Download Net force changes the motion - University of South Alabama

Newton’s Second Law of Motion
PH 104 w/ dr. g
Lec 5
FORCE
Net force changes the motion
y Acceleration is caused by a net force.
y Mass and Weight are two different things.
y Mass resists acceleration = Inertia!
y Acceleration = Net force / mass
y Free-fall: Acceleration = g
y Nonfree-fall: Acceleration < g
Net force changes the motion.
Net force changes the motion.
Acceleration is caused by a net force.
Mass and Weight are two different things.
y Acceration is directly proportional to Net force
y
y Example: pushing crate against friction, constant v…
y Friction force:
y Acts only when there is force on the object
y Opposite direction as applied force, velocity
y Static friction : keeps object stationary
y Kinetic (sliding) friction : wants object to be stationary
y Solid-solid: mostly independent of contact area, speed
Melanie
Roach, USA Olympic weightlifter, lifting 172 lbs
y Mass versus
weight:
y Weight = gravitational force exerted by the Earth
y Metric: Newtons (N) (a bit less than quarter-pound)
y English:
y Mass = quantity of matter in the object = measure of inertia
y Metric:
y English:
y Mass ~ Weight : Can use weight to measure mass
y Examples: weightlessness, weight on the Moon
y Mass is different from volume.
y In fluid: dependent on speed (example: air drag, later)
Net force changes the motion.
Net force changes the motion.
Mass resists acceleration = Inertia!
Acceleration = Net force / mass
y Larger mass : more difficult to accelerate
y NEWTON’S 2ND LAW OF MOTION:
y Using the same force, acceleration of a mass:
y When force in Newtons, mass in kg, acceleration in m/s2:
acceleration =
• (acc. and mass are “inversely proportional” to each other.)
• Compare masses without weighing?
y Wiggle on frictionless surface : which is harder to move?
y Also know:
acceleration ~ net force …
net force
mass
a=
∑F
m
QUESTION!!
What is the acceleration of a 20-kg pail of
cement that is pulled upward (not sideways!)
with a force of 300 N?
Net force changes the motion.
Free-fall: Acceleration = g
y Why a = -g for ANY mass:
y Galileo did not know why. Newton:
y Came up with a = F/m
y Showed gravitational force = weight ~ Mass (why?...)
(a)200
m/s2
(b)150 m/s2
(c)10 m/s2
(d)5 m/s2
Net force changes the motion.
Nonfree-fall: Acceleration < g.
y Frictionless : a “first order” approximation.
y a = F/m still works! (F includes drag.)
y Drag = air resistance = R:
y Directly proportional with:
y surface area (example: parachute)
y Speed
y Now net force = weight minus R
y a < g, R increases as v increases
y max R = weight: net force = zero: v becomes constant!
y v = terminal velocity
Net force changes the motion
y Acceleration is caused by a net force.
y Mass and Weight are two different things.
y Mass resists acceleration = Inertia!
y Acceleration = Net force / mass
y Free-fall: Acceleration = g
y Nonfree-fall: Acceleration < g
y For forces in N, mass in kg
Weight = F = m g
where g = 9.8 m/s2
a=
F mg
=
=g
m m
QUESTION!!
What is the acceleration of a skydiver
at the moment when air resistance
becomes equal to half her weight?
(a) g
(b) g/2
(c) g/4
(d) Zero (velocity is terminal)