mod 3 dynamics

if net force = 0

velocity remains constant

contact force

when physical contact between objects

field forces

when forces act across empty space

measure strength of force through

deformation of spring

newton's first law

in absence of external force when viewed from inertial frame, an object at rest remains at rest and an object in motion continues w/ constant v

inertial frame

could be relative to Earth, relative to distant stars, or a reference frame moving with constant velocity relative to another inertial frame

no force means

no acceleration (and other way)

newton’s second law

∑F→ = ma→

part of 2nd law: when viewed from inertial frame, accel is directly proportional to

net force acting on an object

part of 2nd law: when viewed from inertial frame, accel is inversely proportional to

an object’s mass

mass

specifies object’s resistance to changes in its velocity, scalar property, DIFFERENT THAN WEIGHT

derivation of gravitational force

from g and 2nd law; F_g→ = mg→

gravitational mass =

inertial mass

newton’s third law

if two DIFFERENT objects interact, there is an action reaction pair of forces that are equal in magnitude and opposite in direction

net force

vector sum of all acting forceson an object

what is not an action reaction pair

normal force and gravitational force

rules of ropes

can only be pulled, massless, pulls with the same tension force on both ends

Fn isn’t always the same as Fg because

there may be an acceleration somewhere

acceleration on an inclined plane

a_x = gsinθ

force of friction 

resistance to motion on a surface or in a viscous medium such as air or water

force of friction depends on

details of surface (composition, material)

|F_f→| =

µ|n→|

friction forces act _ to the frictional interface

parallel

the direction a frictional force acts is the direction

opposite the direction that the system would move without friction

µ_s =

tanθ, where θ is angle where motion just starts (F_g,x = F_f,s)

µ_k =

tanθ', where θ’ is angle where velocity is constant (a_x = 0