Video: Newton’s First Law of Motion

Which of the following statements most correctly describes Newton’s first law of motion? [A] An object will move until a net force acts on the object. [B] An object will not move unless a net force acts on the object. [C] An object will change direction if a net force acts on the object. [D] An object will not change its velocity unless a net force acts on the object.

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Video Transcript

Which of the following statements most correctly describes Newton’s first law of motion?

Now we’ve been given four statements, so let’s go through them one by one and see which one is correct. Number one: An object will move until a net force acts on the object. So what this statement is saying is that an object will continue to move. So let’s say we’ve got this object here, which is a ball. And this ball will continue to move let’s say with a velocity 𝑣 until there’s a net force on the object. So is this true?

Well, no, we don’t just see objects constantly moving around, do we? Now if the object in itself is already moving with a certain velocity, then yes, the only way to stop it moving is to exert a net force on it. But what if the net force wasn’t going against its velocity but was rather going in the direction of its velocity?

Well, then this force would result in the speeding up of the object. And also what if the object was stationary in the first place, it was not moving? But there’s no net force on this object, so should it start moving? No, this statement doesn’t make sense. This is not the answer to our question. So let’s move on to statement number two.

Statement two says that an object will not move unless a net force acts on the object. So what this statement is saying is that we’ve got our object here and it’s stationary, and the only way to make it move is if there’s a net force on it.

Now if the object is indeed stationary and we exert a force on it, then that is the only way to get it to move. However, what if the object is already moving? Well, an object can be moving even if there are no net forces on it. What it will do is to continue to move with the same velocity in the same direction. In other words, if the object is already moving, we don’t need to exert an extra force on it to keep it moving. So this statement doesn’t make sense either. This is not the answer to our question. So let’s move swiftly on to number three.

Statement three says an object will change direction if a net force acts on the object. So here’s our object moving towards the right with a velocity 𝑣. And we exert a net force on it, let’s say in this direction. Well, in that case, what the object is going to do is to turn towards the direction of the force. So this statement seems to be adding up so far. But what if we have our object moving towards the right with a velocity 𝑣 and we exert a net force on it in the same direction as its velocity? Is it going to change direction?

Well, no, all it will do is accelerate in the same direction as its velocity. So we found a possible net force on the object that will not change its direction. Therefore, this statement is not the right answer either. And so it looks like number four should be our solution, but let’s go through it to make sure.

Number four: An object will not change its velocity unless a net force acts on the object. Now yes, this does make sense. Let’s consider one scenario first of all where the object is stationary; in other words, its velocity is zero.

The only way for the object to change its velocity is if there’s a net force acting on it. So let’s say we have a net force acting towards the right. This object will now accelerate, it will speed up, in the direction of the force. And so it will start having a velocity towards the right. But what if we had an object that was already moving towards the right with a velocity 𝑣?

Well, what that object will continue to do if we don’t exert a net force on it is to move in the same direction with the same velocity. In other words, it will continue to move towards the right with a velocity 𝑣 unless and until we exert a net force on it. And this net force can be towards this direction that we’ve drawn and that will slow down the velocity, or it could be the same direction and that will increase the velocity, or it could be in any other direction, whichever direction we want.

That force will result in an acceleration in the direction of the force. But hang on, this doesn’t quite add up. We could say that we want to roll our object along the ground. Now let’s say we start rolling it towards the right and it’s got this velocity 𝑣. Well, what happens?

Well, the ball continues to roll and then it slows down and stops. But the statement says that if there’s no net force, then velocity will stay the same and the ball will continue to move towards the right. So what gives? Well, in this case, there is a net force on the object. It’s the friction between the ball and the surface that’s slowing down the ball, not only that but the air resistance as well.

The ball is traveling through some air, and this air provides resistance, further slowing down the ball. And therefore, it’s very difficult on Earth to generate conditions where an object doesn’t have a net force on it, which is why to us it seems like we always need to exert a force to keep an object moving. In reality, we don’t. If there is no net force on the object, it will continue to move in the same direction with the same velocity. But as we already said, these conditions are very very difficult to generate on Earth.

However, we can test this in space where there’s no air resistance or no friction and also no force of gravity to exert a net force. If we launch a spacecraft out into space and then turn off its engine so there is no net force on it, no gravity, no air resistance, no friction, the aircraft does continue to move in the same direction with the same velocity. So it looks like we found our final answer. The most correct description of Newton’s first law of motion is that an object will not change its velocity unless a net force acts on the object.

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