Dynamics- AP Physics l

Equilibrium

An object is in equilibrium if it is moving in a straight line at constant speed or is at rest

Net force in equilibrium

The net force on an object in equilibrium is zero

Force

A push or a pull acting on an object

Net force (same direction)

Forces in the same direction add together

Net force (opposite directions)

Forces in opposite directions subtract

Direction of net force

The net force is always in the direction of acceleration

Mass

Tells how much material is contained in an object

Weight

The force of gravity acting on an object

Gravitational field on Earth

10 N/kg

Weight of 1 kg on Earth

10 N

Normal force

Force of a surface on an object in contact with it

Direction of normal force

Acts perpendicular to the surface

Platform scale reading

The normal force

x- and y-components

Any diagonal force can be broken into perpendicular x- and y-components

Component method

Draw force to scale, drop dotted lines to axes, and measure components

Horizontal component of force

Fcosθ

Vertical component of force

Fsinθ

Friction force

Force of a surface acting along the surface

Direction of friction force

Opposite the direction of motion

Coefficient of friction (μ)

Not a force; a number describing surface roughness

Friction equation

Ff = μFn

Kinetic friction

Used when an object is moving

Static friction

Used when an object is not moving

Static friction values

Can vary up to a maximum value

Static vs kinetic friction

Maximum static friction is greater than kinetic friction

Walking and friction

Friction pushes the person forward (Newton’s 3rd Law)

Gravitational mass

Measured using weight = mg

Inertial mass

Measured using Fnet = ma

Gravitational vs inertial mass

They are equal in all experiments

Inclined plane force breakdown

Break weight into components parallel and perpendicular to incline

Weight component parallel to incline

mg sinθ

Weight component perpendicular to incline

mg cosθ

Newton’s Third Law

Force of A on B equals force of B on A

Third law force pair

Two equal and opposite forces acting on different objects

Third law rule

Third law force pairs never act on the same object

Reaction force

Often the force that accelerates an object

Spring force

A spring pulls harder the more it is stretched or compressed

Spring force equation

F = kx

Spring constant (k)

A property of the spring and always the same for that spring

Units of spring constant

N/m

Circular motion acceleration

v²/r toward the center of the circle

Direction of centripetal force

Always toward the center

Sign convention in circular motion

Forces toward center are positive

Gravitation

All massive objects attract each other

Gravitational force equation

FG = G(m1m2)/d²

Universal gravitation constant

G = 6.7 × 10⁻¹¹ N·m²/kg²

Gravitational field equation

g = GM/d²

Weight near a planet

Weight = mg

Gravitational field vs free fall

g equals free-fall acceleration

Equilibrium problem step 1

Draw a free-body diagram

Equilibrium problem step 2

Break angled forces into components

Equilibrium equations

Up − Down = 0 and Left − Right = 0

Net force problem step 1

Draw a free-body diagram

Net force problem step 2

Determine net force in each direction

Net force equations

Fnet = ma (vertical) and Fnet = ma (horizontal)

Free-body diagram arrows

Arrows start on the object and point in force direction

Free-body diagram labels

List the force, the object applying it, and the object experiencing it