Balanced and Unbalanced Forces

Transcription

1 Balanced and Unbalanced Forces Developed by: Ahmad Aena Deepa Doshi Nancy Han Dwithiya Krishnan Directed by: Nellie Tsipoura GRADE LEVEL: 8 th TIME REQUIRED: 45 MINS KEY TERMS: force, balanced force, unbalanced force, mass, gravity, center of gravity, weight, pulley OVERVIEW: A force is a push or pull that gives energy to an object, sometimes causing a change in the motion of the object. Forces are either balanced or unbalanced in nature. Balanced forces result in no motion. The game of tug-of-war illustrates a balanced force when both opponents do not move in either direction. Unbalanced forces result in motion. The game of tug-of-war also shows an unbalanced force when a team wins and pulls their opponent in the direction of their force (pull). To help demonstrate the concepts of balanced and unbalanced forces, a pulley system is useful. A pulley is a rope, belt, or chain wrapped around a wheel that can either change the level of force or the direction of the force. In a pulley system, the force that pulls on the objects is gravity. Gravity is the force of attraction that depends on the mass of two objects and the distance between them. It is responsible for accelerating an object toward the earth. The Center of Gravity of an object is the point at which an object s weight is concentrated. In standing humans, for example, the center of gravity is located in the midpoint of their bodies, namely, the navel. A concept needed to further understand gravity is that of mass. Mass is a physical quantity that measures the amount of matter, usually in the units of kilograms. Mass does not change in any situation. A concept related to mass is weight. It is incorrect to use the two terms interchangeably since weight is the measure of the force of gravity on an object. For example, if one were to compare his mass on the earth to that on the moon, it would be the same. On the other hand, if he were to measure his weight on the earth and the moon and compare the two quantities, he would reach the conclusion that he weighs 1/6 the amount on the moon that he does on the earth. This is due to the fact that the moon has 1/6 the force of gravity than the earth. OBJECTIVES: To illustrate to the students the concepts of balanced and unbalanced forces using pulleys, weights, and their bodies. Also, to show how different masses act and react upon one another. SKILLS: Ability to use a pulley, group brainstorming, using the scientific method, verbal communication, hands-on activity, understanding of balanced/unbalanced forces and center of gravity

2 MATERIALS: TEAM ROLES: PROCEDURE: (one for each group of students): Various weights with hooks ranging from 10g-100g Large metal pulley String (must be strong enough to support pulley and weights) Chairs Wall space Balance Tennis Ball Pulley holder, pulley-stringer, weight adjuster, recorder, presenter 1) Introduce different key terms 2) Give examples of balanced/unbalanced forces (see overview 3) Explain how students will be doing three experiments a) balancing a tennis ball with weights (demonstrate how to string a pulley) b) two experiments demonstrating center of gravity 4) Divide students into groups of five 5) Steps for experiment 1: (pulley and weights) a) distribute materials to each group b) have pulley-stringer string the pulley c) have pulley-holder hold the pulley d) have weight-adjuster attach equal weights on either side of the pulley to show balanced forces e) remove the two weights f) have students observe the characteristics of the tennis ball paying close attention to its weight (recorder should record observations under Observations of data sheet see attached sheet (#1) or under ASSESSMENTS below) and have students estimate the mass of the tennis ball and record under Hypothesis of data sheet g) have students use the trial-and-error method to try and balance the weight of the tennis ball with the various weights (record data under Results of data sheet). Also have students write (under Results ) whether each trial represents a balanced or unbalanced force. h) have students weigh the tennis ball to find if their results are accurate and record under Results of data sheet 6) Steps for experiment 2: (Balanced force while sitting) a) have each student sit straight on a chair with their back against the chair and their hands along the side of the chair

3 b) have each student attempt to stand up without leaning forward c) record outcome under Results of data sheet (teacher s note**: Students will be unable to stand because their center of gravity is pulling down and an equal amount of normal force is pushing up. A shift of weight will enable one to stand by displacing the center of gravity.) 7) Steps for experiment 3: (Balanced force while standing) a) have each student lean their left shoulder and left foot against a empty sturdy wall b) have each student try and raise their right foot c) record outcome under Results of data sheet (teacher s note**: Students will not be able to raise their right foot because they can not shift their center of gravity because of the wall.) ASSESSMENT: The purpose of this activity is to show different balanced and unbalanced forces. A vocabulary sheet (see attached sheet (#2) for example) of key terms keeps the students motivated in the learning of the key terms. The data sheet (attached, #1, or below) uses the steps of the scientific method to provide organization of the activity and flow of procedure. Data Sheet: Observations: (What do you see?) Characteristics of the tennis ball: Hypothesis: (What do you predict?) Estimate the mass of the tennis ball: Results: (What happened when you did the experiment?) Weight needed to balance tennis ball: Mass of tennis ball (measured from balance): Outcome of Experiment 2: Outcome of Experiment 3: CONCLUSION: The conclusion should be conducted within each group. Have each student write down his or her final statements on the lab. Have them describe the results and analyze how and why they got them. Make sure they include in their explanations where balanced and unbalanced forces were seen in each experiment. After conclusions are made, have the presenter from each group present the group s results. SUGGESTIONS TO ENHANCE ACTIVITY: In order to avoid collisions of the masses, it is easier to use two pulleys instead of one. If time permits, use objects with different weights (other than just the tennis ball) and have students balance the weights with the provided weights.

4 RESOURCES: Hopkins, Jean, et al. Motion, Forces, and Energy. Prentice Hall: Upper Saddle River, NJ, Best of Wonderscience. Delmar Publishers: Albany, 1997.

5 (#1) Name Period Date Balanced and Unbalanced Forces Data Sheet Complete each blank appropriately. Observations: (What do you see?) Characteristics of the tennis ball: Hypothesis: (What do you predict?) Estimate the mass of the tennis ball: Results: (What happened when you did the experiment?) Weight needed to balance tennis ball: Mass of tennis ball (measured from balance): Outcome of Experiment 2: Outcome of Experiment 3: Conclusion: (What are your final statements on the lab? Describe your results, and how and why you got them. Be prepared to share this with the class.)

6 (#2) Vocabulary Sheet Force: Balanced Force: Unbalanced Force: Pulley: Gravity: Center of Gravity: Mass: Weight: