P201 Menu


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.


Ftotal = m a

But what are real forces on the mass m?

Non-contact forces

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

Interactive Physics

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.

  1. Law of Action-Reaction If one body (A) exerts a force FAB on another body (B), then body (B) exerts an EQUAL and OPPOSITE force FBA on body (A).


Sorting out the implications of the laws