AP Physics: Dynamics

Force

A push or a pull on an object that can cause acceleration

What are the forces

Tension (F_t), Friction (F_f), Applied (F_app), Normal (F_n), Spring (F_s)

Free Body Diagram

Shows magnitude and direction of all forces acting on an object/system

C.O.M.

A point on an object where all forces can be applied without causing rotation (dot represents C.O.M.)

How to draw forces on a Free Body Diagram?

Draw forces as arrows that start on dot and point away; if two forces point in same direction, make dot bigger and draw arrows side by side

Unit for all forces

Newton (N) (kg x m/s²)

IMPORTANT Equation for weight:

F_g= mg

1st Law: “Law of Inertia”

An object will move at a constant velocity unless acted upon by a net external force (Inertia = resistance to acceleration = mass; net force = vector sum of all forces)

External Force

A force from an object not in the system

2nd Law:

ΣF_system= m_systema_{system (apply equation to x & y directions separately)}

3rd Law

F_{object 1 on object 2} = -F_{object 1 on object 2} (size of objects doesn’t matter, action-reaction forces only apply to 2 objects)

C.O.M. Formula

X_com=Σmixi/Σmi = m₁x_{1 +}m₂x_{2 +…./} m₁+m₂+_… (Make 1 mass at x=0m)

What does a scale measure?

Normal force

On an elevator, scale will be greatest when…

acceleration points up

On an elevator, scale will be least when…

acceleration points downward