October 27, 1998
Changing from Kinematics to Dynamics introduce mass, m, of object that is moving with velocity, V, and has acceleration, a.
Interactive Physics: Freefall; ... Pendulum.
Multiplying V and a vectors by mass m
m V is called "motion" or momentum, usually replaced by ... p = m V
m a is called "net force" .... Sum F = m a
Newton stated that the net external force on a body causes acceleration of the body
a = DV/Dt
Multiply both sides by the mass m of the object moving
ma = net external force on the body
mv = momentum of the body eg football team of mass m and moving downfield with velocity v has momentum.
m a = D(mV)/Dt
Sum F = m a = D(mV)/Dt
And, just as we divided the acceleration into apath and aperp, we now divide Newton's Law into
Sum Fpath = m apath
Sum Fperp = m aperp
Interactive Physics with Pendulum
a and V
Adding to the picture Ftotal.
But what are real forces on the mass m?
Gravity Fg = G M m/r2 = m g
Contact forces String FT
Add Gravity and String forces
Now can YOU identify the real forces that act along the path and cause apath of the pendulum?
And can YOU identify the real forces that act perpendicular to the path and cause aperp of the pendulum?
Other types of forces that can act on bodies.
Surfaces can push perpendicular to surface: N called the Normal force
can push parallel to surface: f called the friction force
Hands, springs, electricity, magnetism
Newton's Law used to EXPLAIN Galileo and Kepler findings.
Chapter 5 the Apple & the Moon
why do all objects fall with the same acceleration?
is the moon freely falling? most important 1/20".
Newton's THREE Laws we have started with the second law but it is important to see how they fit together and give a beautiful view of nature.
1. Law of Inertia A body moving inertially (its natural motion) is moving in straight line with constant velocity Note that zero velocity is included Such a body has no NET external force on it. Sum F = 0.
2. Law of Non-inertia. A body moving non-inertially (not according to its nature) is accelerating which includes apath and/or aperp. This REQUIRES a NET external force on it. Sum F = ma.
Sorting out the implications of the laws
Can a horse pull a wagon without violating the laws?
Can friction actually help a body move forward?
Let's hitch them up and see what Newton's Law tells us. Can the horse pull harder on the wagon than the wagon pulls back?
Bugs hitting your car: who wins the law of forces?
Newton's laws for satellites how do they "sit" at one point above the earth the physics of Telestar 4.
Weightlessness why is an astronaut weightless?
does it mean no gravity?
have you experienced it?
Do you REALLY understand Newton's Laws? Let's try a little group quiz