For every action there is an equal and opposite reaction?

Continuing our study guides for lock down we look at different properties of water including one of Newton's Laws


This is the tendency of mass (an object) to resist changes in motion. I.e. a mass at rest tends to remain at rest; a moving mass tends to remain moving at a constant speed unless acted upon by a force. In order to overcome inertia a force must be applied.

The force to overcome inertia may be used to increase intensity of a work out by repeatedly applying force to overcome inertia. I.e. changing direction, moving off the spot, or changing muscle group used.

Changing steps frequently will increase the intensity of a workout.

Changing steps requires more time and more energy to overcome inertia and start a new movement.

  • Stationary inertia = beginning from a static position

  • Moving inertia = already moving but needs to continue to apply force to keep moving.

  • Inertia lag = drag or friction - loss of forward momentum and requires extra force to overcome it and continue to move.


The acceleration of a mass depends on the size of the mass and the force applied. The principle of acceleration applies at the start of the movement. A strong person jumping forward in the water will cover more ground more quickly than a week one and exert more energy. However a smaller person may be able to accelerate more quickly because of less mass but they would use less energy and cover less ground. A large person would have to use more energy to accelerate at the same speed.

A strong person therefore or a person putting in more effort will achieve a higher intensity of workout.


Newton’s third law of motion states: “For every action there is an equal and opposite reaction.

Propulsion harnesses this property and can be used to:

  • Create directional movement

  • Remain stationary

  • Aid balance (sculling)

  • Maintain floatation (treading water)

Muscles may be required to fixate heavily to prevent an “opposite action”.

Intensity can be increased by using resisted arm movement or decrease it by using assisting arm movement. E.g. jogging backwards with arm movements sweeping forward = assisted arm movement.

Consider alignment in resisted arm movement - if the water resist the arms as well as the body, the head, shoulders and feet may move forward faster than the body and impede safety and alignment.

Strong individuals who can maintain alignment may play with this principle and inertia to increase intensity.

28 views0 comments

Recent Posts

See All

Subscribe Form

0203 326 3009

©2019 by Hydro-actif. Proudly created with