A rocket becomes progressively easier to accelerate as it travels upward from the ground mainly because?

Answer 1

The rocket's acceleration is created by the net force acting on it. There are three forces acting on the rocket: the thrust provided by the engines, gravity or weight, and air resistance. The acceleration is inversely proportional to the rocket's mass. This is Newton's Second Law:

(acceleration) = (net force) / (mass)

We need to think about the direction of the forces. The thrust acts upward (call this positive), and both gravity and air resistance acts downward (call these negative). So we get

(acceleration) = (thrust - weight - air resistance) / mass

A typical rocket engine will provide constant thrust as long as the fuel lasts. But as the engine consumes fuel, expelling the exhaust products out the back of the rocket, the rocket's mass decreases. This tends to increase the rocket's acceleration since acceleration is inversely proportional to the mass.

In addition to the decreasing mass, the rocket's weight decreases as it moves farther from the center of the Earth--- this effect is described by Newton's Law of Gravity. The rocket's decreasing weight tends to increase its upward acceleration.

The action of air resistance is more complicated, and ordinarily we ignore air resistance in simple models just to avoid the complication air resistance gives to the problem. In the standard air resistance model, air resistance scales with the square of the rocket's speed and the air density. The rocket is moving faster and faster, but the air density is also decreasing as it rises through the atmosphere. I think we can safely say the air resistance force decreases as the rocket gains altitude, but a detailed answer illustrating precisely how this force changes would require a numerical simulation.

Hope this helps!