6.1. The Big Idea http://www.ck12.org
6.1 The Big Idea
In the absence of a net force, objects move in a straight line. If they turn —that is, if their velocity changes,even only
in direction—there must be an applied force. Forces which cause objects to turn around continuously in a circle
are known ascentripetalforces. When an object moves in a circle its velocity at any particular instant points in a
directiontangentto the circle. The acceleration points towards the center of the circle, and so does the force acting
on it. This is only natural, when you think about it —if you feel a force pushing you towards your left as you walk
forward, you will walk in a circle, always turning left.
The reason the Earth orbits the sun is due to the Universal Law of Gravity and centripetal motion. The Universal
Law of Gravity was a great achievement in the history of mankind. It unified the ’heavenly bodies’ and the ’Earthly
bodies’. Isaac Newton showed that an apple falling to the ground and the moon orbiting Earth are both explained by
the same laws of physics.
MEDIA
Click image to the left for more content.Key Equations
Centripetal Force
FC=
mv^2
r
m mass (in kilograms, kg)
v speed (in meters per second, m/s)
r radius of circleGravitational Force
FG=
Gm 1 m 2
r^2
m 1 ,m 2 Masses of objects
G Gravitational Constant, 6. 67300 × 10 −^11 m^3 kg−^1 s−^2
r Distance between objectsKey Concepts
- An orbital period,T, is the time it takes to make one complete rotation.