Video Transcript
In this video, we will be
discussing Newton’s first law of motion. This is one of Isaac Newton’s three
famous laws that form a cornerstone of classical physics. But before we look at a statement
of the first law, let’s imagine the following scenario.
Let’s say we’ve got a ball and this
ball has a mass, which we will call 𝑚, of one kilogram. For now, the ball is just sitting
here. It’s stationary; it’s not
moving. But then let’s just say that
someone comes along and pushes the ball from left to right. In other words, this person’s hand
exerts a force on the ball. We can represent the force that the
person exerts on the ball with an arrow acting on the ball to the right. Now, the magnitude or size of a
force is most commonly measured in a unit called the newton. So let’s say that the person exerts
a force of one newton on the ball.
In this scenario, what happens to
our ball now? Well, intuitively, we might think
that the ball will begin to move. After all, somebody’s pushed it
from left to right. So we think the ball will move to
the right, and this would be correct. Exactly how the ball moves — that
is, how quickly and how rapidly it accelerates — we will see in a moment. But a general statement we can make
here is that the ball starts moving to the right when initially it was
stationary. And that is all because of the
force exerted by this person on the ball.
Okay, now, let’s imagine the same
ball once again, the ball with a mass of one kilogram, and it’s initially
stationary, just like before. Now, instead of having just one
person exerting a force on our ball, let’s see what happens when two people exert a
force on our ball, specifically when one person exerts a force on the ball to the
right and the other person exerts a force on the ball to the left. Importantly, in this scenario, we
will assume that the force being exerted to the right is one newton, and the force
being exerted to the left is also one newton.
In other words then, both people
pushing the ball are pushing with the same magnitude or size of force. They’re just pushing in opposite
directions because it looks like this person was going to push the ball once again
to the right, as we saw here. But then this person drawn in blue
decided to prevent this by exerting a force in the opposite direction of the same
magnitude. In this situation, the force being
exerted to the right is being exactly canceled out by the force being exerted to the
left. They both have the same magnitude
of one newton.
Is this therefore effective in
keeping the ball stationary exactly where it is? Well, this is when Newton’s first
law of motion can come into the picture. Newton’s first law of motion tells
us that an object at rest will remain at rest. And an object moving with a
constant velocity will continue to travel at that velocity unless acted on by an
unbalanced force. Now that’s a lot to take in. So let’s break this sentence down
bit by bit. The first part of the sentence
talks about an object at rest. This means it’s talking about an
object initially stationary, just like the ball that we were discussing here. And this law is saying that the
object at rest will remain at rest. And we’ll ignore the bit about
constant velocity for now. So our object will remain at rest
unless it’s acted on by an unbalanced force.
Now what do we mean by an
unbalanced force? Well, an unbalanced force is simply
one that is not balanced by other forces. That is when forces acting on an
object do not completely cancel each other out. In the first scenario that we
considered, we had one hand exerting a one-newton force to the right on our
ball. And that one-newton force was not
counteracted by any other forces. Therefore, this one-newton force is
indeed an unbalanced force. It is worth noting, though, that an
unbalanced force does not have to simply be one force acting on an object. We could, for example, have lots of
forces acting on our object.
In this case, we’ve shown three
forces all acting to the right. And the net or overall effect of
these three forces acting together, we can call this the net force on the ball, is
in this case equal to a very large force acting to the right which has a magnitude
equal to the magnitudes of the three pink forces added together. And even this net force counts as
an unbalanced force because there’s no other force acting in the opposite direction
to completely balance it out or to counteract it. So if our object has an unbalanced
force acting on it, then the object will no longer remain at rest. This is what Newton’s first law of
motion is telling us.
And remember, the object in this
particular scenario, our ball, was initially at rest. So it started out stationary and
then we exerted a net force on it, in which case the ball now starts to move. It stops being at rest. Specifically, it accelerates or
changes velocity in the direction of the net force. And as we saw with this ball over
here, when we pushed it to the right, the ball started moving toward the right.
However, if we now consider our
ball that was initially stationary and we exert another force on it that exactly
counteracts the first force to the right — so let’s say the force to the right has a
magnitude of one newton and the force to the left also has a magnitude of one
newton, kind of like this scenario here — then the net or overall effect of these
two forces combined, the one-newton force to the right and the one-newton force to
the left, is equivalent to if the ball had no forces acting on it at all.
We describe this by saying that the
net or resultant force on our object is equal to zero. And this is when the forces on the
ball are balanced. The one-newton force to the right
is balanced completely by the one-newton forced to the left or, in other words,
canceled out completely because both these forces have the same magnitude, but they
act in opposite directions. So because the forces on our ball
are balanced, Newton’s first law of motion tells us that the ball which was
initially at rest continues to remain at rest. In other words, it’s not going to
start moving in any particular direction.
So for this scenario here, where we
had a ball that was initially stationary — and we had two people exerting force is
on our ball, one to the right and one to the left, and those forces were exactly the
same magnitude but obviously were acting in opposite directions — we can say that
our ball did not have any unbalanced forces acting on it. All of the forces on the ball were
balanced. And therefore, the ball initially
at rest continue to remain at rest. So we’ve seen that an initially
stationary object continues to remain stationary if the forces acting on it are
balanced. And if an unbalanced force acts on
this initially stationary object, the object accelerates. It stops being stationary.
