Static Friction (College Board AP® Physics 1: Algebra-Based)

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Static friction may occur between the contact surfaces of two objects that are not moving relative to each other
- If an object is static, then it is not moving; it is stationary
Static friction is present up until the point at which the object begins to move
For example, when applying a forward force to try to move a heavy object, static friction:
- acts in the opposite direction to the applied force
- increases to match the magnitude of the applied force
- prevents the object from moving
- continues to increase until it is finally overcome by the magnitude of the applied force, which results in the object beginning to move
Once the object is moving, then kinetic friction is present

When an object moves initially, it seems to slip suddenly
- This sudden slip indicates the point at which kinetic friction takes over from static friction
Less force is required to keep an object moving than to initially make it move
- A smaller frictional force is required to keep an object in motion than to keep it stationary
- The force of kinetic friction is smaller than the force of static friction

$|F_{f, k}| ⩽ |F_{f, s}|$

Where:
- $|F_{f, k}| =$ magnitude of kinetic friction, measured in $N$
- $|F_{f, s}| =$ magnitude of static friction, measured in $N$

Making an object heavier will increase the magnitude of the static friction and increase the magnitude of the forward force required to get it moving

the (exam) results speak for themselves:

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