AP Physics 1: Unit 2: Dynamics/Forces

Newton’s 1st law

The law of Inertia: An object at rest will stay at rest and an object in motion will stay in motion unless acted upon by an unbalanced force

Newton’s 2nd law

The force applied to a mass is proportional to the acceleration the mass will experience

F=ma

Newton’s 3rd law

For every action there is an equal and opposite reaction

Action Reaction forces

Each force on a force diagram will have an action reaction pair

ex: Action: tire pushes on road Reaction: road pushes the tire

Fg

mass (always down)

Fn

Surface (perpendicular)

Ft

Rope, string (along the rope)

Ff

Friction (brakes on a car, not the engine) (opposite of motion)

Fa

In contact that makes object move (direction of push/pull)

Fg=

mg

Fnet=

ma

Fair

Only if problem mentions it (Opposite direction of motion)

Balanced forces

No acceleration (constant or 0 v), net force is 0

Inertia

Tendency of an object to resist changes in its motion.

Frictional force

Amount of force opposing motion

Unit: N

Kinetic friction

the object is in motion

Static friction

the object is motionless

Coefficient of friction

Ratio of force of friction to force normal

Unit: N/N or nothing

Variable: μ

Fs

Range of values dependent on the magnitude of the horizontal force being applied

Fk=

μFn

Fs < or equal to

μFn

Coefficient of friction (μ) is the slope

Can’t be greater than 1

Whenever two objects interact

each exerts a force on the other as a result of the interaction

Pairs of forces in interaction are:

1. Equal in magnitude

2. Opposite in direction

3. Same in kind

An object in equilibrium (balanced forces)

1. remains at rest

2. moves in constant speed/straight line

Newton’s 2nd law of motion a=

fnet/m (fnet is also =mgsin)

According to Newton’s 2nd law of motion, (when the angle changes)

a increases when the angle increases, because mgsin also increases

net force and a have a direct relationship

According to Newton’s 2nd law of motion, (when the mass changes)

a decreases, when mass increases

increasing mass also increases the net force

inversely related

Elevator problem: a=

(fn-fg)/m

Elevator problem: a is upward

Fn>Fg

Elevator problem: a is downward

Fn<Fg

If either (multiple) mass increases

then the gravitational force between the two objects increases by the same proportion (direct relationship)

If the distance between the centers of mass increases,

then the gravitational force between the two objects decreases by the square of the proportion

fgy=mgsin

points down the ramp (causes sliding)

fgx=mgcos

pushes into the ramp (balanced by the normal force)

“Constant” force means

acceleration