<aside> 1️⃣ If the external net force on an object is zero, the object will remain at rest or continue to move at a constant velocity

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<aside> 2️⃣ If the net external force on an object is not zero, the object will accelerate in the direction of the net force. The magnitude of the acceleration is directly proportional to the magnitude of the net force and inversely proportional to the object's mass $\Sigma\overrightarrow{F} = m\overrightarrow{a}$, or $\overrightarrow{F_g} = m\overrightarrow{g}$

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• note that normal force is also equal to gravity, so $F_N =mg$

<aside> 3️⃣ For every action force, there exists a simultaneous reaction force that is equal in magnitude but opposite in direction

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• an object is in equilibrium when the net force on it is sero
• for objects experiencing forces in two dimensions, break the motion into perpendicular components, which can be analyzed independently
• once you have determined the net force using components, use NEwton's second law to determine the acceleration

Solving pulley-mass problems

Two masses hanging from a pulley (video) | Khan Academy

$|\overrightarrow{a}{sys} = \frac{\Sigma{F}{net}}{M_{total}}$

the positive direction of force is the direction in which motion is directed, in this case, down

• where $\overrightarrow{a}$ is the acceleration of the entire pulley system
• $m$ is the total mass of both objects
• $\Sigma{F}$ is the total net force
  • pay attention to direction and sign when solving
  • "add up" both objects for the net force

For tension, focus on one object, and use $a=\frac{\Sigma{F}}{m}$

Solving three box systems

Three box system problem (video) | Khan Academy