A MATCHED SET (Newton’s 3rd Law)
Likewise, with the 3rd Law, students can regurgitate it, but do they understand what it means? In this section, we will discuss forces as “matched pairs”. Small connections can also be made to momentum that can be built upon in the classroom later. This section is very demonstration heavy, as for students to really understand Newton’s 3rd Law, they REALLY need to see it.
9. You Can’t Do It Alone |
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 In this simple demonstration, spring scales are used to show that forces do not occur in isolation, but as interactions between objects. Holding a scale does not get the scale to read anything. However, when a scale is used to pull on another object, it then registers a force. Introducing other scales demonstrates that each scale actually reads the same force, regardless of who is doing the “pulling”, each scale pulls equally and against the initial pull. |
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10. Getting Traction |
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 Using identical RC Cars controlled with one remote, action-reaction pairing between the tires and the surface can be seen. When tires rub against a surface, the tires push back on the surface and the surface will push forward on the tires. One car will be placed on a set of rollers and a lightweight board; the other will be placed directly on the table. When the cars are set into motion, the one on rollers will shoot the board back before it can progress forward. |
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11. You Can’t Blow Your Own Sails |
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 Using a gigantic version of a fan cart, it will be demonstrated that matched pairs will not affect overall motion within an isolated system. Students will watch the fan push a cart while mounted on the cart (matched pair: fan pushes on air, air pushes on fan). Then a “sail” will be placed on the cart, and the cart will no longer be able to move – the fan will push the air into sail, and the sail will push back on the air, yielding no net motion).
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12. Momentum Carts |
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| In this demonstration, carts of equal mass will be placed against each other on a level track. A spring will “explode” the carts apart, and it will be observed that the carts are moving with the same speed. Then mass will be added to one cart, and it can be easily noted that more massive cart has significantly less speed. | |
13. Ball Toss |
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 To further demonstrate this, we will then toss two different balls back and forth. One person will be “coupled” to the ground, and the mass of each ball will not affect them much. The other person will be on skates, and not coupled to the ground. While the forces are equal and opposite, the mass of one object is significantly different, and so the size of those pairs and the skater accelerations observed dramatically change. |
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14. Rocket Power |
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 Finally, utilizing a rocket powered cart, Newton’s 3rd Law will be demonstrated. As the fuel leaves the extinguisher, it will propel the cart forward. A wall of boxes will be knocked down by the thrust of the extinguisher to prove that there is gas leaving the cart and propelling it forward.
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