Work and Machines

Prior to the reading, offer students a chance to turn and talk about whether they think Cooper or Olivia is doing "more work." Treat this question as a brain teaser. After a brief period of discussion, let them know that by the end of today's activity they will know a method of finding an answer.

Assign three students to read the parts of Hamza, Olivia, and Cooper. This dialog is an introduction to the concept of machines and work.

As the fictional students point out in the dialogue, the terms machine, force, and work have both common and scientific meanings. Early in the dialogue, Cooper gives an informal definition of work. It may help to have your student read this definition twice.

Many of your students will follow Cooper’s example and turn their pencil into a windshield wiper. Take a moment to do this as a class. Tell them that the pencil is acting as a lever.

As the scene unfolds, Cooper draws a sketch of how one type of elevator works and the group discusses the concepts.

Consider taking a few minutes to discuss the drawing.

Ask:

• What is holding the elevator up?
• Olivia mentions that there is a pulley. Can someone point out the pulley?
• One end of the cable is attached to the elevator. What is the other end attached to?
• Cooper says that the elevator is a complex machine. What does he mean by complex machine?

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Setting: Cooper, Olivia, and Hamza are hanging out in the library working on their homework.

Hamza: I never have the energy to do my homework.

Olivia: I keep telling you, Hammy, that’s an attitude problem, not an energy problem.

Hamza: I wish I had a machine to do all that work for me.

Cooper: If you had done your homework, you’d know what work machines really do. Hint: not homework.

Hamza: If you say so, Nerd Cooper. What is this “work” that machines do?

Olivia: I thought work was what energy did.

Hamza: (sarcastically) Is your name Nerd Cooper?

Cooper: Be nice. And she’s right. Work is when you change how something is moving: speed it up, slow it down, change its direction or how it's spinning—like when you kick a soccer ball.

Olivia: See Hamza, work can be fun!

Cooper: Basically it can be any movement that lines up with a push or pull. I mean that’s the science term “work.” But in everyday English we use the word “work” for a lot of different things.

Hamza: So, scientifically speaking, if I kick a ball and the ball knocks over a chair, I did the work of making the ball go, and then the ball did the work of knocking over the chair?

Cooper: Hmmm...

Olivia: I think so, Ham. You had energy from eating food, and then you exerted a force when you kicked the ball. Your kick gave energy to the ball, and the ball used the energy to knock over the chair.

Cooper: It’s cool to think about how something simple like kicking a ball has all these things going on.

Hamza: (daydreaming) I’m a machine on the soccer field.

Olivia: Well, they actually don’t allow machines on a soccer field, but they do in a hockey rink.

Hamza: You mean the Zamboni machine that smoothes the ice?

Cooper: Actually, she’s talking about the hockey stick.

Olivia: And you would know that if...

Hamza: I know! I know! If I did my homework. Wait, how is a stick a machine? It doesn’t take gas or electricity. That can’t be right.

Cooper: It is a machine! The way to figure it out is to think about whether it can transmit work.

Olivia: Coop, did you get the thing on the homework about pretending your pencil was a windshield wiper?

Hamza: What!? Oh great. Now I’m curious. I can’t believe I’m curious about homework. What have you two done to me?

Cooper: Here. Try it. Hold your pencil in front of you by the end and shift it back and forth like a windshield wiper. A small motion of your fingers causes a large motion at the other end of the pencil. That means it’s a machine. A lever to be exact.

Hamza: Oh, that’s kind of cool. I see the connection to the hockey stick. A player moves the stick at one end, but the stick moves a lot more at the other end and applies force to the puck so it goes faster than the hand moves. Wham!

Olivia: I guess golf and baseball are similar. They use levers. But I know people think of machines as things like cars or elevators. Not sticks.

Cooper: Well, it’s not that cars and elevators aren’t machines. They’re more like a bunch of machines put together into a complex machine. Look:

Cooper sketches a car and an elevator.

In this part of the activity, an example of lifting books from the floor onto shelves demonstrates how force and work can be measured.

First, students read an informal definition of a newton. In the formal definition, gravity makes 1 kilogram of mass exert about 9.8 newtons of force. Students are asked to consider that if one apple weighs 1 newton then a book weighs 10 newtons.

Students typically confuse the terms force and work. When a student pushes against a door, force is involved. Consider asking students, “What is a force that is pulling you down all the time? This force is pulling everything down, and we never think about it.” The answer is gravity.

In physics, work is defined as force times a distance traveled. Returning to the example of a student pushing against a door, if the door doesn’t open, there is no work being done, only a force against the door. But if the door opens, there is work. Work must include some movement. Work is measured in joules. One joule equals 1 newton times 1 meter of travel.

The “Turn and Talk” asks students to determine from the sketches and description who is doing the most work and who is doing the least. You may need to clarify that each book weighs the same. Hamza is applying force (lifting) one book at a time and raising each book 1 meter in one second, so he is doing the least amount of work. Cooper is lifting 1 book at a time, but 2 meters high. He is clearly working twice as much as Hamza. Olivia’s work is equal to the work done by Cooper because she is lifting 2 books at a time but only 1 meter high.

Consider having student groups with different answers briefly debate who is correct. Then, if students do not readily understand, tell them that they will now use multiplication to calculate the work done by each student.

The last third of the activity introduces joules, the standard unit of measurement for work. Students may not immediately realize that work is the product of the force of an object times the distance the object moved. To arrive at the joules of an object, the force must be in newtons, and the distance in meters. So 1 joule is 1 newton * 1 meter.

Students read that Hamza expended 10 joules each time he lifted a book. Moving a force of 10 newtons, 1 meter high can be written as 10 N * 1 m = 10 joules.

Olivia lifted 2 books (10 newtons each so combined force of 20 newtons) 1 meter high, so she worked 20 joules.

Cooper's work equaled 10 newtons (for 1 book) * 2 meters = 20 joules.

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