Download Active Physics 2.6 - Ms. Juengel`s Class

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
 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
Teacher Demo
 Chair v Wall
Class demo
 Two students on
skateboards
 Push against each other
with the same force – what
happens?
In lab groups
 Using the spring scales,
try to have one person
exert more force than
the other (see
investigation in Physics
book)
Observing a meter stick push back
 Weight
 Books
 Meter stick
Physics Talk 2.6
 What always accompanies
 An unbalanced force
acceleration?
 What was the force that
pushed the person away
from the wall in the chair?
 The wall exerted a force on
the person that caused her
to accelerate away from the
wall.
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
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.
Newton’s Third Law
 What are some examples  The person pushed (applied a
force) on the wall and the wall
of Newton’s Third Law?
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.
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.
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
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
Free Body Diagrams
 How do you draw the force
 The arrow emerges from
 Center of mass
 The point at which all the
of weight?
 How do you draw other
forces?
the object’s center of mass.
mass of an object is
considered to be
concentrated.
 Arrows are drawn by
arrows emerging from the
point of contact
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.
Challenging Newton’s Third Law
 Scenario: If I pull on the chair  False! There are additional forces at
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?
work.
 The force on the ground is larger
than the force on the chair, which
makes the person move forward.
What do you think now?
 P 207 answer WDYTN in
your journal
Homework
 P 208 #1, 4-7