First Law: Inertia

An object in motion tends to stay in motion unless acted on by an outside force.

An object at rest tends to stay in rest unless acted on by an outside force.

Objects with more mass also have more inertia.

Second Law:

F_net = ma

Acceleration of an object is proportional to the net force acting on it. An object’s acceleration is inversely related to the mass of the object.

The net force of an object is the sum of all forces acting on it. If the forces are in the same direction, they are simply added together to calculate the net force.

Example: 10 N (to the left) + 10 N (to the left) = 20 N (to the left)

If they are in opposite directions, the forces would have different signs, so positive and negative numbers add together, or they can be subtracted to get the same result:

Example: 15 N (to the right) + 8 N (to the left) = 7 N (to the right)

Similarly: (+15 N) + (-8 N) = (+7 N)

Third Law

Every force has an equal and opposite force.

For every action, there is an equal and opposite reaction.

Forces always occur in equal, opposite pairs.

Examples:

• If a cyclist’s foot pushes on a bike pedal, the pedal pushes back against the cyclist’s foot.
• A person’s weight pushes against the floor through their foot; the floor pushes back against the foot.
• The sun’s gravity pulls on the earth, and the earth’s gravity pulls on the sun.
• A proton’s positive electrical charge pulls on an electron’s negative charge. The electron pulls back on the proton.
• A kid jumps up, pushing against the earth. The earth pushes the kid up into the air. The kid accelerates upward a noticeable amount. The earth accelerates in the opposite direction… but it  hardly accelerates at all since it has so much mass (inertia).