Exactly how an unbalanced force
affects the acceleration of an object is actually given by Newton’s second law of
motion, which tells us that the net unbalanced force on an object is equal to the
mass of the object multiplied by the acceleration it experiences in the direction of
the unbalanced force. However, our focus here is on
Newton’s first law of motion. And there is still one scenario we
haven’t considered yet. Specifically, Newton’s first law of
motion talks about an object initially moving with a constant velocity.
So let’s imagine our two pink balls
once again, except that this time the top ball is initially moving with a velocity
to the right, which we will call 𝑣. And let’s say that this velocity 𝑣
is equal to one meter per second. So this ball is just minding its
own business, moving toward the right at a constant velocity, and then we come in
and exert a force to the right on this ball of a magnitude one newton. Now, as we’ve already seen, this is
an unbalanced force. And Newton’s first law of motion
states that an object initially moving with a constant velocity continues to travel
at a constant velocity unless acted on by an unbalanced force.
Now, in this scenario, we do have
an unbalanced force acting on our object. Therefore, a ball will no longer
continue to travel with that constant velocity of one meter per second. In fact, the ball will accelerate
in the direction of the unbalanced force, as defined by Newton’s second law of
motion, which we saw a little bit earlier. So we can say that exerting a
one-newton force to the right on our ball, which was initially traveling at one
meter per second to the right, results in the ball accelerating or gaining velocity
in the direction of the unbalanced force.
So in this particular case, our
ball at some later time will have a velocity toward the right that is greater than
one meter per second because of the force we’ve exerted on it. However, if we were to instead
exert the force of one newton in the opposite direction to the ball’s initial
velocity, then the ball would accelerate in the direction of the unbalanced force,
meaning some time later the ball would have a velocity of less than one meter per
second to the right. And eventually, the ball would
actually stop and then start moving toward the left. Because the one-newton force is
acting in the opposite direction to the ball’s initial velocity, so in this
particular case, it will slow the ball down first and then start it moving toward
the left.
Now, let’s consider our second
ball. Once again, it’s moving toward the
right with a velocity of one meter per second. And this time, we exert one force
to the right on this ball with a magnitude of one newton and another force to the
left with a magnitude of one newton. Once again, as we’ve seen
previously, these forces are balanced. Therefore, the net force or overall
force or resultant force on our ball is equal to zero. Now, for this scenario, Newton’s
first law tells us something that might be rather surprising. It tells us that an object that was
initially moving with a constant velocity continues to move with that same constant
velocity if there’s no net force acting on the object.
So in this particular scenario, our
ball continues to move to the right with a velocity of one meter per second. Now this fact, as we’ve mentioned
already, might seem a little bit surprising to us because we might imagine, for
example, rolling a ball along the ground with an initial velocity toward the right
of one meter per second. Why does this ball not just
continue to roll at one meter per second forever? Why does it eventually slow down
and stop as it’s running along the ground?
Well, the reason for this is that
there is actually an unbalanced force acting on the ball, specifically, the friction
between the ball and the ground. This frictional force acts in the
opposite direction to the balls initial velocity. And therefore, this scenario is
more like the first scenario that we considered where there is an unbalanced force
acting on the object. And that is why, as the ball moves
from left to right, it eventually slows down and stops.
Lastly, there is one other
consideration to be made about Newton’s first law of motion. Specifically when talking about an
object initially moving at a constant velocity, we’ve been told that an unbalanced
force acting on this ball will cause the velocity of the ball to change. We’ve already considered what
happens to a ball if we exert an unbalanced force in the same direction as the
object’s velocity and if we exert an unbalanced force in the opposite direction as
the object’s velocity. However, both velocities and forces
are vector quantities. And the definition of a vector
quantity is one which has magnitude or size and direction.
We’ve already seen how a force
acting in the same direction as the object’s initial velocity causes it to speed up
and a force acting in the opposite direction as the objects initial velocity causes
it to slow down. But what if we exert an unbalanced
force on the object that acts in a direction perpendicular to its initial
velocity? Well, in this situation, the
magnitude of the object’s velocity will not change. It won’t speed up or slow down. However, the direction of motion of
the object will indeed change. In fact, this set-up here, where
the force is constantly perpendicular to the velocity, is a prerequisite for
circular motion. In other words, the object will
move along a circular path if at every point along the path, the velocity is
perpendicular to the direction of the force exerted.
And note here that this is not
breaking Newton’s first law of motion. An object which had an initial
velocity of 𝑣 to the right will at some point later have a different velocity,
specifically a velocity with the magnitude 𝑣 still, but say at this point pointing
downwards. And this is all because of the
unbalanced force we’ve exerted on the object. In other words then, an unbalanced
force doesn’t necessarily have to cause an object to speed up or slow down. It could also cause it to change
direction. And at this point, we’ve covered
the basics of Newton’s first law of motion.
So now that we’ve stated this law
in a fair amount of detail, let’s summarize what we’ve talked about in this
lesson. We firstly saw that Newton’s first
law of motion states that an object initially at rest will remain at rest and an
object initially moving with a constant velocity will continue to travel at that
velocity unless an unbalanced force acts on the object. We also saw that an object has an
unbalanced force acting on it if all the forces acting on the object do not combine
together to give a net force of zero. So that is a summary of Newton’s
first law of motion.
