Video: Newton’s First Law of Motion

In this video, we will learn how to use Newton’s first law of motion and how to apply it to objects that are stationary, as well as objects moving at a constant velocity.

12:52

Video Transcript

In this video, we will be looking at Isaac Newton’s First Law of Motion. It is one of his three famous laws, which form a cornerstone of classical physics. So let’s take a look at what this law says. Newton’s First Law of Motion tells us that an object at rest will continue to 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. Okay, so we can see that the statement basically has two parts to it.

Firstly, if we’re considering an object, so let’s say this ball is our object, and this object is not moving. It’s at rest. In other words, its velocity is zero. Well, in this situation, Newton’s First Law of Motion is telling us that this object will continue to remain at rest. Or in other words, it will keep sitting still in the same position unless it is acted on by an unbalanced force. So what do we mean by an unbalanced force? Well, if we take into account all of the forces acting on the object, then an unbalanced force is acting on the object if the net or overall force on the object is nonzero.

So let’s start by considering this object just sitting there with no forces acting on it. Well, in that situation, the net or overall force on the object is indeed zero. There’s no forces acting on it. And so this is not a situation where the forces on the object are unbalanced. But now, let’s think about a situation where two people start pulling on the object. One of them pulls towards the left and the other pulls towards the right. Let’s say that both of these people pulling on the object exert the exact same force in terms of the magnitude of the force. Both of them exert a force 𝐹.

Well, in this situation, the object does have forces acting on it. However, the two forces are exactly the same in magnitude and acting in opposite directions. Therefore, overall, they cancel each other out. And so the net force on the object is zero. In other words, this situation is equivalent to if there were no forces acting on the object at all. Now, it’s important to note that it’s not necessary for there to be two forces acting in the opposite directions with exactly the same magnitude in order for there to be a net force of zero on the object.

For example, we can consider that there would be a large force acting in this direction on the object and then two smaller forces, one acting in this direction and one acting in this direction as well. Well, in this situation, if we were to think of the large force acting at an angle on the object and we break it down into a horizontal component and a vertical component and then if this horizontal component was exactly equal in size to the force acting towards the left. And similarly, the upward component exactly balanced out the downward component, then, overall, the forces would cancel each other out. And once again, we would have a net force of zero acting on the object.

So in this situation, the forces are balanced on the object. And therefore, the object will continue to remain at rest. Now, this is an important point because there are forces acting on the object. And yet the object is not moving. And once again, to clarify the reason, it’s because all of the forces act to cancel each other out.

However, let’s now consider the situation where we’ve got a very small force acting towards the left and a very large force acting towards the right. Let’s say the force towards the left is 𝐹 one and the force towards the right is 𝐹 two. Well, in this situation, the two forces are acting against each other. But 𝐹 two massively overpowers 𝐹 one. And therefore, the overall or net force on the object is equal to 𝐹 two minus 𝐹 one towards the right. In other words, the situation where there are two forces acting in opposite directions but one is larger than the other is equivalent to the situation where there’s only one force acting on the object, but with a magnitude 𝐹 two minus 𝐹 one.

Well, now, in this situation, the forces are not balanced. In other words, we have an unbalanced force on the object. And Newton’s First Law tells us that when there’s an unbalanced force acting on the object, then the object will no longer continue to remain at rest. It will start to accelerate in the direction of the unbalanced force. In other words, an unbalanced force results in an acceleration of the object. And of course, we should recall that acceleration basically means a change in velocity. Therefore, in this situation, the velocity of the object is going to increase towards the right.

So we’ve covered the first scenario that’s mentioned in Newton’s First Law of Motion. Let’s now think about the second scenario. This scenario talks about an object, this time not at rest. We’ve been told that an object moving with a constant velocity, so let’s say this object is moving at a velocity 𝑉 towards the right, will continue to move at that same velocity unless acted on by an unbalanced force. In other words, if we have two forces acting on the object in opposite directions and those two forces have the same magnitude, then the net force on the object is zero. And the forces are balanced. And therefore, even though the object has forces acting on it, it will still continue to move towards the right with a velocity 𝑉.

