|According to the third Newton's law: every action
has an equal and opposite reaction. Then, both
action should get cancelled and the body should
remain constant. My question is, why does a bullet
move from a gun according to this law?|
|Question Date: 2017-06-23|
An excellent question!
You are correct in stating Newton's third law of
motion that every action as an equal and opposite
Newton's third law is a law of the forces
acting on an object, which are the product of
mass and acceleration.
So let's take a closer look at your bullet
Initially the bullet and gun are in contact with
each other, so we consider them as a single
closed system. When the gun is fired, a
combustion reaction heats up the air causing it to
expand rapidly and shooting the bullet.
We can recast Newton's third law from forces to
change in momentum. Momentum is defined as mass
Before the gun is fired, everything is at rest.
The bullet experiences a change in momentum that
must be matched by a change in momentum of the
gun. This is Newton's third law in action. The
change in momentum of the gun is called
recoil. Because the gun is much more massive
compared to the bullet, it does not recoil as
noticeably. You can see this whenever someone
shoots a gun; they need to ground themselves to
hedge against the recoil.
You can apply the same principle to how a
rocket moves in space. The fuel that is combusting
at the tail-end of the rocket is what is pushing
on the rocket and propelling it forward (the same
is true when the rocket is being launched, unlike
the misconception that the rocket pushes off the
You have almost stated Newton's Third Law
correctly! What you forgot is that the action
is applied from object A onto object B and the
reaction arises from object B to A! So, what
we refer to as "action-reaction" is a pair
of forces that is applied to two objects, not one,
and this is why they cannot cancel.
Let's think of two examples to convince you.
Let's assume you play pool (billiards) and all the
balls have the same mass. And let's say I hit a
striped ball on a solid one. Initially, the
striped ball is moving, and the solid is
stationary. But after the collision, the striped
will be stationary and the solid will move.
How did the solid ball move? It moved
because the striped ball applied a force on it, in
Medieval terms called "an action".
How did the striped ball stop? It
stopped moving because the solid ball applied a
force on it, in Medieval terms, "a reaction".
"Action" and "reaction" are nothing more than two
forces. But Newton gave them this name because
they are special forces, they have same
magnitude and opposite directions (and are applied
at different centers of mass). And this is the
reason why if the two billiard balls have the same
mass, the striped ball will stop immediately and
the solid will move with the velocity that the
striped had, assuming the collision happened at
the center line. If not, new physics of rotation
had to be discovered!
Now, the second example, is more complex in
reality but may help you to think of
"action-reaction" pairs is easy. Imagine you are
taking your dog (or other favorite animal) for a
walk, so there is a leash in between you. The
leash can only sustain tension, not compression.
You pull the dog towards you because you want to
keep moving (you perform the action). But the dog
resists (reacts) and pulls towards its favorite
tree / street corner. The dog experiences your
force (action) via the leash, and you experience
its force (reaction). But these forces can only be
the same in magnitude! But you are definitely
getting pulled by the leash (and the dog) and only
the friction of the ground and your muscle power
helps you to stay put and not fall ! So,
definitely, you see that you who applies the
action, experiences a force that is not being
A nice answer is on this
The gun moves, too, but in the opposite direction
and with the opposite force. If you ever fire a
gun, you will learn this, as the gun will have
something called a "kick", and the more
the gun, the more powerful the kick. This kick is
the movement of the gun backward as a consequence
of propelling the bullet forward.
The thing you should be aware of: the
opposite reaction is not applied to the same
object. If you treat the two objects (or more)
as a whole system, then the opposite reactions
will get cancelled. But individually, object A
exerts force F to object B; object B will exert
the opposite force F to A (not to B). So
individually, the total force on A or B is not
balanced! As for the gun, it is the same
principle. The explosive chemicals will expel the
bullet out, at the same time, you will also feel
the backlash (the opposite force). That's
why you have to hold the gun firmly while opening
fire. If no one is holding the gun, then the
bullet will be going forward and the gun will move
It’s great that you’re really trying to use
you’re understanding of Newton’s laws to
explain everyday phenomenon, but you’re missing a
key idea. When the gun fires, it pushes the
bullet in one direction and the gun in the
other. Both the bullet and the gun experience
the same force but in opposite directions. These
two forces are equal and opposite of each other.
This opposite force manifests itself as
recoil, which is where the gun pushes back
whenever it is fired. You may be wondering why the
bullet travels so much faster if both the gun and
bullet experience the same force. This is because
the heavier the object is, the less velocity it
will experience when forced. The exact numbers are
a bit more complicated than that but the basic
idea is there. Thank you for your question!
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