Newton’s Laws of Motion:
1. A mass continues its uniform motion unless acted upon by y Uniform: constant‐velocity motion or at rest
2. Acceleration = net force / mass
y
Get net force first!
3. For every action, there is an equal and opposite reaction.
y
y
Momentum
= inertia of a moving object
PH 104 w/ dr. g
Lec 7
Cannot exert more force than force exerted back.
Accelerations not necessarily equal/opposite: 2nd law
Momentum = inertia of a moving
object.
y Momentum = mass x velocity.
y Impulse = Force x time interval.
y Impulse = change in momentum.
y Collisions: momentum is conserved.
Momentum = inertia of a moving object.
Momentum = mass x velocity.
y INERTIA of an object at rest =
y Larger mass : harder to get moving
y INERTIA of an object in uniform motion =
Momentum = inertia of a moving object.
Impulse = Force x time interval.
y Newton’s 2nd Law: velocity is changed using FORCE.
y Here: momentum is changed using IMPULSE:
y Momentum = mass * velocity
y 1‐ton car moving at 100 km/h :
y
Momentum = ton‐km/h
y 2‐ton truck moving at 50 km/h?
y
Momentum = ton‐km/h
y Moving object contains momentum (not force)
y Moving object can exert a force… (collision…)
y Momentum gained ~ F = force applied
y Momentum gained ~ t = how long F is applied
Momentum = inertia of a moving object.
Momentum = inertia of a moving object.
Impulse = change in momentum.
Impulse = change in momentum.
F * t = Δ(m * v) = m * (Δv)
y Scenario 1: Increasing object’s mv
y Applying big F
y Applying F over longer t :
y follow-thru: batters, tennis players, golf
F * t = Δ(m * v) = m * (Δv)
yScenario 2: Decreasing object’s mv
y Make t long:
boxer’s “roll”, parachutist’s landing
y Make t short: higher F: karate chop
Decreasing object’s mv quickly
Momentum = inertia of a moving object.
Momentum = inertia of a moving object.
Collisions: momentum is conserved.
Collisions: momentum is conserved.
y Collision between two objects: no external impulses
y System = the two objects. “External” = outside system.
y Force/impulse on each object due only to other object.
y Net force over both objects =
Change in the TOTAL momentum =
In other words: Total momentum is
y Total momentum of system:
y Momentum: a vector (add up like forces)
y total(mv) before collision = total(mv) after collision
A
B
C. same
y
Elastic = bounce off each other, Inelastic = stick together
y Momentum conservation: good way to analyze collisions
y Do not need to know forces involved (just: elastic/inelastic)
y Bouncing has more impulse than stopping.