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## Pixar in a Box

### Unit 7: Lesson 2

The physics of particle systems- Start here!
- Graphing motion over time
- Position, velocity and acceleration
- Vector addition
- Velocity and acceleration vectors
- Understanding net forces
- Net forces
- Force and acceleration
- Applying gravity to a particle
- Particle collisions
- Particle collisions
- Animating particles
- Particle calculations

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# Understanding net forces

When multiple forces are acting on a particle we'll need to express them as a single force known as a net force.

## Want to join the conversation?

- Can you use net forces in JavaScript?(5 votes)
- You can certainly represent forces in Javascript and calculate what the net force would be when you combine them.(7 votes)

- Would it matter what order you put the forces on when using the head to tail method?(4 votes)
- No. Adding forces is like adding numbers - it doesn't matter what order you do it in. The mathematical term for this is commutative.(6 votes)

- what even is a net force? well I didn't fully pay attention and I'm in 4th grade.(0 votes)
- A net force gives the direction a particle is moving, when more than one force is acting upon it. To find a net force, you must add all the forces together, as it states at1:04in the video.(10 votes)

- What Is The Direction Of The Net Force When One Force Is Pulling Down And One Force Is Pushing To The Right?(2 votes)
- which
is used there**software**(2 votes) - Say the force of wind is 5 units and the force of gravity is 7 units, would you add the two unit measurements to get 12? And would the length of the net force be 12 units? I'm just confused on how you would find the length of the net force.(2 votes)
- Can you make a video on the equations you have to do to get the net force because i just started this and I don’t know how to do it(1 vote)
- how do I find net force?(1 vote)
- how large is the earth(1 vote)
- How do you determine which force is the net force in an unbalanced force. The net force is all the forces added together so how can it be a singular force?(1 vote)

## 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)