Forces and Motion Ch. 2.3 Newton's third law relates action and reaction forces.
Newton made an important observation that explains the motion of the jellyfish. He noticed that
forces always act in pairs. ​
Newton's third law ​
states that every time one object exerts a force
on another object, the second object exerts a force that is equal in size and opposite in direction
back on the first object. As the jellyfish contracts its body, it applies a downward force on the
water. The water applies an equal force back on the jellyfish. It is this equal and opposite force
on the jellyfish that pushes it up. This is similar to what happens when a blown-up balloon is
released. The balloon pushes air out the end, and the air pushes back on the balloon and moves
it forward.
Action and Reaction Pairs
The force that is exerted on an object and the force that the object exerts back are known together as an
action/reaction force pair. One force in the pair is called the action force, and the other is called the reaction force. For
instance, if the jellyfish pushing on the water is the action force, the water pushing back on the jellyfish is the reaction
force. Likewise, if the balloon pushing the air backward is the action force, the air pushing the balloon forward is the
reaction force.
You can see many examples of action and reaction forces in the world around you. Here are three:
•
You may have watched the liftoffs of the space shuttle on television. When the booster
rockets carrying the space shuttle take off, their engines push fuel exhaust downward. The
exhaust pushes back on the rockets, sending them upward.
•
When you bang your toe into the leg of a table, the same amount of force that you exert on
the table is exerted back on your toe.
•
Action and reaction forces do not always result in motion. For example, if you press down on
a table, the table resists the push with the same amount of force, even though nothing
moves.
Action and Reaction Forces Versus Balanced Forces
Because action and reaction forces are equal and opposite, they may be confused with balanced forces. Keep in mind
that balanced forces act on a single object, while action and reaction forces act on different objects.
Balanced Forces​
If you and a friend pull on opposite sides of a backpack with the same amount of force, the
backpack doesn't move, because the forces acting on it are balanced. In this case, both forces are exerted on one
object—the backpack.
Action and Reaction​
As you drag a heavy backpack across a floor, you can feel the backpack pulling on you with an
equal amount of force. The action force and the reaction force are acting on two different things—one is acting on the
backpack, and the other is acting on you.
The illustration below summarizes Newton's third law. The girl exerts an action force on the boy by pushing him.
Even though the boy is not trying to push the girl, an equal and opposite reaction force acts upon the girl, causing her
to move as well.
Newton's three laws describe and predict motion.
Newton's three laws can explain the motion of almost any object, including the motion of animals. The illustrations on
page 60 show how all three of Newton's laws can be used to describe how kangaroos move. The three laws are not
independent of one another; they are used together to explain the motion of objects.
You can use the laws of motion to explain how other animals move as well. For example, Newton's laws explain why
a squid moves forward while squirting water out behind it. These laws also explain that a bird is exerting force when it
speeds up to fly away or when it changes its direction in the air.
You can also use Newton's laws to make predictions about motion. If you know the force acting upon an object, then
you can predict how that object's motion will change. For example, if you want to send a spacecraft to Mars, you must
be able to predict exactly where Mars will be by the time the spacecraft reaches it. You must also be able to control
the force on your spacecraft so that it will arrive at the right place at the right time.
Knowing how Newton's three laws work together can also help you win a canoe race. In order to start the canoe
moving, you need to apply a force to overcome its inertia. Newton's second law might affect your choice of canoes,
because a less massive canoe is easier to accelerate than a more massive one. You can also predict the best position
for your paddle in the water. If you want to move straight ahead, you push backward on the paddle so that the canoe
moves forward. Together, Newton's laws can help you explain and predict how the canoe, or any object, will move.