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