5th Grade Science Unit 3

Vermilion Parish Schools "Connecting Students, Home and Schools"

Activity 3: Balanced and Unbalanced Forces (SI GLEs: 16, 18, 19, 22; PS GLE: 9)

Materials List: Force, Motion, and Energy Vocabulary Chart BLM (Activity 1), book, table, large soft kickball, large rope for tug of war, bandana, two playground cones or markers, materials for designing another activity to show unbalanced forces, science learning log.

Safety Note: Remind students of appropriate behavior when playing Tug of War and kicking balls around other students.

Review with students the definition of a balanced force. Have students add vocabulary from Activity 2 to their Force, Motion, and Energy Vocabulary Chart BLM from Activity 1 prior to beginning this activity.

To help students understand the concept, point to a book sitting on a table. Ask students to identify the two forces at work (the table pushes up on the book and the book pushes down on the table). Ask students if these forces are balanced or unbalanced. (Since there is no movement, the forces must be balanced.) Ask students what needs to be done to make the forces unbalanced (slant the table). Which force is becoming stronger as the table is slanted? (The force of gravity is able to exert a force over a greater vertical distance and it overcomes the force that is pushing up on the book and the force of friction that is acting against the bookís movement down the table).  Direct students to read about balanced and unbalanced forces in their textbook and carefully examine the examples that are offered. Instruct students to work in small groups to discuss examples of activities that demonstrate balanced forces at work (e.g., a boat or other object floating on water, a person standing on a flat surface, water droplets suspended in the air in the form of clouds, etc.). Have each group draw an illustration of the activity in their science learning logs (view literacy strategy descriptions) and identify the opposing forces. Each entry in the science learning log should be dated. Science learning logs are journals that are used to record observations and illustrations, predictions, data collection, conclusions, etc. about what is being learned. Students should also explain to their classmates why this example is correct. Allow other students to critique classmatesí explanations and refute faulty reasoning.

Next, demonstrate unbalanced forces for the students. Unbalanced forces occur when one force is greater than its opposite force. Review safety procedures to follow when kicking, spinning, pushing, pulling, and dropping objects. If students created a related safety poster in Unit 1, have those students explain the safety issues involved. If none was created, spend a few minutes discussing safety rules before continuing with the activity and add the rules to the safety chart in Unit 1. To demonstrate unbalanced forces, have a student gently kick a large, soft ball toward you; then kick the ball another direction, and with more force, so it will have greater acceleration. Ask What was the greater force? (the kick that made the ball move the farthest) How can you tell? (The ball changed direction so the force that kicked the ball must have been greater than the kick that started the ball moving and the force of friction that was opposing the movement of the ball. The force of gravity that was holding the ball to the Earth was equal to the force of the Earth pushing up on the ball, so these two forces were equal and had no effect on the ballís movement.) Explain to students that the force applied to an object to move it can be shown with arrows. The length of the arrows and the direction in which they are drawn indicate the magnitude of the force (see example below). The example shows that the magnitude of the force to the right is stronger than the magnitude of the force to the left. Therefore, the forces are unbalanced and the ball will move to the right.

Have the students form two Tug of War teams with an equal number of students on each side. Remind students of the earlier discussion about safety when playing the game. Position the bandana in the middle of the rope and place one cone on each side of the rope directly in front of the two opposing lead players.  Have both teams lift the rope and hold it taut. Ask students if this is a balanced or unbalanced force (balanced, because the bandana is not moving its position). Ask students to identify which forces are acting on the rope (gravity pulls down, students pull up, each team is also pulling the rope toward them with enough force to make the rope taut).The goal of the game is to pull the rope so that the bandana crosses over the cone. Have the two teams play a game of Tug of War and have the other students carefully note which forces are acting on the students and the rope (the students are pulling up on the rope, gravity is pulling the rope downward, both teams are pulling the rope toward their teams, and friction is acting in opposition to each teamís forward movement). After one side wins, ask students to determine if the forces remained balanced throughout the game and explain how they were able to determine their answer (the bandana moved over to one teamís side which showed that their pulling force and the force of friction opposing their forward movement was stronger than the opposing teamís, since there was movement, the forces had to be unbalanced). Have students draw arrows to indicate the magnitude of each force acting on the rope, making sure to draw larger arrows for the stronger forces. Students should be sure to indicate the force of friction of the studentsí feet acting against the forward movement of the rope, as well as the upward force of students pulling up on the rope and the downward pull of gravity. Friction and pulling are the two major forces at work in the game of tug-of-war and should be given primary emphasis in the drawing. Student should also be sure to always to show forces acting in pairs against each other. Ask students to devise a way to make all forces balanced and try out their plan. Compare the two games. Have the students reflect in their science learning logs (view literacy strategy descriptions), describing the difference between balanced and unbalanced forces and how they would use what they learned to choose teams for a field day game of Tug of War. 

Working in groups, have students design another example of an unbalanced force (e.g., use a rubber stopper tied to a string, a moving marble, and a dropped ball). They will then share their examples with classmates and describe what is happening. Using class demonstrations or classmate examples, students will draw pictures in their science learning logs, using arrows to illustrate opposing forces and explain why the forces are unbalanced. The teacher should lead a class discussion to include the following questions: 

  • What observation indicates that forces are balanced?  (lack of movement)

  • How is a balanced force different from an unbalanced force? (balanced forces cause no movement and unbalanced forces cause movement to occur)

  • What is one activity that people do everyday that uses an unbalanced force? (walking, moving around, getting up, and sitting down)

  • What athletic events use unbalanced forces? (football, soccer, baseball, basketball, swimming, croquet, volleyball, golf, etc.)

  • Ask students to try and name one athletic event that does not use unbalanced forces. (none known)

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