However, if we exert an unbalanced force on the object, for example, if we just have one force acting towards the right, then the object will no longer remain stationary. It will start to accelerate or change its velocity in the direction of the force. Now, in this situation, because the force is acting towards the right and the velocity was already towards the right, all that’s gonna happen is that the object is gonna speed up. It’s no longer gonna be travelling at a velocity 𝑉.

However, the key point in this situation is that Newton’s law tells us that an unbalanced force will cause an object to change its velocity. We’re not just talking about speed. We can recall that velocity is a vector quantity. In other words, it has magnitude or size and direction. What this means in practice is that if we were to exert, for example, a net force in this direction, then the object will start to accelerate in that direction. And not only will the magnitude of the object’s velocity change, but so will its direction. So sometime later, the object will travel with a different speed, so let’s say 𝑤, and be travelling in a different direction as well.

Now, it is entirely possible for an object to continue to travel with the same speed but still have a net force on it. For example, a planet orbiting a star can very easily be moving at the same speed, let’s call the speed 𝑠, all the way around its orbit. And yet, there’s a net force acting on the planet. The gravitational force from the star is pulling the planet towards the star. However, the reason that this situation is not breaking Newton’s First Law of Motion is because even though the speed of the object is staying the same, its velocity is changing. And this is because the direction in which the object is moving is also changing. And velocity is a vector quantity.

So the key thing to take away from Newton’s First Law of Motion is that an unbalanced force will cause an object to change its velocity. And equally is importantly, if all of the forces on the object are balanced, then the object will not change its velocity. It will continue to travel in the same direction at the same speed whether that speed is, for example, 𝑉 towards the right or 𝑉 is equal to zero.

And it’s at this point that we realize that the 𝑉 is equal to zero situation, the first scenario talked about in Newton’s First Law of Motion, is just a special case of the situation where the object’s velocity is constant because, remember, zero velocity is also a constant velocity. And that constant velocity will remain constant unless there’s an unbalanced force acting on the object. However, the reason that Newton’s First Law of Motion makes a distinction between the 𝑉 is equal to zero case and the situation where the object is moving with a certain velocity is because we very often have to deal with objects not moving at all. And so it’s very convenient to mention the case where the object is not moving, just as a little reminder to ourselves.

Now, coming back to this situation here, where the object will continue to travel at the same velocity if there isn’t an unbalanced force acting on it, we might ask the question, why don’t we see objects travelling with a constant velocity forever and ever, all that often. For example, if we roll a ball along the ground, then why do we see that ball eventually slow down and stop at some point.

Well, the reason is because the ball is in contact with the ground. So there is a friction force acting on the ball. And a friction force, by nature, works against the motion of a moving object. Therefore, the friction force is basically accelerating the ball in this direction or, in other words, decelerating the ball because it’s actually travelling in this direction. And this results in the ball slowing down and stopping.

Now, as well as the friction force, there is an air resistance force on the ball because the ball is trying to move through some air which contains all these air molecules. And the ball colliding with these air molecules as it moves through the air results in the air molecules trying to slow the ball down. And so we actually have two forces that we can think about that are trying to slow the ball down. If we want the ball to continue to travel at a constant velocity forever and ever, then we have to find a way of balancing these two forces.

For example, if we were to push the ball with our hands such that the force we exerted with our hands exactly cancelled out the two forces acting towards the left, then the ball would continue to travel at a constant speed for as long as we exerted the force with our hands. So generating a situation where all of the forces on an object are completely balanced is actually harder than we’d imagine it to be. And having realized this, let’s now look at an example question to familiarize ourselves a little bit more with Newton’s First Law of Motion.

A book at rest on a table has a weight of eight newtons. At what rate is the book accelerating? What is the net force acting on the book? What magnitude force does the table apply to the book?

