Forces and Motion

Free Body Diagrams - object is always a closed dot, the tail of the force if touching the dot, and the forces need a label and direction, don’t add extra forces

Don’t include drag (air resistance) or friction unless it says “not negligible” or an object is coming to a stop

Causing acceleration will be the positive direction (even if its down or left), retarding or resistive force is negative direction

Net Force - all the force added together

Weight - always happens, always down, always draw first, symbols - fg, w, mg

Normal Force - only happens on a surface, always perpendicular to the surface, symbol N

When in freefall apparent weight (Normal Force) is 0

Scale on a elevator tells us the normal force

Tension - happens on string or rope, always away from object, symbol - T

Tension is always equal on the same rope

Tension will cancel out in every pulley question (Will not be used in Net Force Equations)

Pulley rope ?s - Acceleration = Difference in Weights / Total Mass

Thrust - force that puts you in the direction of motion

N (newtons) - Kg * m/s2

Newton’s third law - every force is a pair, the pair has equal and opposite magnitudes (action / reaction)

Tip for answering 3rd Law ?

Identify the 2 interacting objects and make sure they are in your answer

Normal Force and Weight will never be an action reaction pair

Weight force is caused by the earth

Static equilibrium- no net force, motionless or at rest

Dynamic equilibrium - no net force, no acceleration

FRQ rules - no components in a free body diagrams, need equations (first with variables then plug in numbers) and labels

Kinetic Friction = fk = ukn, depends on Normal Force

Static Friction - fs <= usN

Coefficient of static friction is always greater then coefficient of Kinetic Friction

Coefficient of static friction has a max value of 1 since it is a ratio (fs - k) / N

Drag - proportional to velocity (when velocity goes down drag goes down vise versa)

Terminal Velocity - Weight = Drag (which means 0 net force and 0 acceleration) still has velocity

For pushing box questions - to quickly solve find the percentage of weight on the boxes and use 2nd equation to solve (for both x and y direction)