Science/8/Focus on Physical Science 2.0 Unbalanced forces cause changes in velocity. As a basis for understanding this concept:
a. Students know a force has both direction and magnitude.
b. Students know when an object is subject to two or more forces at once, the result is the cumulative effect of all the forces.
c. Students know when the forces on an object are balanced, the motion of the object does not change.
d. Students know how to identify separately the two or more forces that are acting on a single static object, including gravity, elastic forces due to tension or compression in matter, and friction.
e. Students know that when the forces on an object are unbalanced, the object will change its velocity (that is, it will speed up, slow down, or change direction).
f. Students know the greater the mass of an object, the more force is needed to achieve the same rate of change in motion.
g. Students know the role of gravity in forming and maintaining the shapes of planets, stars, and the solar system.
Introduces students to Newton’s 3rd Law. Focuses on misconceptions regarding action and reaction pairs, and emphasizes that force pairs occur simultaneously.
Through demonstrations, activities, and discussion students will formalize an understanding of Newton’s 3rd Law and apply it to a variety of situations.
Additionally, students will clarify any misconceptions stemming from the commonly quoted phrase “for every action, there is an equal and opposite reaction.” Specifically, students will recognize that the force pairs are simultaneous rather than a sequence of events, and also that although force pairs are equal in size, the consequence of the forces are usually not “equal.”
Students will be able to:
State and explain Newton’s 3rd Law. Forces always occur in pairs that are equal in size and opposite in direction.
Give real life examples of Newton’s 3rd Law.
Identify the force pairs in a variety of examples.
Explain, in their own words using examples, that although force pairs are equal in magnitude, that the consequences of these forces are usually not equal.
Use Newton’s 3rd Law to analyze the physics of an open field tackle in football.
2 spring scales (or a class set)
2 bathroom scales
2 rolling chairs or skateboards
2 dynamics carts
Anticipatory Set (Lead-in):
How many of you have ever been in a skirmish with a sibling, after which you each told your parents that “they pushed me first”? In today’s lesson, Newton will help you understand that you were both wrong. First however, you will also learn a bit about football, rockets, and many other phenomena in your daily lives.
Lesson Plan Procedure:
Question 0: Define a “force” as a push or pull
Come to a class consensus on the definition of a force, “a push or pull,” and document the definition on Question 0 of the activity worksheet.
Question 1: Establishing that forces occur in pairs
Explain and demonstrate that one method to measure a force is to use a spring scale. In this case, a scale measures how much it pulls on an object.
Attach 2 spring scales together and ask for one or two volunteers (you can be one of them if it will help demonstrate the point). Explain that you will know if a person is applying a force to the other if the spring scale registers the force.
Now do a series of scenarios to make it clear to the class that one person (spring scale) can not apply a force to a second person (spring scale) unless the second also applies a force to the first. You can do this by asking one of them to pull (you can specify a particular force within the range of their spring scale), but asking the second person to NOT pull. You can repeat this and let them try again, or have the participants reverse roles. In all cases, they will find that one can only register a force if the other one does also.
You can reinforce this idea with two bathroom scales that can be pushed together. Scale 1 cannot push scale 2 if scale 2 does not provide resistance (push back), and vice versa.
Ask the students to list other examples where forces occur in pairs.
NOW ask for the students to try and think of an example where forces do NOT occur in pairs. That is, can any of the students think of an example where an object applies a force to a second, and the second does not apply a force back? Help them to see that in each case there are always force pairs (later in this discussion you will cover the concept that although forces are equal, the results/consequences of these forces are not equal, but you don’t need to cover that in detail at this point).
On Question 1 of their activity sheets, have them record that “forces always occur in pairs.”
Question 2: These force pairs occur simultaneously
Perhaps one of the biggest misconceptions regarding the 3rd Law is that, it implies an action and then a reaction. It should be made clear to the students that the two forces occur simultaneously. Generally the students will easily come to a consensus if posed the question described below:
Ask the students to discuss with a neighbor or in small groups the following question: Consider a force pair of your choice and determine whether the two forces occur simultaneously or if one happens and then the other happens. Have the students share their answers either on small whiteboards or orally. In each case, if necessary, assist the students to understand that the force pairs always occur simultaneously.
