Begin the analysis of the experiment by asking the students to use the Summary tool from the Cognitive Toolbox.

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Explain to students that before they can analyze the data collected in the Lab, they should first refresh their memory of what was done. One good way to do this is to use the Cognitive Tool, Summarize. The Summarize tool can be used here as follows.

Ask Students: What is a Summary? Students should recall that a summary is a statement that contains the most important information while leaving out the details that are not important to understand what happened.

Tell students to look back at the two trials they conducted in the Lab. Ask Students: Can you summarize what we did in the first trial? Student summaries may vary. The first trial involved holding objects of different weights in order to feel the force of gravity acting on those objects. Forces were represented as arrows, whose length and direction depicted the amount and direction of the force.

Ask Students: Can you summarize what we did in the second trial? Student summaries may vary. The second trial involved finding the mass of the different objects, then using a spring scale to record the force in Newtons required to hold the object stationary in the air.

Tell students that now they can begin to think about why they obtained the results that they did.

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Assist students in compiling their data by creating a line graph.

Instruct students to construct a line graph in the blank space in Problem 5 of the Student Data Records. Remind students of the steps from the Procedural Toolbox tool, Creation and Completion of a Line Graph if necessary:

• Draw an x-axis and a y-axis.
• Decide which variable to plot on the x-axis and which variable to plot on the y-axis.
• Label the axes appropriately.
• Decide the beginning and ending numbers to place on the x-axis and y-axis. The numbers should allow all the points to be plotted on the graph.
• Decide the scale for each axis.
• Graph the data and connect the points with a line.

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Ask students to look at their graph and to think about what their data are showing them, with regard to the relationship between mass and the force of gravity. Encourage students to use the Rules Cognitive Tool, in which they examine data and try to come to a conclusion about the relationship between variables that can be stated in the form of a rule. 

Guide students through this process as follows:

• Explain to students that often, graphs show a rule or relationship between variables.

Ask: What are the variables on the graph? Students should indicate that mass of the object is on the x-axis and the force of gravity acting on the object is on the y-axis.

Ask: Does the graph show you a rule about the relationship between the mass of an object and the force of gravity? Does one change when the other changes? How? Students should indicate that the graph shows that as the mass increases, the force of gravity increases.

Encourage students to state the relationship in the form of a rule. Student answers may vary. The force of gravity is directly related to the mass of an object. The greater the mass of an object, the greater the force of gravity that acts on the object.

• Use the completed graph to help students extrapolate beyond their data. For example, 

Ask students: what would happen if the mass of the object doubled or tripled? If the mass of the object doubled, the force of gravity would also double. If the mass of the object tripled, the force of gravity would triple.

• Direct students to complete Problem 6 in their Student Data Record.

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Use the following explanation to further students’ understanding of the concept of gravity, and their ability to distinguish the concepts of weight, mass, and gravity.

Explain to students that every object exerts a force of gravity with every other object. For example, the students, books, and desks in the room all exert a force of gravity on each other. The reason that you do not feel these forces is that gravity itself is a weak force.

Ask students: Why is the force of gravity between the Earth and objects on the Earth so large? Student answers will vary.

• Explain that the force of gravity is dependent on the mass of both objects and that the Earth has a very large mass, while the objects in the room have a relatively small mass.
• The force of gravity is also related to the distance between the two objects. The greater the distance between the objects the smaller the force of gravity between them. As you get further from Earth the force of gravity gets less.

Ask students: What property of matter does the force of gravity give an object? Student answers will vary. Weight.

Explain to students that the weight of an object is equal to the force of gravity on the object. Although students may be used to measuring weight in grams (actually a measure of mass) or measuring their own weight in pounds (Imperial units for weight), tell students that weight can also be measured in Newtons. Refer students to the Table in their Student Data Record for confirmation that they measured the weight of, for example, the bucket in Newtons.

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Explain that if the force of gravity changes, for example if a student was to travel to another planet, then their weight would change. Similarly, without a force of gravity, an object would not have a weight.

Tell students that on the moon the force of gravity is 1/6th of the force of gravity on Earth.

Ask students: Why is the force of gravity less on the moon than on Earth? Student should indicate the moon has less mass.

Remind students that they have measured weight in units of Newtons. Ask students: If the force of gravity on Joe is 600 N on Earth, what would his weight be on the moon?  It may help to solve the math word problem on the board.

Ask students: Can you define mass? Student should indicate that mass is the quantity of matter in an object or substance.

Ask students: Is a person’s mass different on the Moon compared with on Earth? Students should indicate that the amount of matter is constant, so mass would remain the same on the moon and on Earth.

Explain that the mass of an object is due to the amount of matter an object consists of, and therefore it remains constant. 

The weight of an object is due to the force of gravity that exists between objects. The force of gravity is dependent on the amount of matter each object consists of (i.e. mass), hence if one object has a large mass the force of gravity between the objects is large. Also, if the objects are far apart the force of gravity will be smaller. For example, the force of gravity due to the Moon is relatively small on objects on Earth, because the force of gravity exerted by the Earth is so large.

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Return to the concept of balanced and unbalanced forces, in order to integrate students’ understanding of the force of gravity with the concepts of acceleration and motion.

1. Discuss the objects measured in the lab. Ask students: Were the forces balanced or unbalanced when you held the bucket? How do you know? Students should indicate that the forces were balanced because the bucket did not move. If the forces were unbalanced, the bucket would either have fallen towards the floor, or been lifted higher into the air.
2. Explain to the students that the force they held the objects with was equal to the weight or the force of gravity of the object. If they had applied less or greater force, then the forces would have been unbalanced and the bucket would have accelerated, by either falling toward the ground, or lifting into the air.
3. Direct students to complete Problem 7 in their Student Data Records. Students should indicate in their answer that the forces were balanced because the bucket did not move.
4. Direct students’ attention to Problem 8. Review the notation of forces as being represented by arrows with a particular length and direction. Students may work in pairs to complete Problem 8, in which they will indicate the forces on the bucket by drawing arrows to represent the direction and sizes of the forces.

• To conclude this Investigation, tell students that they will continue to study the effects of forces on the movement of objects in the next Investigation.