Okay, so in this question, we’ve been told that we’ve got a book at rest on a table. In other words, the book is not moving. It’s just sitting there on the table. And we’ve been told that it has a weight, which will act in a downward direction on the book. And this weight is eight newtons. So now that we’ve labelled all of the information we have so far onto the diagram, let’s look at the first part of the question.

We’ve been asked to find the rate at which the book is accelerating. Now, to answer this, we can recall that acceleration is defined as the rate of change of velocity. In other words, it’s the change in velocity of an object divided by the interval of time over which that change in velocity occurs.

However, in this particular scenario, we’ve been told that the book is at rest. In other words, it’s not moving at all. And so it has a constant velocity, which we’ll call 𝑉, of zero. And hence, we can see that, actually, there is no change in velocity of the book because the velocity of the book is not changing. In other words, Δ𝑉 is zero. And if the velocity of the book is not changing over any given period of time, then the acceleration of the book is also zero. Therefore, as our answer to the first part of the question, we can say that the rate at which the book is accelerating is zero meters per second squared.

Moving on to the second part of the question then, we’ve been asked to find the net force acting on the book, in other words, the overall or resultant force acting on the book. Now, in the question, we’ve been told that the weight of the book is eight newtons. And so we might think that the net force on the book is eight newtons as well since this is the only force we’ve labeled in the diagram so far. However, we need to be a little bit careful here.

To answer this question, we need to recall Newton’s First Law of Motion. Newton’s First Law of Motion tells us that an object at rest will remain at rest and an object moving at a constant velocity will continue to travel with that velocity unless acted on by an unbalanced force. In other words, unless an unbalanced force acts on an object, which in this situation is at rest, the object will continue to remain at rest. And this is exactly what we’ve been told is happening in this situation.

We’ve been told that the book is at rest on the table. And in order for that to be true, the forces on the object must be balanced. In other words, there must be some force exerted by the table on the book to counteract this eight-newton weight of the book. And so we can say that the table will exert an upward eight-newton force on the book in order to cancel out the downward eight-newton force. This type of force is known as the normal force. And it is called this because the force is normal to or perpendicular to or at right angles to the surface that is actually exerting the force. That’s the top of the table.

Now, this upward eight-newton force is also sometimes known as the contact force because it occurs due to the contact between the book and the table. But then, the point is that the net force on the book must be zero because, otherwise, the book would start to accelerate. And we can see that this is true because the eight-newton force upward exactly cancels out the eight-newton force downward. And so our answer to the second part of the question is that the net force acting on the book is zero newtons.

So now, we can move on to the final part of the question, “What magnitude force does the table apply to the book?” Well, luckily, we’ve just discussed this. The table applies an eight-newton upward force onto the book. And this is known as the normal force or the contact force. However, we’ve only been asked to state the magnitude of the force, in other words, the size of the force. We don’t need to worry about the direction. And so we can say that the table applies an eight-newton force onto the book.

Okay, so now that we’ve had a look at an example question, let’s summarise what we’ve talked about in this lesson. We’ve seen firstly that an object at rest remains at rest and an object moving with a constant velocity continues to travel with that velocity unless acted on by an unbalanced force. This is a statement of Newton’s First Law of Motion. And the important thing in the situation where an object is travelling with a constant velocity is that unless there’s an unbalanced force acting on the object, the object will continue to travel with the same velocity. That means both the object’s speed and direction will stay the same.

Now, an important point of Newton’s First Law of Motion is that an unbalanced force causes an object to accelerate. And we can recall that an acceleration is the same as a change in velocity divided by the time taken for that change in velocity to occur. In other words, an unbalanced force can either cause an object to speed up or slow down or to change direction and travel at the same speed or to change direction and change speed. But as long as there’s a change in velocity, then that means that there’s an acceleration. And therefore, an unbalanced force is acting on the object.

And of course, when we talk about an unbalanced force, what we actually mean is a nonzero net force on the object. In other words, when we consider all of the forces acting on the object and find that the overall resultant force is not zero, then we say that the object has an unbalanced force acting on it.