direction to be considered work.
2.2. Work and Motion
If force is exerted and an object does not move, then work does not occur. For example, if you try to push a very heavy object, but the object does not move, then work is not done.
2.3. Work and Direction
If the force exerted is in a different direction than the movement of an object, then work does not occur. When you pick up a box, the exerted force and movement of the box are in the same direction, so work is done. However, when you carry a box, you are exerting force in a vertical direction to hold the box, but the box moves in a horizontal direction as you walk. Carrying the box is not considered work.
2.4. Calculating Work
The equation for work is force times distance. For example, if you use 5 Newtons of force to push a cart 10 meters, then the amount of work is equal to 50 Newton meters. If the cart becomes heavier and you need to exert 10 Newtons of force to move it the same distance, then the amount of work is equal to 100 Newton meters.
2.5. A Joule Is a Unit of Work
A Newton meter is also known as a joule. One joule is equal to the amount of work when one Newton is exerted to move an object one meter.

2. Pause and Interact

3.1. Review
Use the whiteboard text tools to solve the problems.

3. Power

4.1. Work and Power
The amount of work that is accomplished is not related to time. For instance, during a five km bike ride, one person may pedal slowly, while another person may pedal quickly. In both cases, an equal amount of work is done. However, unlike work, power is affected by time. In this example, the more powerful cyclist is the one who pedals faster and arrives at the destination more quickly.
4.2. Definition of Power
Power is the rate at which work is done. A lawn mower that completes the job in 10 minutes is more powerful than a lawn mower that takes 30 minutes to complete the same job. Over the time period of an hour, the more powerful lawn mower can mow six lawns in an hour, while the less powerful mower can only mow two.
4.3. Calculating Power
Power is calculated by dividing the amount of work done by the amount of time it takes to do the work. Knowing that work equals force times distance, you can rewrite the equation as force times distance divided by time.
4.4. Power Example
Let’s calculate the power of a football player that uses 240 Newtons to move the training equipment 10 meters in 8 seconds. Using the equation for power to solve this problem, we learn that the power is equal to 300 Newton meters per second, or 300 joules per second.
4.5. A Watt Is a Unit of Power
Power is measured in units of joules per second, which are also known as watts. One thousand watts of power is equal to one kilowatt. Here are some examples of the amount of power required to run different household appliances and tools.

4. Pause and Interact

5.1. Review
Use the whiteboard text tools to solve the problems.

5. Machines and Work

6.1. What Is a Machine?
A machine is a device that is designed to make work easier. Machines are not necessarily complex. For example, shovels and hammers are types of simple machines.
6.2. Input and Output Forces and Work
Work is done when a force causes an object to move. The work you do on a machine is called work input, and the force you exert is known as input force. The work the machine does is called work output, and the machine’s force exerted on an object is known as output force.
6.3. Calculating Work Input and Output
The input force times the input distance equals the work input. Similarly, the output force times output distance equals work output. The work output of a machine will never be greater than the work input.
6.4. How Machines Make Work Easier
Machines don’t change the amount of work that is done, but they do make work easier. A machine can help us by changing the size or the direction of the input force, or both.
6.5. Decreasing Input Force
Some machines make work easier by allowing us to exert a smaller input force, while achieving a greater output force. To do this, the input force must be exerted over a greater distance. For example, when loading a heavy box onto a truck, it is easier to push the box up a ramp than to pick it up. Pushing the box requires less input force, but it is applied over a greater distance.
6.6. Increasing the Input Force
Some machines make work easier by allowing us to exert a greater input force over a shorter distance, in order to achieve a greater output distance. For example, when using a hammer, you exert a large amount of input for a short distance on the handle, while the head of the hammer exerts a smaller output force that is applied over a greater distance.
6.7. Changing the Direction of the Force
For some machines, the direction of the exerted force is changed, while the input and output forces and distances remain equal. In these machines, changing the direction makes it easier to accomplish the work. For example, a pulley allows us to lift an object by pulling down on a rope rather than having to lift the object up.

6. Pause and Interact

7.1. Review
Use the whiteboard text tools to describe how each of these simple machines makes work easier.

7. Mechanical Advantage and Efficiency

8.1. Mechanical Advantage
The amount of help a machine provides is measured by its mechanical advantage. This can be determined by comparing the output force to the input force. In this example, the lever with an input force of one Newton and an output force of three Newtons, provides a mechanical advantage by tripling the output force.
8.2. Calculating Mechanical Advantage
The formula for mechanical advantage is output force divided by input force. This lever has an input force of two Newtons and an output force of ten Newtons. It has a mechanical advantage equal to five. A person using chopsticks exerts three Newtons of input force over a short distance, while the output force decreases to one Newton over a greater distance. The chopsticks have a mechanical advantage of .33.
8.3. Efficiency of Machines
In an ideal situation, the total amount of work done by a machine should equal the amount of work put into it. However, due to friction, the output force is usually less than the input force. Machines with greater efficiency do a better job of overcoming the force of friction. For example, a pulley with 90% mechanical efficiency uses 90% of the work input to lift a load, while the other 10% is used to overcome friction.
8.4. Calculating Mechanical Efficiency
To calculate mechanical efficiency, divide work output by work input and multiple by 100. For example, you do 3,000 joules of work pushing a box up a ramp. The work done by the ramp is 2,400 joules. The mechanical efficiency of the ramp is 2,400 divided by 3,000, times 100. This equals 80%. That means the other 20% of the work input was used to overcome friction.

