Why Buzz Lightyear’s Rocket Launch Looks Better Than Reality

This graph says that the vertical position of the rocket increases by a (nearly) constant amount from one frame to the next. In physics, we call that “constant velocity.” Since this is a plot of position vs. time, the slope of the line will be equal to this constant vertical velocity. From the graph above, you can see this puts the launch speed of the rocket at 192 meters per second (m/s). That’s pretty darn fast—but is that fast enough to actually reach space? The answer is both yes and no. Here’s why.

Let me give a brief overview of escape velocity. Suppose you take an apple and toss it up in the air with a velocity of 10 meters per second. (That’s fairly fast for an apple.) As that apple moves upward, it’s going to slow down. Eventually, thanks to the pull of gravity, it will stop and then start falling back toward Earth.

But let’s say the apple is moving super fast, at 11.186 kilometers per second. Then it will get high enough such that the gravitational force won’t be strong enough to stop it. That apple will escape.

Buzz Lightyear’s rocket is fast—but not that fast. Remember, we calculated that it’s moving at 192 meters per second. But that’s not a problem, because you don’t need to worry about escape velocity if you have a rocket. The engine will keep pushing the spaceship to overcome that pull and keep it moving at a constant speed, so it won’t fall back to Earth.

In the case of Buzz’s rocket, there are essentially three force interactions during this part of the motion. First, there’s the thrust from the engines. A conventional chemical engine combusts propellants to create exhaust gasses. All forces come in pairs, so when the exhaust is ejected from the engine, it pushes the rocket in the opposite direction. (The nice thing about rocket engines is that they work both in Earth’s atmosphere and in space, where there is no air.)

The other two forces on the spacecraft are the downward-pulling gravitational force due to its interaction with the Earth, and an air resistance force pushing in the opposite direction as the ship. Air resistance is caused by the collisions between the rocket and the air.

As the spacecraft leaves the ground, both of these forces will eventually become insignificantly small. That’s because moving farther from the center of the Earth means that the strength of the gravitational force pulling on the ship decreases. And once the rocket gets beyond the atmosphere, there will no longer be air resistance, because there won’t be any air. The only force remaining will be the thrust from the engines, so the speed of the spaceship should increase.

But … this isn’t how real rockets work. Normally, a rocket engine produces a thrust force that is greater than the gravitational force. This means that a rocket traveling upward would accelerate and not just travel at a constant velocity.