Peoples Physics Book Version-2

http://www.ck12.org Chapter 7. Centripetal Forces Version 2

7.1 Forces so Far

In the absence of a net applied force, moving objects travel in a straight lines; this is Newton’s First Law. If their
velocity changes,even only in direction, there must be an applied force. When something experiences a net applied
force, there are several possibilities:

1. The force is constant in magnitude and direction and points along the line of motion, or the object is at rest.
   In this case, the object will accelerate or decelerate in the direction of the force. The object’s position and
   velocity can be found using the so called ’big three’ equations.


2. The force is constant in magnitude and direction and acting at some non-right angle to the direction of motion.
   In this case, the object’s velocity vector can be broken down into perpendicular components in such a way that
   on is parallel to the force. Then the problem is reduced to two one dimensional problems — along the force
   and perpendicular to it — which can be solved according to case 1 above. An example of this is parabolic
   motion under the influence of gravity.


3. The force is constant in magnitude, the object is in motion, and the force isalways perpendicular to the
   velocity vector. In this case, the object will move in a circle. This is a kind of ’opposite’ of case 1: then, the
   object’s speed changed — whether increased or decreased — by the largest possible amount for a given force,
   while now the object’s speeddoes not change at all.


4. The force is not constant
   a. The force is due to a compressed or extended spring. We will cover this later.
   b. All other cases: beyond the scope of this book.