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Course: Middle school physics>Unit 1

Lesson 3: Action and reaction forces

Action 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.

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• 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!
• 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!
• 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 ?
• 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.
• If a rock falls to the ground what will the ground do?
• The ground will do nothing.
• btw the airplane is a british airways 787 and the rocket is a falcon heavy :)
• 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?
• Ion even kno😭🙏❌
• 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.
• 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
• How did he know there was a table infront of me?...
• magic sal is magic
• Why do the force act on different things (points) and not on the same things (points)?
• 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?"
• 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.