Teaching Physics with the Physics Suite

Edward F. Redish

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A two-stage rocket

Students in a school rocketry club have prepared a two stage rocket. The rocket has two small engines. The first will fire for a time, getting the rocket up part ways. Then the first stage engine drops off revealing a second engine. After a little time, that engine will fire and take the rocket up even higher.

The rocket starts firing its engines at a time t = 0. From that instant, it begins to move upwards with a constant acceleration. This continues until a time t1. The rocket drops the first stage and continues upward briefly until a time t2 at which point the second stage begins to fire and the rocket again accelerates upward, this time with a larger (but again constant) acceleration. Sometime during this second period of acceleration, our recording apparatus stops.

On the graphs below, sketch qualitatively accurate (i.e., we don't care about the values but we do about the shape) graphs of each of the height of the rocket, y, its velocity, v, its acceleration, a, the force on the rocket that results from the firing of the engine, F, and the net force on the rocket, Fnet. Take the positive direction as upward.

Note to the instructor: The students must infer gravity in the middle segment of the rocket's path -- a way to check whether they are mapping from a physical picture of what's happening to the graphs or not. This is also a terrific opportunity to work with them on consistency -- between position and velocity, between velocity and acceleration, between net force and acceleration, and between component forces and net force.

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Page last modified November 3, 2002: D20