What are some differences between doing your homework and doing mechanical work? How do you think scientists measure and calculate work?

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1 You may be reading this as part of your homework. Do you think reading is work? No matter how difficult the reading is, it is not work in the scientific sense of the word. Of course, you are using your muscles to lift and hold the paper and to move your pen, which is work. However, reading words and thinking about what you have read is not work in the same way that lifting the paper or moving a pen are work. Scientists sometimes call this kind of work mechanical work. What are some differences between doing your homework and doing mechanical work? How do you think scientists measure and calculate work? Work is done when a force moves an object. Reading this companion is not work because you are not using forces to move objects. (Recall that a force is a push or pull.) Whenever work is done on an object, the object s energy changes. Several different forms of energy can change because of work. If a force causes an object to gain speed, then its kinetic energy has increased. Kinetic energy is energy of motion. Consider a car accelerating from a standstill to 60 miles per hour (mph). While the car accelerates, the engine is doing work by applying force that causes the car to gain kinetic energy. You can also do work by increasing an object s potential energy. Potential energy is stored energy. An object gains potential energy when it is raised to a higher elevation. As long as an object is up at some height, is has the potential to release energy or do work as it falls back down. What force causes an object to fall down? The force of gravity, which is equal to the weight of the object. gravity: the force that attracts anobject with mass to another object with mass (such as Earth) Picture a 20-pound watermelon resting on the kitchen table. You grasp the melon, carry it outside, walk around the block, and return the melon to the table. Now imagine you carry the melon upstairs to the second floor. The walk around the block would probably seem harder because you carry the watermelon a greater distance. However, carrying the melon upstairs produces more work. Why? During your walk around the block, the melon remained the same height above the ground. Therefore, it gained no potential energy as you walked. Holding a watermelon above the ground increases its potential energy. If you were to drop the watermelon, its potential energy would change to kinetic energy as it fell. 1

2 In comparison, the watermelon gained a lot of potential energy when you raised it from the first floor to the second floor. The more an object s energy changes, the more work has been done to it. Everyday Life: Plants at Work Even plants have to work sometimes. Plant cells have strong cell walls. When plants need to, they can make their cells absorb extra water. The extra water puts pressure on the plant s cell walls, which makes the cells rigid. This keeps the plant erect. When plants don t get enough water, they wilt and collapse. If more water becomes available, the plant s cells will swell and the plant will raise itself up, which is work. This force that plants can apply is called turgor force. Some plants can exert so much of this force they can crack pavement and split rocks! We can measure and calculate work. Work equals force times distance. You can write the work formula like this: W = Fd Scientists measure force in units called newtons (N). (One newton equals about ¼ pound.) Scientists measure distance in units of meters (m). So, work is measured in units called newtonmeters (N m). Another unit for measuring work is the joule (J). One joule equals one newton times one meter, or one newton-meter. Suppose you used a force of 100 N to lift a watermelon a distance of 5 m. How much work did you do? Use the work formula: W = Fd = (100 N)(5 m) = 500 N m = 500 J A plant can exert enough force tobreak pavement. You did 500 J of work on the watermelon. Scientists also use joules to measure energy. So, you can also say the watermelon gained 500 J of energy. 2

3 If a force is applied to an object but the object does not move, no work is done on the object. Consider these two gymnasts: The woman is doing pull-ups, and the man is holding a pose without moving. Both actions require a lot of strength, but only the woman is moving, so only the woman is doing work! The woman is doing work because the force is producing motion. The man is doing no work because the force is producing no motion. Each pull-up produces work equal to the woman s weight (force) times the length of her arms (distance). 3

4 We can use simple machines to help us do work. Simple machines are devices that make work easier and have few or no moving parts. These machines are usually classified into six types, shown below. Simple machines don t do work for you. They don t even reduce the amount of work. They simply make work easier. Recall that work equals force times distance. If force increases, distance must decrease; if force decreases, distance must increase. Most simple machines help us by decreasing the applied force needed to do work. However, the force must be applied over a longer distance. 4

5 For example, a loading ramp is a type of inclined plane that is useful for raising a heavy load. Suppose we want to move a heavy piano 1 m off the ground and into a truck bed. Lifting the piano 1 m straight up would require a huge amount of force. A ramp allows us to push the piano gradually off the ground, using a much smaller force. However, we have to push the piano a greater distance to get it up the ramp and into the truck. Another example of a simple machine is a claw hammer. We could not pull a nail out of a board with our fingers the amount of force needed is too great. However, we can use the claws of the hammer to pull out the nail easily. A claw hammer is a type of lever. It decreases the force needed to remove the nail. However, we must move the hammer over a greater distance. An inclined plane such as a ramp (left) makes work easier by reducing the force needed to lift a load off the ground. A lever such as a claw hammer (right) reduces the force needed to remove a nail from a board. Getting Technical: Compound Machines Machines that are made up of several simple machines are called compound machines. A bicycle is a combination of pulleys, levers, screws, and wheels and axles. A water screw consists of a screw placed inside a wheel and axle. This machine has been used for thousands of years to lift water from rivers and move it over distances. This ancient compound machine is a type of water screw. Water is trapped in the lower compartments of the machine. Turning the screw moves the water from lower to higher compartments. 5

6 What are some other simple or compound machines you use every day? Think of the tools you use to cut or sharpen things. Think of the tools you use to lift things off the ground or move things over a distance. What do you know? Each row includes two related tasks, task A and task B. Does one task do more work, or is the amount of work the same between task A and task B? Write your answers in the third column. Task A Task B Which does more work? Lifting a book from the floor to the top of a bookshelf Carrying the book from one end of the room to the other Running one mile around Running half-a-mile uphill a track Carrying a heavy box up a ramp to the bed of a truck Lifting the box off the ground into the truck bed 6

7 Analyzing Everyday Machines With your child, explore the relationship between machines and work by identifying different machines used in and around your home. Many of the tools that we use in our everyday lives are compound machines rather than simple ones. Remind your child that a compound machine is made up of simple machines. An easy example is a pair of scissors. Scissors combine wedges and levers to create a tool that cuts more easily than either a wedge or a lever by itself. Instruct your child to compare cutting a paper without scissors to cutting a paper with scissors: How does this compound machine make the job of cutting easier? Here are some other everyday compound machines: Axe: Combines a wedge (the axe head) with a lever (the handle). Swinging the lever transfers a powerful force to the wedge, Scissor blades are wedges, while the handles act as levers. Moving the handles brings together the blades, generating a force that makes the work of cutting easier. which cuts more deeply into an object than if you were merely to press the wedge against the object. Stapler: Combines a wedge (the staples) with a lever (the stapler top). Pressing down on the lever drives the wedges into a stack of paper, holding together the individual papers. Hand-operated can opener: Combines a wedge (the blade) with a lever (the handle) attached to a wheel and axle. Rotating the lever transfers the force to the wheel and axle, which transfers the force to the wedge, which cuts into and around the lid of the can. Wheelbarrow: Combines levers (the handles) with an inclined plane (the bucket) and a wheel and axle. Lifting the levers transfers the force to the inclined plane and the wheel and axle, allowing us to push a load more easily than if we were to carry it. You probably also have examples of simple machines such as screws (screw), hammers (lever), and pulls for opening and closing curtains (pulleys). For each simple or compound machine, ask your child how each machine makes doing work easier. Here are some questions to discuss with your child: What kind of simple machine is this? How does it make work easier? Is this actually a compound machine? If so, what simple machines is it made of? Do these machines cause you to do less work? Explain your answer. 7