Transcript Action and Reaction Pairs

BELLWORK 11/12/2015
 A car has a mass of 1.50 x 103kg. If the
force acting on the car is 6.75 x 103N to the
east, what is the car’s acceleration?
ANSWER
 4.50 m/s2, east
The Third Law: Action and Reaction Pairs
 “For every action there is an
equal and opposite reaction.”
 This statement is known as
Newton’s third law of motion.
 Newton’s third law discusses
Action and Reaction Pairs.
Forces occur in pairs
 The two forces in a pair are
called action and reaction.
 Anytime you have one, you
also have the other.
 If you know the strength of
one you also know the
strength of the other since
both forces are always
equal.
Name the Action – Reaction Force
Pairs
 A baseball pushes a
glove left.
Example #2
 A bowling ball
pushes a pin to the
left
Example #3
 The exhaust gases
from the rocket push
down against the
ground.
Example #4
 Identify the two pairs
of action-reaction
forces.
 Use the notation
"foot A", "foot C",
and "ball B" in your
statements.
ANSWER
 The first pair of action-reaction force pairs
is: foot A pushes ball B to the right; and
ball B pushes foot A to the left. The
second pair of action-reaction force pairs
is: foot C pushes ball B to the left; and ball
B pushes foot C to the right.
Newton's Third Law
 Newton’s third law states that for every action
force there has to be a reaction force that is
equal in strength and opposite in direction.
 Action and reaction forces act on different
objects, not on the same object.
 Action and Reaction forces do not cancel each
other out because they act on different objects.
EXPLAIN
 Name the action and
reaction forces present
when a hammer hits a
nail.
 If the force that the nail
exerts on the hammer is
equal to the force that
the hammer exerts on
the nail, why doesn’t the
nail remain at rest?
Now
 Read Page 133 – Action and Reaction
forces each act on different objects.
Hmmmmmm………..
 Astronauts go on spacewalks when they need to work
on their space shuttle or perform experiments. Once
they exit the space shuttle, how can they begin their
motion if there is nothing for them to push on?
ANSWER
 One solution is to throw something
opposite the direction you want to move.
Try this Problem……..
 While driving down the road, a firefly strikes
the windshield of a bus and makes a quite
obvious mess in front of the face of the
driver. This is a clear case of Newton's third
law of motion. The firefly hit the bus and the
bus hits the firefly. Which of the two forces
is greater: the force on the firefly or the
force on the bus?
ANSWER
 They are the same…For every action,
there is an equal and opposite reaction.
CLASSWORK
 Read Pages 132- 134
 Answer #’s 1-3 on Page 134.
BELLWORK 11/13/15
 A boy pushes a lawnmower
to the right
 A. Name the reaction force.
 B. Draw 2 FBD’s to show
how the action and reaction
forces cancel out AND why
both the boy and the
lawnmower don’t just stay
still.
ANSWER
REACTION FORCE
F lawnmower
on boy
ACTION FORCE
F ground
pushing foot
forward
F friction
Fpush from boy
HMMM………..
 Action and Reaction Pairs do not cancel
each other out because each force acts on
a different object. They would both have
to act on the same object to cancel each
other out.
 Horse and Wagon Problem