## When you push someone on a swing, why don’t the forces cancel?

### If you push someone on a swing with 50 N of force and they push back with 50 N of force, then why does the person still move? Shouldn’t they stay motionless if all the forces are cancelled out?

You’re struggling with the most common misconception about Newton’s third law of motion — the law stating that for every force object A exerts on object B, there is an equal but oppositely directed force exerted by object B on object A. The pair of forces described in Newton’s third law always act on different objects and therefore never cancel one another. Object A’s force on object B acts only on object B and can cause object B to accelerate. Object B’s force on object A acts only on object A and can cause object A to accelerate.

When you push someone on the swing, your force on the person affects that person and will affect their swinging motion. The person does indeed push back equally hard on you, but that force affects you! If you are wearing roller skates, you will accelerate backward and drift away from the swing.

## Does a rocket push up on itself?

### Does a rocket push up on itself?

No. An object cannot push on itself, meaning that the entire rocket cannot push on the entire rocket.

But part of the rocket can push on another part of the rocket, and that’s exactly what it does. The ship-part of the rocket pushes on the fuel-part of the rocket and the two parts accelerate in opposite directions as a result. The plume of exhaust rushing out of the tail of the rocket is the fuel-part that has accelerated downward to an exceptionally high speed. That fuel-part has been pushed downward hard by the ship-part of the rocket. The ship-part of the rocket has been pushed upward equally hard and it accelerates upward. Gravity introduces a complication, in that it pulls all of the parts downward, but the upward push on the ship-part typically dwarfs gravity and so the ship-part accelerates upward rapidly.