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Course: Middle school physics - NGSS > Unit 1
Lesson 2: Forces and accelerationNet force
Forces can accelerate an object. When equal forces push an object in opposite directions, they cancel each other out, and the object doesn't move. But when an additional force is added in one direction, the object starts to accelerate in that direction. Created by Sal Khan.
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the true question is where are they getting all these rockets?(35 votes)
it's not a rocket, it's just an engine and a fuel tank, no nosecone(11 votes)
The thing that bothers me is "How does that tiny rocket move that giant hunk of rock?"(18 votes)- you can also push that asteroid with your own hands (not a big push) if there is no stronger force pushing it toward you, so any rocket can do it(10 votes)
So what Sal's saying is the longer the rocket runs attached to the asteroid, the more momentum it gains.
Agree or Disagree?(10 votes)- I agree with you(5 votes)
- If I have a rocket that goes on forever and I have an infinite amount of a perfect vacuum, and I attach the rocket to an asteroid, would at one point the asteroid be accelerated to the speed of light?(7 votes)
That is an interesting question. Luckily, I know the answer:
According to our current understanding of physics, it is not possible for an object with mass (such as an asteroid) to reach or exceed the speed of light. This concept is described in Albert Einstein's theory of relativity, which outlines that as an object with mass approaches the speed of light, its mass would increase infinitely, requiring an infinite amount of energy to propel it further.
As a result, even if you had a rocket that could continuously provide thrust in a perfect vacuum, the mass of the asteroid would increase as it approached the speed of light, making it increasingly difficult to accelerate to that speed. Eventually, the amount of energy required to continue accelerating the asteroid would become so immense that it would be unattainable.
Furthermore, it is important to note that reaching the speed of light is not just a matter of a hypothetical rocket's power but also involves the fundamental nature of spacetime, as described by relativity. So, regardless of infinite thrust or any other means, it is considered impossible for a massive object to reach or surpass the speed of light.
It is worth mentioning that these concepts are based on our current understanding of physics, and future discoveries might provide new insights. However, as of now, according to the principles of relativity, reaching the speed of light remains beyond the realm of possibility for objects with mass.(5 votes)
If two rockets of equivalent force push the asteroid on opposite sides and there's another one on the top, would the asteroid move downwards, or what would happen?(6 votes)
In this scenario, the two horizontal forces would cancel each other out, so the net force would just be downwards. The asteroid's acceleration would therefore be downwards. It wouldn't necessarily move downwards, though. For example, if the rocket already had an upward velocity, the downwards acceleration would just be slowing it down at first.
Does that help?(8 votes)
dang i didn't know that me and my friend where in space.(6 votes)- If acceleration didn’t exist, would everything be still?(4 votes)
It depends... Is matter initially in movement? Or is matter initially not in movement?(4 votes)
Wait, I forgot what net force is- can someone explain?(4 votes)- Net force is just all of the forces added together,
or ΣF (Σ means sum) You can also use F=ma to calculate it because it adds the force up to make a larger/smaller force.(4 votes)
- If one force was acting on the asteroid, pushing is sideways, and another force pushing it down from above, would the asteroid move diagonally?(2 votes)
Yes, it would go exactly diagonally if the 2 forces are the same and no other force is acting on the asteroid. Hope it helps. ^v^(6 votes)
- if you added 5 to the right of the asteroid you will end up having 3 more rockets which means 3 times the boost will make it move to the left. but then they will explode.(4 votes)
Video transcript
- [Instructor] 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. And what we wanna do is
we wanna 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
this super-heated 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. And 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. And 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, you have 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 happened 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. And so 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 also this new force, which I will make in purple,
'cause it's a purple rocket. And so that new force,
you could draw like this, to show, all right, that
will now be the net force, 'cause you have the
equivalent of two rockets going in the left direction, and one rocket going
in the right direction. Or 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. And 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 wanna think about how something might be accelerated,
and in what direction, you need to think about the net force, because some of the forces
could counteract each other.