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Forces and Motion

What is the Motion of a Flying Rocket?
Continued from the previous page.

When the rocket takes off, there is a large thrust
from the engine but very little drag because the
rocket is moving slowly and air resistance is small.
The forces are unbalanced because there is a
large upward force but only a small downward force
and so the rocket has a large acceleration
in the direction of the larger force (upwards).

As the rocket gets faster,
the drag increases and acceleration decreases
until the thrust and drag are equal in size.
Now the forces are balanced
and the rocket will continue with a constant velocity.

The motion of the rocket
is shown on the velocity - time graph below.

The red curve shows acceleration changing from large to small.
The blue line shows constant velocity.

This is the same shape of velocity - time graph
as a falling object reaching its terminal velocity.

The next page shows how to calculate forces and acceleration.

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