Work: Force through a Distance
''Work'' is a term used in physics to indicate that a force has caused an object to move. A tennis player hitting a tennis ball with a racket is an example of work because the tennis ball moves when struck by the racket. But work is not done every time a force is applied. For example, if you push against a building for several minutes, you may become tired, but no work has been done. This is because the building did not move. The amount of work done is determined by multiplying the force applied to an object by the distance the object moves in the direction of the force. The SI (internationally agreed-upon method of using the metric system of measurement) unit for work is joule if the SI units for force and distance, respectively, are newton and meter.
In this project, you will learn how to measure the work done on an object. You will determine the effect that a simple machine has on work. You will also determine the effect of the direction of the force on work.
Purpose: To measure the work done on an object.
- brick (or any object of comparable weight)
- shoe box large enough to hold the brick
- paper hole-punch
- 5-pound (2200-g) spring scale with a hook
- yardstick (meterstick)
- masking tape
- Place the brick inside the box.
- Use the paper hole-punch to cut a hole in the end of the box. Attach the scale's hook through the hole in the box (see Figure 5.1).
- Place the box at one end of a table. Place a piece of masking tape 6 inches (15 cm) in front of the box. This will be the starting line.
- Measure the distance from the starting line to the end of the table toward which you are pulling in meters (m)—an SI unit for distance. Record this as distance traveled (d) in a Work Data table, like Table 5.1.
- Pull the scale so that the scale and box move horizontally across the table at a constant speed. Determine the scale reading when the box crosses the start line. This force must be measured in newtons (N)—an SI unit for force. Record the newton force in the Work Data table. If your scale measures in pounds, convert pounds to newtons using this conversion: 1 pound = 4.45 N. For example, if the measurement is 2 pounds, the newton force would be 2 pounds × 4.45 N/1 pound = 8.9 N. If the scale measures in grams, convert grams to newtons using this conversion: 1 g = 0.0098 N. For example, if the measurement is 908 g, the newton force would be 908 g × 0.0098 N/1g = 8.9 N.
- Repeat step 5 four times and average the measurements.
- Calculate the work done on the box using this equation: w = f × d.
In the equation, w equals work, f is the force acting on the box in newtons, and d is the distance (in meters) the box moves during the time the force is being measured. For example, if f = 8.9 N and d = 0.5 m, then w = 8.9 N x 0.5 m = 4.45 Nm.
Note: 1 Nm = 1 joule (J). Joule (J) is an SI unit for work.
So for this example the work done is 4.45 J.