Newton's third law of motion

Philip is working outside, but now he's out of fuel in his jet pack. How can he get back to the spaceship, before he runs out of air? To find the solution we go back to earth. Here are Kim and Jenny! Kim pushes Jenny, but what happens?

They both move. They both accelerate! This is how it works: Kim is acting on Jenny with a force, in this direction. But, since Kim also moves, a force must be acting on Kim as well. No change in velocity without the action of a force.

Newton's First Law! So, both Kim and Jenny are affected by one force each. Two forces, equal in magnitude, but opposite in direction. This is an action-reaction force pair. You'll have seen, or felt, this, many times: This is what makes ...

Your hand hurt, if you punch it into the wall; ... a helicopter fly; ... a swimmer move forward, by moving the arms; ... and a gun recoil, move backwards, when it is fired. All of these are caused by action reaction force pairs.

And that is what Newton's Third Law is all about. It tells us, that: When an body exerts a force onto another body, it is itself affected, by another force, equal in magnitude and opposite in direction. The hand pushes the wall, and the wall... ... pushes the hand. The helicopter pushes the air downwards, The air pushes... ...

the helicopter upwards. ... the swimmer pushes the water backwards, The water pushes... ... the swimmer forwards. The gun pushes the bullet forwards, the bullet... ... pushes the gun backwards.

Same thing here. When Kim is pushing Jenny, Jenny pushes back, in the opposite direction. The force affecting Kim is exactly as strong as the one affecting Jenny. Since they both weigh the same, they move the same distance. But what if we replace Jenny with something heavier?

The reaction force is of same magnitude as Kim's push... ... however the elephant only moves a little bit. This has to do with Newton's Second Law of motion. It takes more force to accelerate a large mass, than a small one. Pause the film, and try working it out for yourself!

Okay, what about this one? Kim is pushing a wall! And the wall isn't moving at all? Same thing here. The forces are of the same size, but the wall is fixed to the ground.

And the ground is fixed to the Earth! The acceleration is equal to the force divided by the mass. And the earth has a lot more mass, so we don't even notice that it's accelerating. Now Kim has an idea, that can help Philip, using Newton's three laws of motion! Kim exerts a force on the ball, and it changes velocity, and flies away.

Newton's First Law. By throwing it hard, the force is strong, and the ball accelerates a lot. Newton's Second Law. An equally strong reaction force affects Kim, in the opposite direction. Newton's Third Law. ...

and then there's friction, slowing down the movement. Now, how does this help Philip? He can do the same thing! Throw something! And use the reaction force, to get back to the space ship!

Something small will do. Yeah, the spanner! In what direction should you throw it? Wooow! Exactly!

For you to move towards the ship, you throw the spanner in the other direction! When Philip throws the spanner, the reaction force pushes him in the opposite direction. Philip has more mass than the spanner, so he only accelerates a little bit. But it doesn't matter, because there's no force of friction here to slow him down. That's that.

Newton's three laws of motion together helped Philip. Now let's hope the spanner, which is moving at constant speed through space, doesn't crash into something important.