You are here: start » activities » guides » rflinacc

Navigate back to the activity.

## Linear Acceleration: Instructor's Guide

### Main Ideas

Accelerating reference frames cause simple motion to appear complicated.

### Students' Task

*Estimated Time: 20 minutes, including wrap-up*

Students are asked to draw the trajectory of a ball thrown straight up on the station platform as seen from a train accelerating through the station, for several values of the (constant) acceleration.

### Prerequisite Knowledge

Basic familiarity with Newtonian mechanics for constant acceleration. Know the definition of trajectory (can be defined quickly). Optional but helpful: motion diagrams (object is represented as a dot for equal time intervals)

### Props/Equipment

- Small whiteboards with markers
- Pictures (transparencies or electronic) of several sample trajectories to show as solutions to students

### Student Conversations

- Some students draw motion diagrams, other draw trajectory graphs, but the students who draw motion diagrams seem to have a better understanding that the coordinate system is changing position in a time dependent way.
- Some students don't recognize the parabolic shape is preserved (just rotated) - some students don't want to call this a parabola because it does not pass the vertical line test.

### Activity: Wrap-up

- First discuss the case of zero acceleration, but nonzero velocity, that the trajectory a parabola (the same as if the train is not moving but the ball has a non-zero horizontal component to the velocity).
- Then slowly increase the acceleration (opposite to the velocity) (the parabola rotates so that the symmetry line is no longer vertical).
- Is a boomerang trajectory possible? (Boomerang here means stops at the same location that it started - it actually looks like a line rather than a loop.)
- Show prepared pictures/graphs if appropriate.

See this page for further discussion.

### Extensions

Which way does gravity (appear to) point on the train?