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Explanation of Centripetal Force by Ron Kurtus - Succeed in Understanding Physics. Key words: physical science, Law of Inertia, swinging ball, velocity, perpendicular, Moon, Earth, gravity, centrifugal fictitious, pseudo, Ron Kurtus, School for Champions. Copyright © Restrictions
Centripetal Force
by Ron Kurtus (16 March 2010)
Centripetal force is a force applied perpendicular to the motion of an object over a period of time, causing the object to move in a curve path.
Depending on the way centripetal force is applied, the path of the object may be a slight curve to a circle or other conic section. The Law of Inertia causes a centrifugal inertia force, which is equal and opposite to the centripetal force.
Examples of applications of centripetal force include swinging a ball on a string, the motion of the Moon around the Earth and a car going around a curve. There is an equation of centripetal force as a function of the mass and velocity of an object and its radius of curvature.
Questions you may have include:
- What direction should centripetal force be with respect to the motion?
- What are some examples of centripetal force?
- What is the centripetal force equation?
This lesson will answer those questions.
Useful tool: Metric-English Conversion
Centripetal force must be perpendicular
A centripetal force is a force that is applied perpendicular or orthogonal to the direction of motion of an object. The force is applied for a period of time, thus causing the object to move in a curved path. If it was only an instantaneous force, the object would simply change directions and continue in a straight line.
This follows from the Law of Inertia or First Law of Motion defines a property of matter and space, where an object in motion will follow a straight line unless acted upon by some sideways force.
Different curved paths
Centripetal force can simply cause the path of an object to curve for a short distance, or it can be continuous and cause the object to follow a circular path around some center point. There are some cases where the centripetal force can cause an object to follow an elliptical, parabolic or hyperbolic path. These paths are called conic sections.
Centrifugal force
The tendency of the object to continue in a straight line results in what is called its centrifugal force. This force acts in the opposite direction of the centripetal force, pulling the object outward. Centrifugal force is not a real force but is instead an inertial force. It is also called a fictitious or pseudo force.
(See Centrifugal Force is Not Real for more information on that subject.)
Examples of centripetal force
An object being swung around on a rope, the motion of the Moon around the Earth and an automobile going around a curve are examples of a centripetal force being applied.
Swing object on rope
When you swing a ball around on a rope, you must hold onto the rope and pull on it with a force. Otherwise the ball and rope will fly off according the Law of Inertia, which wants to have an object move in a straight line. The force you are pulling on the rope is the centripetal force.
Ball swung on rope requires centripetal force
to keep ball from flying away
Motion of Moon around Earth
The Moon is kept in orbit around the Earth through centripetal force caused by the constant gravitational force between the Moon and the Earth.
If the gravitational force would suddenly vanish, the Moon would shoot off in a straight line, tangent to its previous orbit around the Earth.
Car going around a curve
When an automobile moves along a road, it will tend to move on a straight line, due to its inertia. However, if it comes to a curve in the road, the driver turns the steering wheel to aim the front wheels in a direction following the curve in the road.
Tires provide centripetal force for car going around a curve
The friction between the front tires and the road create a force that is perpendicular to the direction of motion. That friction force is the centripetal force, causing the automobile to go on a curved path.
Force required
The equation for the centripetal force required to cause an object to follow a curved path is:
F = mv2/r
where
- F is the force
- m is the mass of the object
- v is the straight line velocity of the object
- r is the radius of curvature cause by the force
Summary
Centripetal force is perpendicular to the motion of an object over a period of time, causing the object to move in a curve path. That path may be a slight curve, a circle or curved path. The centrifugal inertia force is equal and opposite to the centripetal force. There are various possible applications to the centripetal force. The equation for the force is: F = mv2/r.
Work beyond your abilities
Resources and references
Websites
Books
Forces In Nature by Liz Sonneborn Rosen; Publishing Group (2004) $25.25 - Understanding gravitational, electrical and magnetic force
The Science of Forces by Steve Parker; Heinemann (2005) $29.29 - Projects with experiments with forces and machines
Glencoe Science: Motion, Forces, and Energy, by McGraw-Hill; Glencoe/McGraw-Hill (2001) $19.32 - Student edition (Hardcover)
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Centripetal Force