Science 8 – 2.2

Forces in Fluids


• explore and compare objects that describe movement in terms of balanced and unbalanced forces (210- 13, 210-14, 309-2)

Fluid Dynamics

Hydraulics and Pneumatics

Balanced forces

Balance forces are two forces acting in opposite directions on an object, and equal in size. Anytime there is a balanced force on an abject, the object stays still or continues moving continues to move at the same speed and in the same direction. It is important to note that an object can be in motion even if there are no forces acting on it.

example-of-balanced-forcesHanging objects
Take a look at this hanging glass bulb shade. The weight of the bulb shade pulls down and the tension in the cable pulls up. The forces pulling down and pulling up can be said to be in balance.


balanced-forces-exampleFloating objects Take a look at this log floating on a pool of water. It is floating because the weight of the log is balanced by the upthrust from the water. If more weight is tied to the log, the force pulling it down may be more and will cause it to sink.


reaction-forceStanding/Sitting on a surface
Consider a metal block resting on a surface of a table. Its’ weight is balanced by the reaction force from the surface. The surface pushes up against the metal block, balancing out the weight (force) of the metal block.


Unbalanced Forces

Unlike balanced forces, we say unbalanced forces when two forces acting on an object are not equal in size.
Unbalanced forces causes can cause:
pressure bulleta still object to move
pressure bulleta moving object to speed up or slow down
pressure bulleta moving object to stop
pressure bulleta moving object to change direction
Unbalanced forces make the wagon in the diagram speed up.