8. Pause and Interact

9.1. Review
Use the whiteboard text tools to solve the problems.

9. Types of Simple Machines

10.1. Types of Simple Machines
There are several types of simple machines, including an inclined plane, a wedge, a screw, a lever, a wheel and axle and a pulley. All of these machines make work easier for us.
10.2. Inclined Plane
An inclined plane is a straight, sloped surface that increases the distance over which the input force is applied. This simple machine reduces the amount of input force needed to perform work. Examples of inclined planes include ramps, slides and staircases. The mechanical advantage of an inclined plane is equal to the length of the incline divided by the height of the incline.
10.3. Wedge
A wedge is a moving inclined plane that increases the distance over which the input force is applied. This type of simple machine reduces the amount of input force needed to perform work. An axe, a knife and a shovel are all examples of wedges. The mechanical advantage of a wedge is calculated by dividing the length of the wedge by its greatest width. Mechanical advantage increases as the wedge becomes longer and thinner.
10.4. Screw
A screw is an inclined plane that is wrapped around a cylinder. It increases the distance over which the input force is applied, reducing the amount of input force needed. If you think about turning a screw, it is fairly easy to turn, but you have to turn it many times. Other examples of screws include a screw lid jar, a drill bit and a light bulb. Threads that are close together increase the mechanical advantage of a screw.
10.5. Lever
A lever is a bar or board that pivots around a fixed point called a fulcrum. Seesaws, bottle openers, wheelbarrows and hockey sticks are all examples of levers. The mechanical advantage of a lever is calculated by dividing the distance from the fulcrum to the input force by the distance from the fulcrum to the output force.
10.6. Classes of Levers
There are three classes of levers, depending on where the fulcrum is located. In a first-class lever, like a seesaw, the fulcrum is located between the input and output force. The direction of the input force is opposite to the output force. In a second-class lever, like a wheelbarrow, the output force is between the fulcrum and the input force. The direction of the input force does not change, but as the force is applied over a distance, the output force is increased. In a third-class lever, like a hockey stick, the input force is between the fulcrum and the output force. The direction of the input force does not change, but the distance of the output force is increased.
10.7. Wheel and Axle
A wheel and axle is a simple machine made up of one large cylinder, the wheel, attached to a smaller cylinder, the axle. If the input force is applied to the axle which has a smaller diameter, then the output force is spread over a greater distance on the wheel which has a larger diameter. Examples include a vehicle wheel, a rolling pin and a doorknob. The mechanical advantage of a wheel and axle is calculated by dividing the radius of the wheel by the radius of the axle.
10.8. Pulley
A pulley is a type of simple machine that consists of a cable placed around a grooved wheel. A simple, fixed pulley changes the direction of the input force, making the work easier. Movable pulleys and pulley systems require less input force applied over a longer distance in order to increase the output force. Pulleys are used in many ways and can be found on flagpoles, window blinds and boat sails.
10.9. Types of Pulleys
A pulley can be fixed or movable. A fixed pulley, such as the one found at the top of a flagpole, is attached to a structure. Fixed pulleys have a mechanical advantage of 1. A movable pulley moves along with the load that is being carried. It has a mechanical advantage of 2. A block and tackle pulley system combines a fixed and movable pulley. This type of system can often be found on cranes. The mechanical advantage of this block and tackle is 4.

10. Pause and Interact

11.1. Review: Pulley Types
Use the whiteboard text tools to complete the activity.
11.2. Review: Mechanical Advantage
Use the whiteboard text tools to complete the table.
11.3. Simple Machines
Drag each example to the correct type of simple machine.

11. Simple Machines in the Body

12.1. Simple Machines in the Body
We normally think about simple machines as tools that we use in our daily lives. However, there are also simple machines found in our bodies. Our arms and legs are types of levers where the elbow and kneecap serve as fulcrums. Our teeth work as wedges when they bite into food, such as an apple.

12. Compound Machines

13.1. Compound Machines
Two or more simple machines that are combined together are called a compound machine. For example, a bicycle is constructed using many simple machines, including a wheel and axle, pulleys, screws, and levers.

13. Vocabulary Review

14.1. Vocabulary Matching Review
There are several types of simple machines, including an inclined plane, a wedge, a screw, a lever, a wheel and axle and a pulley. All of these machines make work easier for us.

14. Virtual Investigation

15.1. Exploring Pulleys
In this virtual investigation you will experiment with different types of pulleys and load weights to better understand the mechanical advantage of each pulley. A pulley or pulley system will only lift a load if the input force is large enough to move the weight of each load. In this activity you can select an adult or a child to lift the load. Each person is limited by the maximum amount of effort (input force) he can exert. A pulley with a greater mechanical advantage will make it easier to lift the load.Directions Select a load and a pulley. Then select the adult or the child and click the LIFT button to see if the load is lifted up to the tree house. Explore how pulleys work by using different loads and pulleys, and by changing the person who is using the pulley. Collect data and record it on the pop-up data sheet.

15. Assessment

16.1. Work, Power and Simple Machines
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