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## The physics of particle systems

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# Understanding net forces

## Video transcript

(bouncing) - Sir Isaac Newton famously observed that objects at rest stay at rest unless they're acted on by a force. And objects which are in motion
have an unchanging velocity. For example, a particle
floating through empty space would never stop. So whether a particle is motionless or traveling at a constant velocity, if no forces are applied to the particle, then no change in velocity occurs. That's called Newton's First Law. In order to accelerate our particle, that is, to change its velocity, a net force must be applied. As with velocity and acceleration,
forces are also vectors. The force due to gravity for
instance, will point downward, and its length will depend on
how much gravity is present. If this gravitational
force is the only force acting on the particle, then
its velocity will change, meaning it will accelerate
in the direction of the force vector. But what if there are multiple forces at work on the particle? Like if I add a wind force
in addition to gravity. Suppose for instance, that the
wind is blowing to the right half as strongly as the force
of gravity pointing downward. We need to add these forces
together using vector addition to determine the net force. Written as an equation, net force is equal to gravity plus wind. Since the net force is now
pointing down and to the right, that also dictates the direction that that particle will accelerate. If there are more forces at work, we would add those forces all
up to determine the net force. And that would determine the direction the particle would accelerate. For instance, suppose the
particle is sitting on a table. Gravity is pulling the particle downward, and the wind is pushing the particle to the right, as before. But there's another force at
work on the particle, too. The table is pushing up on the
particle to resist gravity. So our force diagram looks like this. The forces of gravity and the
table, cancel each other out, leaving the wind force as a net force. Okay, in the next exercise
you'll get some practice working with net forces. (water splashing)