All forces on objects result in net force {resultant, forces}|. Force-vector sum equals resultant-force vector. Vector resultants explain motions in sailing, billiard balls colliding, planes flying, crowbar leveraging, wrenches twisting, gears turning, wrecking balls breaking, roller coaster riding, and car rocking to get out of mud or snow.
Parallel forces {couple, physics}| can act in opposite directions on axis ends, to tend to cause rotation.
All system forces can add to zero {equilibrium, physics}|. All system velocities can add to zero {static equilibrium}. Objects have gravity centers. When objects move, gravity centers can rise {stable equilibrium}, fall {unstable equilibrium}, or stay the same {neutral equilibrium}.
Pressure, temperature, concentration, or force can change over distance {gradient, physics}|. Gradients are like forces and cause flow in resistant medium. Flow increases until reaching terminal velocity. Terminal velocity determines flux.
Force pushes or pulls objects along line between interacting-object centers. When one object exerts force in a direction on another object, second object exerts same force in opposite direction on first object {reaction, forces}. Relation is symmetric.
examples
Rockets burn fuel. Hot gas pushes rocket forward, while rocket pushes gas backward in opposite direction.
For objects on coil springs, coil spring pushes object up, while object pushes coil spring down.
When walking, foot pushes back against ground, and ground pushes back on person, to send body forward.
An hourglass on a balance has dropping sand, but balance stays still, because weight is constant. Downward collisions balance upward force.
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Date Modified: 2022.0225