## 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.

## Does space dust settle on orbiting space shuttles?

#### Does space dust settle on orbiting space shuttles? — A, Troy, MT

What a great question! I love it. The answer is no, but there’s much more to the story.

I’ll begin to looking at how dust settles in calm air near the ground. That dust experiences its weight due to gravity, so it tends to descend. Each particle would fall like a rock except that it’s so tiny that it experiences overwhelming air resistance. Instead of falling, it descends at an incredibly slow terminal velocity, typically only millimeters per second. It eventually lands on whatever is beneath it, so a room’s floor gradually accumulates dust. But dust also accumulates on vertical walls and even on ceilings. That dust is held in place not by its weight but by electrostatic or chemical forces. When you go into an abandoned attic, most of the dust is on the floor, but there’s a little on the walls and on the ceiling.

OK, now to the space shuttle. The shuttle is orbiting the earth, which means that although it has weight and is falling freely, it never actually reaches the earth because it’s heading sideways so fast. Without gravity, its inertia would carry it horizontally out into space along a straight line path. Gravity, however, bends that straight line path into an elliptical arc that loops around the earth as an orbit.

So far no real surprises: dust near ground level settles in calm air and the shuttle orbits the earth. The surprise is that particles of space dust particles also orbit the earth! The shuttle orbits above the atmosphere, where there is virtual no air. Without air to produce air resistance, the dust particles also fall freely. Those with little horizontal speed simply drop into the atmosphere and are lost. But many dust particles have tremendous horizontal speeds and orbit the earth like tiny space shuttles or satellites.

Whether they are dropping toward atmosphere or orbiting the earth, these space dust particles are typically traveling at velocities that are quite different in speed or direction from the velocity of the space shuttle. The relative speed between a dust particle and the shuttle can easily exceed 10,000 mph. When such a fast-moving dust particle hits the space shuttle, it doesn’t “settle.” Rather, it collides violently with the shuttle’s surface. These dust-shuttle collisions erode the surfaces of the shuttle and necessitate occasional repairs or replacements of damaged windows and sensors. Astronauts on spacewalks also experience these fast collisions with space dust and rely on their suits to handle all the impacts.

Without any air to slow the relative speeds and cushion the impacts, its rare that a particle of space dust lands gracefully on the shuttle’s surface. In any case, gravity won’t hold a dust particle in place on the shuttle because both the shuttle and dust are falling freely and gravity doesn’t press one against the other. But electrostatic and chemical attractions can hold some dust particles in place once they do land. So the shuttle probably does accumulate a very small amount of accumulated space dust during its travels.

## If you were out in space and could see every individual person clearly, would it…

#### If you were out in space and could see every individual person clearly, would it look like they were walking at a slant? — KD, McMinnville, OR

To the astronauts orbiting the earth, up and down have very little meaning. Because they are falling all the time, these astronauts have no feeling of weight and can’t tell up from down without looking. If an astronaut were to look at a person walking on the ground below, that person might easily appear at a strange angle, depending on the astronaut’s orientation and point of view.

## How might an ion engine work?

#### How might an ion engine work? — DAA, San Diego, CA

One possible ion engine uses mercury as a propellant. The mercury starts as a liquid in a small tank, but its atoms slowly evaporate to form a low-density gas. An electric discharge through this gas, such as occurs inside a fluorescent lamp, knocks electrons off some of the mercury atoms. When a mercury atom loses an electron, it becomes a positively charged mercury ion and can be accelerated from the discharge by electric fields. In the ion propulsion engine, an electric field extracts and accelerates the mercury ions toward a hole in the side of a spaceship. The mercury ions are ejected into space at enormous speeds. As they accelerate, the mercury ions exert reaction forces on the engine and these forces are what propel the spaceship forward. Overall, the mercury ions accelerate in one direction while the spaceship accelerates in the other direction. To keep the spaceship electrically neutral, the engine also ejects electrons into space. However, mercury ions provide most of the engine’s thrust.

## If you have four carts of equal weights, one with small wheels, one with large w…

#### If you have four carts of equal weights, one with small wheels, one with large wheels, one with small wheels in front and large wheels in back, and one with large wheels in front and small wheels in back, which cart will be easiest to move? — PK

The cart with the small wheels will be easiest to move. That’s because, as the cart starts moving, each kilogram of mass in the wheels acquires twice as much energy as each kilogram of mass in the cart itself. Keeping the mass of the wheels low by making the wheels small reduces the energy in the overall cart and makes it easier to start or stop.

## Can a rocket, starting back toward the earth from 30,000 feet, reach the speed o…

#### Can a rocket, starting back toward the earth from 30,000 feet, reach the speed of sound before reaching the earth? — WJT, Crystal, MN

Some rockets probably reach the speed of sound in a few hundred feet heading upward, so that reaching the speed of sound in 30,000 feet heading downward would be a simple task. In fact, if you dropped a highly aerodynamic object such as a rocket from 30,000 feet, it could reach the speed of sound even without any propulsion! Gravity alone will accelerate it to about 130% of the speed of sound.

## Why does the tower of Pisa lean?

#### Why does the tower of Pisa lean? — CM, Edison, NJ

The tower was built long ago on unstable ground that was unsuitable for supporting such a tall and heavy masonry structure. For an object to remain upright indefinitely, its center of gravity must lie above its base of support and that base of support must be firm at all its edges. The tower’s base of support had at least one edge that wasn’t firm and that began to sink downward under the weight of the tower. Once this edge sunk a small distance, the tower’s center of gravity shifted sideways so that it was above that weak portion of the base of support. This shift in the tower’s center of gravity put even more stress on the weak part of the ground and caused additional sinking, additional tipping, and even more shifting of the tower’s center of gravity. This process might have toppled the tower over by now were it not for recent efforts to stop the tipping. The base of the tower has been reinforced to prevent further tipping.

## Is there any gravitational force between two atoms?

#### Is there any gravitational force between two atoms? — AW, Karachi, Pakistan

Yes, everything in the universe exerts gravitational forces on everything else in the universe. However, those forces are usually so small that they are undetectable. The gravitational forces between two bowling balls are only barely measurable in a laboratory. The gravitational forces between two atoms are so small as to be hopelessly undetectable.

## How do rockets work?

#### How do rockets work?

Rockets push stored materials in one direction and experience a thrust force in the opposite direction. They make use of the observation that whenever one object pushes on a second object, the second object exerts an equal but oppositely directed force back on the first object. This statement is the famous “action-reaction” concept that is generally known as Newton’s third law. While it seems sensible that when you push on a wall it pushes back on you, this situation is extraordinarily general. For example, if you push a passing car forward, that car will still push backward on you with an equal but oppositely directed force. If you push on your neighbor, your neighbor will push back on you with an equal but oppositely directed force even if your neighbor is asleep! In the case of a rocket, the rocket pushes burning fuel downward and the burning fuel pushes upward on the rocket with an equal but oppositely direct force. If the rocket pushes its fuel downward hard enough, the fuel will push up on the rocket hard enough to overcome the rocket’s weight and accelerate it upward into the sky and beyond.