Download 3rd Law Powerpoint

Newton’s Laws of Motion
Number 3
• Newton’s Third Law of
Motion states that for
every action force, there
is an equal and opposite
reaction force.
• Forces always occur in
interacting force pairs.
• Interacting force pairs are
equal in size and
opposite in direction.
Is the force of the bullet forward
equal to the force of the rifle
backward? Yes
Are the accelerations equal? Why
No, the mass of the rifle is much
greater than the mass of the bullet.
Force is the same, so…
if  Mass bullet
then  Acceleration bullet
if  Mass rifle
then  Acceleration rifle
Interacting Force Pairs
• Why don’t force pairs cancel each
other out?
– Force pairs do not act on the
same object.
• 1st object exerts action force on
2nd object.
• 2nd object exerts reaction force on
1st object (equal in size, opposite
in direction).
• Equal forces don’t always have
equal effects.
• For example, the action force of Earth
pulling on an object and causing it to
fall is much more obvious than the
equal and opposite reaction force of
the falling object pulling on Earth.
Identifying Interacting Force Pairs
• Action Force =
– Bowling ball pushes
pin left.
• What is the reaction
force?
– Pin pushes bowling
ball right.
Identifying Interacting Force Pairs
• Action Force =
– Enclosed air particles
push balloon wall
outwards.
• What is the reaction
force?
– Balloon wall pushes air
particles inward.
Identifying Interacting Force Pairs
• Identify at least six force pairs in this picture.
• Momentum is a quantity
defined as the product of
the mass and velocity of
an object.
momentum = mass x velocity
p = mv
• Moving objects have
momentum.
• For a given velocity, the more
mass an object has, the
greater its momentum is.
• Likewise, the faster an object
is moving, the greater its
momentum is.
• Force is related to change
in momentum.
• When you force an object to
change its motion, you force
it to change its momentum.
• Momentum is conserved in
collisions.
• The Law of Conservation of
Momentum states that the
total amount of momentum
in an isolated system is
conserved.
• Momentum is transferred.
• When a moving object hits a
second object, some or all of the
momentum of the first object is
transferred to the second
object.
• Momentum can be transferred in
collisions, but the total
momentum before and after a
collision is the same.
• A semi and a car collide… Which
will get hit farther?
• If they stick together… Which
direction will they move?
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EXIT TICKET: #37-40
37.“For every action force, there is a reaction force that is ___ in
size and ___ in direction.”
a. opposite; equal
b. equal; opposite
c. equal; equal
d. opposite; opposite
38. If 2 spheres were pulled to the left on a Newton’s Cradle and
then released, how many will move to the right?
a. 1
b. 2
c. 3
d. 4
39. If the action force is a hockey stick pushing a puck to the left,
then the reaction force is the ___.
a. puck pulling the hockey stick to the left
b. puck pulling the hockey stick to the right
c. puck pushing the hockey stick to the left
d. puck pushing the hockey stick to the right
40. If the action force is a bowling ball pushing a pin to the right,
then the reaction force is the ___.
a. pin pushing the bowling ball to the left.
b. pin pushing the bowling ball to the right
c. bowling ball pulling the pin to the left
d. bowling ball pulling the pin to the right