Net force | Movement and forces | Middle school physics | Khan Academy

Let's say that we are in deep space, and there is this asteroid here that, compared to us, is stationary or relative to us is stationary. What we want to do is we want to start to move it. So, what we do is we attach a rocket to one side, and then we ignite that rocket, and it starts to send all the superheated gas, all of these particles, to the right. Well, what do you think that's going to do to the asteroid?

Well, it's going to push on the asteroid in that direction, or you could say it's going to exert a force on that asteroid. We could show that force like this, where the strength of that force, or the magnitude of the force, is the length of this line, and then the direction I will specify or show with that arrow. So, fair enough, I will be pushing towards the left. When I push to the left, it doesn't just start to move the asteroid to the left; it actually will accelerate the asteroid to the left.

So, the longer that this rocket is running, it's going to make the asteroid move to the left faster and faster and faster. But let's think about another example. Let's say that you and one of your friends had a little bit of miscommunication, and they went and put an identical rocket on this side of the asteroid, and y'all ignited it at the exact same time.

So, this one is going to push in the other direction. What do you think is going to happen? If these happen at the exact same time, even though there's now twice as much force being exerted on this asteroid, it's going in opposite directions, so they zero out, and so there's zero net force. Therefore, this asteroid won't be accelerated at all.

Now, let’s say that a third friend wanted to correct this situation, and this isn't necessarily the most efficient way to do it. But what they do is they put another identical rocket right over here and let's say ignite that. Now, what will happen?

Well, now you had the original two forces that net out to each other, but now you have this new force, which I will make in purple because it's a purple rocket. And so that new force you could draw like this to show that will now be the net force, because you have the equivalent of two rockets going in the left direction and one rocket going in the right direction.

Another way we could draw that is we have two rockets going in the left direction, so that would have a force that looks like this, and then we have one going in the right direction. So if you were to net it out, this is equivalent to just having one rocket that we originally saw. That's equivalent to just going back to what we originally saw.

So the important takeaway from this video is that it's not just about the force that's being applied. If you want to think about how something might be accelerated and in what direction, you think about the net force, because some of the forces could counteract each other.