Have the students record this conclusion in Question 2: “These force pairs occur simultaneously or at the same time.”
Question 3: These pairs are equal in size
Either as a class demo or in small groups, ask students to compare the magnitude of the force pairs by reading the approximate values on the spring scales (if necessary, define “magnitude” as “size”). The students should conclude that the relative magnitude of the forces is the same. Note: Spring scales tend to be a bit inaccurate, so encourage the students to look at general trends rather than exact scale readings. Also, if available, perhaps the best demonstration of this can be done with 2 electronic force sensors hooked together with graphical readings and one of the readings inverted.
Have the students record the class consensus in Question 3: “Forces occur in pairs that have equal magnitude.”
Question 4: These pairs are opposite in direction
Through discussion, the students should be able to identify that the direction of the force pairs is equal and opposite. You can simply ask them to check with a neighbor to determine the relationship between the directions of each force in the pair. Have the students record the answer under Question 4: “Forces occur in pairs that have opposite directions.”
Question 5: Putting it all together, formalizing the 3rd Law
Demonstration: Blow up a balloon and let it sputter around. Ask the students to explain this using the concepts addressed in Questions 1 through 4. Explain that rockets propel themselves by applying a force to the exhaust gas which in turn applies a force to the rocket.
Now that you have covered the concepts of the 3rd Law, have the students record a formal description for Question 5: “Forces occur simultaneously in pairs that are equal in magnitude and opposite in direction.”
Note: This description of the 3rd Law tends to avoid creating the many misconceptions that are generated by the layperson definition, “For every action there is an equal and opposite reaction.”
Question 6: Alternative description of the 3rd Law
Have the students complete Question 6 with a very useful and functional description of the 3rd Law: “If object A applies a force to object B, then object B simultaneously applies a force to object A that is equal in magnitude and opposite in direction.”
Explain to the students that they can replace “object A” and “object B” with any force pair objects. Give examples, and have the students give examples.
Note: This definition is very useful in helping students apply the 3rd Law to situations. If they can identify the force pair, this definition will help them remember that the forces are simultaneous and equal.
Question 8 and 9: Although the magnitudes of forces are the same, the consequences and results of these forces are often NOT the same.
Use Question 8 to discuss with students that although the force pairs are equal in magnitude, the consequences of these forces are usually NOT the same.
Through discussion and demonstration, you can show that equal forces produce different results. Any of the demonstrations below can get this point across:
• Two people of significantly different sizes sitting on rolling chairs or skateboards push off each other, and the smaller one will have a greater acceleration. Rather than pushing, they can also pull on a rope.
• If available, you can use 2 small demonstration carts with different masses. You can compress something elastic between them and release. You can also connect them to a chain of rubber bands and notice that the cart with less mass will experience greater acceleration.
• To clarify the point that a given force can produce very different results, as in the windshield and bug question, you can drop a brick on a rock and drop the same book on a paper cup lying on its side.
Ask the students to give other examples in which the force pairs create very different results. You can help them by giving a few of your own:
• A raw egg is dropped and hits the ground.
• A car runs into a thick concrete barrier.
• A bullet fires from a gun (the recoil velocity of the gun is much less).
After the above demonstrations and discussions, have the students answer Question 9.
Closure (Reflect Anticipatory Set):
Ask the students to answer the following question: A brother and a sister are in a skirmish. When their mother comes to find out what is going on, the brother says that the sister pushed him first. The sister claims that the brother pushed her first. Use Newton’s 3rd Law to explain whether either of them is technically correct.
Answer: According to Newton’s 3rd Law, if one person pushes a second person, the second person simultaneously pushes the first person with equal magnitude in the opposite direction. However, similar to an egg hitting concrete, the results of these force pairs are not the same. Thus, when you push on a person, despite the fact that their force on you may be the same, that person may fall over while you remain standing.
Assessments & notes
Plan for Independent Practice:
For homework the students can list several examples of force pairs in their daily lives. They can identify the forces and describe whether or not the forces produce equal results.
Assessment Based on Objectives:
Quiz: Newton’s 3rd Law
Adaptations & Extensions:
Newton’s 3rd Law as addressed in this lesson is a natural introduction to the topic of “conservation of momentum.”