# Calculation of minimum speed of projectiles under linear resistance using the geometry of the velocity space

### Abstract

We look at the problem of the minimum speed of projectiles in a constant gravitational field. In the absence of resistance, the problem may be studied in the frame of a high school curriculum. One needs only Newton’s laws and a minimum amount of analytic geometry to compute the orbit, which turns out to be parabolic. Furthermore, in case the projectile is launched upwards, employing the theorem of conservation of mechanical energy we conclude that the minimum speed occurs at the highest point.In the presence of resistance, the system is dissipative and hence, the previous tools are not available. We focus at the case where the resisting force is linear in speed and opposing the velocity vector. It has been observed in numerical experiments that the minimum speed, if any, occurs when the projectile is on the way down, after having achieved the maximum height. We propose a presentation of the solution to this problem using the geometry of the velocity space. As a basic tool, we apply an old idea introduced by Hamilton and Maxwell, the hodograph of motion.

It turns out that the hodograph of motion in this case is a straight line. This fact allows us to describe the values of initial speed and launching angles that will result in an orbit with or without minimum speed. In the former case, the calculation of the value of minimum speed represents the distance of the origin to the hodograph line and is given by elementary manipulations. This approach in the study of physical problems, besides being elegant on its own right, helps college students feel the deep relation between physics and geometry.

### References

M. Papadimitrakis, Professor

Department of Mathematics and Applied Mathematics

University of Crete

Voutes Campus

70013 Heraklion, Greece

Tel: +0302810393840

e-mail: mpapadim@uoc.gr

Dr. John Antoniadis

Max Planck Institute for Radio Astronomy

Auf dem Huegel 69, 53121, Bonn, DE

jmantoniadis@gmail.com

Dr. GEORGIOS PAVLIKAKIS

Teachers’ trainer on didactical methods, innovative methods in education, environmental management, environmental and consumer education

Address: Dardanellion 99, 171 24 Nea Smirni, Greece

Telephone /fax : 0030 210 9353941, 00306932528943 E-mail: pavlikak@otenet.gr

Department of Mathematics and Applied Mathematics

University of Crete

Voutes Campus

70013 Heraklion, Greece

Tel: +0302810393840

e-mail: mpapadim@uoc.gr

Dr. John Antoniadis

Max Planck Institute for Radio Astronomy

Auf dem Huegel 69, 53121, Bonn, DE

jmantoniadis@gmail.com

Dr. GEORGIOS PAVLIKAKIS

Teachers’ trainer on didactical methods, innovative methods in education, environmental management, environmental and consumer education

Address: Dardanellion 99, 171 24 Nea Smirni, Greece

Telephone /fax : 0030 210 9353941, 00306932528943 E-mail: pavlikak@otenet.gr

Published

2019-07-09

How to Cite

PISPINIS, DIMITRIOS.
Calculation of minimum speed of projectiles under linear resistance using the geometry of the velocity space.

**European Journal of Physics Education**, [S.l.], v. 10, n. 3, p. 1-9, july 2019. ISSN 1309-7202. Available at: <http://eu-journal.org/index.php/EJPE/article/view/234>. Date accessed: 17 july 2019.
Issue

Section

Articles

This work is licensed under a Creative Commons Attribution 4.0 International License.

The copyright for all articles belongs to the authors. All other copyright is held by the journal.