Transcript Newton`s 3rd Law of Motion: Action and Reaction

Newton’s 3rd Law
of Motion:
Action and Reaction
Book M
Section 2.4
Pages: 64-69
•Newton realized that forces are not “one-sided.”
•Whenever one object exerts a force on a second object, the
second object exerts a force back on the first object.
•The force exerted by the second object is equal in strength
and opposite in direction to the first force.
•The first force is called the “action” and the other force the
“reaction.”
•Newton’s third law of motion states
that if one object exerts a force on
another object, then the second
object exerts a force of equal
strength in the opposite direction on
the first object.
•You may already be familiar with examples of Newton’s third law
of motion.
•Perhaps you have watched figure skaters and have seen one
skater push on the other.
•As a result, both skaters move—not only the skater who was
pushed.
•The skater who pushed is pushed back with an equal force, but in
the opposite direction.
•The speeds with which the two skaters move depend on their
masses.
•If they have the same mass, they will move at the same speed.
•But if one skater has a greater mass than the other, she will move
backward more slowly.
•Although the action and
reaction forces will be
equal and opposite – the
same force acting on a
greater mass results in
a smaller acceleration.
•Newton’s third law is in action all around you.
•When you walk, you push the ground with your feet.
•The ground pushes back on
your feet with an equal
and opposite force.
•You go forward when
you walk because the
ground is pushing
you!
•A bird flies forward by exerting a force on the air with its wings.
• The air pushes back on those wings with an equal force that
propels the bird forward.
•A squid applies Newton’s third law of motion to move itself
through the water.
•The squid exerts a force on the water that it expels from its body
cavity.
•At the same time, the water exerts an equal and opposite force
on the squid, causing it to move.
•You have already learned that balanced forces, which are equal
and opposite, add up to zero.
•In other words, balanced forces cancel out.
•They produce no change in motion.
•Why then don’t the action and reaction forces in Newton’s third
law of motion cancel out as well?
•After all, they are equal and opposite.
•To answer this question, you have to consider the object on
which the forces are acting.
•Look, for example, at the two volleyball players in the photo
below.
• When they hit the ball from opposite directions, each of their
hands exerts a force
on the ball.
• If the forces are
equal in strength, but
opposite in direction,
the forces cancel out.
•The ball does not
move either to the left
or to the right.
•Red arrows show action
forces. Blue arrows show
reaction forces.
•Newton’s third law, however, refers to forces on two different
objects.
•If only one player hits the ball, as
shown in the photo here, the player
exerts an upward action force on the
ball.
•In return, the ball exerts an equal but
opposite downward reaction force
back on her wrists.
•One force is on the ball, and the other
is on the player.
•The action and reaction forces cannot be added together
because they are acting on different
objects.
•Forces can be added together only
if they are acting on the same object.
•The player's wrists exert the action
force.
•The ball exerts the reaction force.
•Newton also wrote about something that he called the “quantity of
motion.”
•What is this quantity of motion? --Today we call it momentum.
• The momentum (moh men tum) of an object is the product of its
mass and its velocity.
•The more momentum an object
has, the harder it is to stop.
•You can catch a baseball
moving at 20 m/s, for example,
but you cannot
stop a car
moving at the
same speed.
•Why does the car have more momentum than the ball?
•The car has more momentum because it has a greater mass.
•A high velocity also can produce a large momentum, even when
mass is small.
•A bullet shot from a rifle, for example, has a large momentum.
•Even though it has a small mass, it travels at a high speed.
•What is the unit of measurement for momentum?
•Since mass is measured in kilograms and velocity is measured in
meters per second, the unit for momentum is kilogram-meters per
second (kg · m/s).
•Like velocity and acceleration, momentum is described by its
direction as well as its quantity.
•The momentum of an object is in the same direction as its
velocity..
•Momentum is useful for understanding what happens when an
object collides with another object.
•When two objects collide in the absence of friction, momentum is
not lost.
•This fact is called the law of conservation of momentum.
• The law of conservation of momentum states that the total
momentum of the objects that interact does not change.
•The quantity of momentum is the same before and after they
interact.
•The total momentum of any group
of objects remains the same unless
outside forces act on the objects.
•Friction is an example of an
outside force.
•However--the word conservation means something different in
physical science than in everyday usage.
•In everyday usage, conservation means saving resources.
• You might conserve water or fossil fuels, for example.
• In physical science, the word conservation refers to conditions
before and after some event.
•A quantity that is conserved is the same after an event as it was
before the event.
Two Moving Objects
•Two train cars traveling in the same direction on a track shown in the figure
below.
•Car X is traveling at 10 m/s and car Y is traveling at 5 m/s.
•Eventually, car X will catch up with car Y and bump into it.
•During this collision, the speed of each car changes.
•Car X slows down to 5 m/s, and car Y speeds up to 10 m/s.
•Momentum is conserved—the momentum of one train car decreases while the
momentum of the other increases.
One Moving Object
•Suppose that car X moves down the track at 10 m/s and hits car Y, which is not
moving.
•The figure below shows that after the collision, car X is no longer moving, but
car Y is moving.
• Even though the situation has changed, momentum is still conserved.
• The total momentum is the same before and after the collision.
•This time, all of the momentum has been transferred from car X to car Y.
Two Connected Objects
•Now suppose that, instead of bouncing off each other, the two train cars couple
together when they hit.
•Is momentum still conserved? --The answer is yes.
•You can see in the figure below that the total momentum before the collision is
again 300,000 kg · m/s.
Two Connected Objects
•But after the collision, the coupled train cars make one object with a total mass
of 60,000 kilograms (30,000 kilograms + 30,000 kilograms).
•The velocity of the coupled trains is 5 m/s—half the velocity of car X before the
collision.
•Since the mass is doubled, the velocity must be divided in half in order for
momentum to be conserved.