Main content
Course: Middle school physics > Unit 1
Lesson 3: Action and reaction forcesAction and reaction forces
Newton's third law of motion states that every force has an equal and opposite reaction force. This law is demonstrated through examples like a plane flying by pushing air particles, a rocket launching, and an astronaut moving in space. It's also explained how these forces affect us in everyday life and in the universe. Created by Sal Khan.
Want to join the conversation?
- OK, let's accept Sal's explanation. If, however, there's equal forces at play with both the jet plane and the rocket, how do those objects actually move? By the explanation, doesn't one force acting equally against the other prevent some kind of "forward" motion? Any insight is welcome. And thanks Sal!(18 votes)
- if you throw a ball on the floor with a force of 150 newton (newton is the si unit of force) a force of 150 newton is exerted on the molecules of the floor . each molecule then exerts the same force of 150 newton not on itself but the ball so it bounces upward (if the ball doesn't bounce upward it doesn't mean that the reaction force of the molecules balance out but because the reaction force is not enough to move the ball move , the amount of force need to move the ball depends on the mass of the ball) Hope this helps!(34 votes)
- I do not understand this "And it's actually not that the Moon is rotating around the Earth. It's actually, they're both rotating around the center of mass of their combination" could you explain more ?(10 votes)
- The Earth and the Moon are exerting force on each other, but due to Earth's much larger mass, it doesn't move as much. They're still orbiting that center, but the Moon moves much more because its mass is smaller. It appears that the Moon is orbiting the Earth because Earth "orbits" the Moon so little. Earth's mass is greater than the Moon's, so it also has more gravity that pulls the Moon to orbit that center more than the Earth itself.(13 votes)
- If a rock falls to the ground what will the ground do?(7 votes)
- The ground will do nothing.(7 votes)
- btw the airplane is a british airways 787 and the rocket is a falcon heavy :)(7 votes)
- wouldn't the moon be pulled into the earth tho cause it has a lesser mass? But with that logic wouldn't the same happen with mercury, Venus, and the sun?(4 votes)
- Ion even kno😭🙏❌(5 votes)
- How did he know there was a table infront of me?...(4 votes)
- magic sal is magic(3 votes)
- Why do the force act on different things (points) and not on the same things (points)?(4 votes)
- Could you elaborate on your question?(1 vote)
- So all this time, Earth and the moon are rotating around the center of mass of their combination. Earth rotates around itself, around the sun, and around that center of mass between itself and the moon?! So, why in school do they say Earth has only two types of motion(rotation and revolution)? And also, if the Earth and the Moon are rotating around that center of mass because of the reaction and action force, why don't objects do the same thing here with Earth? Why aren't they rotating around Earth?"(3 votes)
- The Moon has high velocity around the Earth while the Earth has low velocity due to inertia. So, if objects do not have velocity, there is no circular motion around the Earth. Hopefully that helps.
Edit: I should also point out that you can imagine a cannon ball shooting out at high speed; that high speed will constantly change direction due to gravity. That's essentially how the moon and artificial satellites orbit Earth.(3 votes)
- So, Newton's 3rd law,
opposite and equal reaction means that something, whatever it may be, is going in the opposite direction than of the force applied but with the same reaction as in equal force/acceleration. Right?
I just want to make sure I've got this straight in my head.(4 votes)- That's correct! According to Newton's third law of motion, when you apply a force on an object, the object exerts an equal and opposite force on you. This means that the object pushes back in the opposite direction with the same magnitude of force or acceleration. So, if you push a box to the right with a certain force, the box will push back on you to the left with an equal force. This law helps explain the interaction between objects and the resulting forces involved.
It does seem confusing at first but you'll get there(1 vote)
- So a plane is heavy and to solve it they made an engine so strong an entire van flies out of the exhaust many meters away?(3 votes)
- Yes, to get a plane off the ground, you need powerful thrusters, apparently ones so powerful that they fly planes at a breathtaking 770 to 930 km/h (480 to 575 mph).(1 vote)
Video transcript
- [Lecturer] You've probably
heard the phrase that for every force, there's an equal and
opposite reaction force. And this is also known as
Newton's third law of motion, but it's also one of the most
misunderstood laws of physics. So that's why we're going
to dig into it a little bit in this video. So I have two examples here
where Newton's third law or this notion of an
action and a reaction force is happening. So over here, you have this plane flying and the plane is able to move forward by pushing air particles
through these jet engines. So these air particles are pushed outward at a very, very high velocity
out the back of the engines. If you are to enlarge one
of those air particles, let's say, this is this
purple dot right over here. There is a force that
is being exerted on it by the jet engine. And that force is going in that direction. So what is the equal and
opposite reaction force? Well, the equal and
opposite reaction force is not also are occurring
on that molecule. It's what the molecule
is doing to the plane. The equal and opposite reaction force is that the molecule
is going to be pushing on the jet engine with an
equal but an opposite force. So it's going to go in
the opposite direction. And that's how the jet is
able to accelerate forward by pushing on these particles
and accelerating them backward by exerting a force on them. The equal and opposite force is the force that the particles, those molecules of air
are exerting on the jet and moving it forward. The same thing here is
going on with this rocket. You have some rocket fuel in there. It gets ignited. It explodes. And as it explodes, there's a force that exerts
on those little molecules and that force is going in this direction. But as it does that, there's an equal and opposite force that the molecules are
exerting on the rocket. The rocket is having a force acted on it, once again, equal and opposite. So it's important to realize
that the reaction force is not on the same object. It's on the other object. If one object is putting
an action force on another, then the second object is
putting a reaction force on the first. The forces do not cancel out. It's also important to realize that both forces are generated in pairs and happen at the exact same time. There's no delay. We can look at other examples of this. This is a scenario that I would
never want to be caught in being drifting through space. Now this astronaut here has
some type of a rocket pack that might help him move around. But let's say that your
rocket pack ran out of fuel and you're just drifting through space. How can you get back to your spaceship? Well, if you have a
wrench or something on you that you can throw, if you can take that wrench, and if you can push that
wrench in that direction, and let's say your spaceship
is over here to the left. Well, the equal and
opposite force is the force that the wrench is going to
exert on you, the astronaut, and then it will push
you in that direction and accelerate you in that direction. So that's a useful thing, if you're ever get caught
drifting through space. But you could do an experiment right now. Press on the table in front of you. When you press on that table, you're clearly putting
a force onto that table. If your table is soft, you
will see it get compressed. But notice your finger itself
is also getting compressed. And the whole reason why
you can even feel that is because your finger
is getting compressed. And that is the equal and opposite force that the table is putting on your finger. And this can happen at very,
very large distances as well. The whole reason why the Moon
is in orbit around the Earth is because there's a gravitational force of Earth's mass acting on the Moon. But there's an equal and
opposite force of the Moon acting on Earth. And it's actually not
that the Moon is rotating around the Earth. It's actually, they're
both rotating around the center of mass of their combination, that just happens to be
so much closer to Earth. It's actually within Earth's volume that it looks like the Moon
is rotating around the Earth. And this isn't just celestial bodies. I weigh 165 pounds. That is the force that Earth
is acting on me due to gravity, but it turns out that there's
an equal and opposite force of 165 pounds that I am
pulling on Earth with. So I will leave you there. Look around the world. This is happening everywhere. For every force, there's an equal and
opposite reaction force. But they don't cancel out because they're acting
on different bodies.