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Active Physics 2.6

Active Physics 2.6. Newton’s Third Law. Welcome! I’m so glad you’re here!. Do Now: WDYS WDYT p 198 Agenda: Investigation 2.6 Physics Talk 2.6 Homework p 208 #1, 4-7. Success Criteria Provide evidence that forces come in pairs, with each force acting on a different object

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Active Physics 2.6

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  1. Active Physics 2.6 Newton’s Third Law

  2. Welcome! I’m so glad you’re here! • Do Now: • WDYS WDYT p 198 • Agenda: • Investigation 2.6 • Physics Talk 2.6 Homework p 208 #1, 4-7 • Success Criteria • Provide evidence that forces come in pairs, with each force acting on a different object • Use Newton’s 3rd Law to analyze situations • Describe how Newton’s 3rd Law explains much of the motion you see in your everyday life • Learning Objective • Explain the forces acting on 2 objects that are interacting

  3. Teacher Demo • Chair v Wall

  4. Class demo • Two students on skateboards • Push against each other with the same force – what happens?

  5. In lab groups • Using the spring scales, try to have one person exert more force than the other (see investigation in Physics book)

  6. Observing a meter stick push back • Weight • Books • Meter stick

  7. Physics Talk 2.6 • What always accompanies acceleration? • What was the force that pushed the person away from the wall in the chair? • An unbalanced force • The wall exerted a force on the person that caused her to accelerate away from the wall.

  8. Physics Talk 2.6 • What were the forces involved when the students pushed each other on the skateboards? • What was the forces that moves you forward when you walk across a room? • Each student exerted an equal force against one another. • Your foot applies a force to the ground and the ground exerts an equal force on you

  9. Newton’s Third Law • What is Newton’s Third Law? • For every applied force, there is an equal and opposite force. The two forces always act on different objects.

  10. Newton’s Third Law • What are some examples of Newton’s Third Law? • The person pushed (applied a force) on the wall and the wall pushes the person • Student A on the skateboard pushed on student B and student B pushed on student A • You push on the floor backwards and the floor pushes you forward. • You pulled on the spring scale. The spring scale pulled on you. These forces were always equal and opposite in direction. • You cannot touch someone without them touching you back.

  11. Newton’s Third Law • How does the ground provide force if it doesn’t look like it’s moving? • Example • The floor actually bends a little bit. If you stand in the center of a trampoline, the bend is quite noticeable; however, floors made of wood or concrete provide less of a bend. • This is like when the meter stick bent under the force of the weight.

  12. Newton’s Third Law • Equivalent expressions of Newton’s Third Law • For every force applied to object A by another object B, there is an equal and opposite for applied to object B by object A • If you push or pull something, that something pushes or pulls back on you with an equal amount of force in the opposite direction ALWAYS • Forces always come in pairs

  13. Free Body Diagrams • How do you draw a free-body diagram? • Each force is represented by an arrow. • The direction of the force matched the direction of the arrow. • Each arrow labels the kind of force. • Objects are often drawn as boxes

  14. Free Body Diagrams • How do you draw the force of weight? • Center of mass • How do you draw other forces? • The arrow emerges from the object’s center of mass. • The point at which all the mass of an object is considered to be concentrated. • Arrows are drawn by arrows emerging from the point of contact

  15. Newton’s law in the meter stick bend • What is the first pair of forces? • What is the second pair of forces? • The meter stick pushes up on the mass and the mass pushes down on the meter stick. • Earth pulls down on the mass with a force of gravity and the mass pulls up on the Earth with an equal force of gravity.

  16. Challenging Newton’s Third Law • Scenario: If I pull on the chair then the chair pulls on me with an equal force. Therefore, the two forces cancel and nothing should move. Newton’s law must be wrong. • True or False? • False! There are additional forces at work. • The force on the ground is larger than the force on the chair, which makes the person move forward.

  17. What do you think now? • P 207 answer WDYTN in your journal

  18. Homework • P 208 #1, 4-7

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